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EOMBRYOOLOGY

VOL. II

BY

J. (_u‘rlLAHAM KEIHR.

pa- MACMILLAN-AND 00., LIMITEI) ST. MARTIN’S STREET, LONDON

1919 ‘ TO THE MEMORY O14‘

THREE CAMBRIDGE NATURALISTS

CHARLES DARWIN, M.A., oimxsjws COLLEGE FRANCIS BALFOUR, M.A., TRINITY COLLEGE

ADAM SEDGWICK, M.A., TRINITY COLLEGE

Preface

THE object of this volume is to sketch in its main outlines the science of Vertebrate Embryology as disclosed by the study of the 11on-mammalian vertebrates. It is not meant as a work of reference as regards details. The facts of embryology are dealt with as illustrating general principles: large masses of data which have no particular bearing, in the present condition of knowledge, are deliberately omitted.

It is believed that a volume upon the lines indicated is greatly needed~———not merely for students intending to specialize in vertebrate morphology but also for students of medicine who desire to know something of the framework of morphological principles which serves to unite together the detailed facts of anatomy. The science of embryology, in fact the science of animal morphology as a whole, has suffered much through the patient but undiscriminating accumulation of masses of mere descriptive detail which have tended to obscure general principles and incidentally to smother interest in one of the most fascinating of sciences. It is hoped that the student who reads through portions of this book will have at least his suspicions aroused that behind the dull facts of anatomical structure there lies a very charming and livi11 opl1y.

It has again been one of the misfortunes of vertebrate embryology that its teaching has been dominated in great part by general ideas based upon insufficient data. In an evolutionary science like morphology the real fundamental principles are to be elicited by enquiry into the more archaic types of existing animal life. But the material for the earlier embryologieal investigations was chosen

not for its archaicism but rather for purely practical reasons such as

V11 W . viii EMBRYOLOGY OF THE LOWER VERTEBRATES

accessibility or case of investigation. It follows that at the present time when we have knowledge of the more archaic subdivisions of the vertebrata not accessible to the early builders of the science, it is necessary to regard the historical foundations of vertebrate embryology rather critically in the light of the fuller knowledge of to-day. In essaying the writing of this volume I have been fortunate in having at my disposal——for the first time in the history of em}iryology——devclopmental material of all three-genera of Dipnoi as well as of ]’oly/pfe7'us-—i1i addition to the more accessible material of the other relatively archaic groups constituted by the Elasmo- branchs, Actinopterygian Ganoids, and Uroclele Amphibians. This has rendered possible an all-round survey of the chief problems of vertebrate embryology which would otherwise have been quite impossible.

As already indicated 1 do not intend this volume as a work of reference on the details of vertebrate embryology: that role is fulfilled by the wonderful and indispensable ][6t7t(flb’llzG]t edited by O. Hertwig-——of which incidentally I have made constant use and to which I must express my sincere aclcnowledgments. Nor do I attempt to give full historical accounts of the development of various parts of .the subject. The literature lists are merely guides to paint the way to the student who desires to extend his reading to original papers. The dates given in these lists are as a rule the dates given on the title-page of the complete volume, and are merely to facilitate finding the particular paper: they must not be taken as giving the actual date of publication of the individual memoir.

I have to express my grateful thanks to various friends. As regards the first three chapters I had the benefit of the wise counsel of Mr. Walter Heape, who unfortunately however found himself com- pelled by the exigencies of war work to withdraw from the Editor-ship.

Various chapters have benefited by the help and advice of my friend and colleague, Dr. W. E. Agar. The entire volume has been read in proof by Mr. James Chumley and Dr. Monica Taylor, to both of whom I am deeply indebted. To Dr. John Love and to Dr. Jane Robertson I am indebted for helpful criticism in regard to special sections of the book. i

The illustrations which forin a marked feature of the volume I PREFACE - ix

owe for the most part to the artistic skill, combined with high scientific accuracy, of Mr. Kirkpatrick Maxwell. Apart from the completely original figures it will be noticed that there are many which have been worked up from illustrations in original papers, but which are practically new figures. In all such cases, however, I have thought it only right to make due acknowledgment of the author of the original figure.

For permission to borrow particular text-book figures l have to thank Professor Frank R. Lillie, Mr. John Murray, Messrs. Masson & Cie, and Messrs. Macmillan. The present unfortunate circum- stances of international strife call for a special acknowledgment of the generous way in which Professor Alf. Greil entrusted to me the originals of his valuable unpublished figures illustrating the development of the heart in the bird. They are reproduced on pages 384 and 385. ,

I have included the name of Charles Darwin in the dedication of this volume to emphasize the fact that Embryology is primarily a branch of synthetic evolutionary science. While the fashion of the day in evolutionary research favours rather experimental research into the phenomena of inheritance and more or less speculative enquiry into the ultimate mechanism of inheritance or into the possible causes of evolutionary ehange—-morphology, and more especially embryology, is steadily at work all the while, mapping out the paths along which the evolution of organisms and their con- stituent organs has taken place. Working away in comparative seclusion, unadvertised, and for the most part unnoticed, embryology is thus building up an important part of the framework of what will be the permanent edifice of evolutionary science.

J. GRAHAM KERR.

Fel)'I"u(w'_z/ 3, 1919.

Contents

CHAPTER I

PAGE SEGMENTATION, GASTRULATION, AND THE FORMATION OF THE GERM

LAYERS . . . . . . . . 1

CHAPTER II

THE SKIN AND ITS DERIVATIVES . . . . . 69

'CHAPTER HI

THE ALIMENTARY CANAL . . . . . . 144‘

CHAPTERIV

THE COELOMIC ORGANSI . . . . . . 197 CHAPTER,V

THE SKELETON . , . . . . . 288 CHAPTER VI

VASCULAR SYSTEM . . . . . ., . 360

CHAPTER VH

THE IXTERNAL FEATURES OF THE BODY . . . 429

CHAPTER VH1

ADAI"].‘ATION To ENVIRONMENTAL CONDITIONS DURING EARLY STAGES

OF DEVELOPMENT . . . . . . 455 xi xii EMBRYOLOGY OF THE LOWER VERTEBRATES

()HAP'1‘El€. IX PAGE SOME 012' THE GENERAL CONSIDERATIONS RELATING TO THE EMBRY-

onouv on‘ runs V ER'.l‘1dBI{.A'1‘A . . . . . 484

CHA PTER X

'[‘nw. PIw:1.'IcAr. STUDY ow THE Emnmronomr on‘ THE COMMON FOWL 508

C IIA PTER X1

ll1N'r.~s 1u«:uAnmN«: vumc PRA(‘!’l‘ICAL STUDY ow THE EMBRYOLOGY on‘

THE VARIOUS 'l‘YP1«3s 01» Lowlm VER’I‘l'}BHA'l‘EH . . . 558 APPENDIX THE GENEIl.AL 1\ImTHo1»s op EMBRYOLOGICAL H.Ebl<JARCII . . 573

INDEX . . . . . . . . 5333




CHAPTER I - Segmentation, Gastrulation, and the Formation of the Germ Layers

THE Vertebrate begins its individual existence in the form of a single cell, the Zygote or fertilized egg, which in turn originates in the process of fertilization by the fusion or conjugation of two gametes.‘ Of these the microgamete or spermatozoon, derived from the male parent, is of relatively insignificant bulk as compared with the macrogamete or unfertilized egg. As a consequence the more obvious features of the Zygote, such.as shape, size, and so on, are simply taken over from the macrogamete-in other words, they are of maternal origin. Such maternal features may remain obvious for some time during early stages of development, so long in fact as the maternal protoplasm remains predominant in bulk as compared with that elaborated under the control of the Zygote nucleus, but it seems unnecessary to assume that this fact has the important bearing upon questions connected with Heredity which has been claimed for it by some workers on Invertebrates.

The Zygote is a typical cell, composed, so far as its living substance is concerned, of cytoplasm and nucleus, the cytoplasm containing a lesser or greater amount of food-material or yolk. In shape it is in the vast majority of cases approximately spherical. In the Myxinoids it is elongated, almost sausage-shaped, and in a certain number of cases, for example Amia, its sh e is literally “ oval.”

The macr0gamete——-and fore the Zygotemdiifers much in size in different Vertebrates, ranging from about '1 mm. in diameter in Amphioacus to as much as 85 mm., or more, in the case of the African Ostrich. In some of the Sharks the size of the Zygote is also very great and this was doubtless the case too, with that of‘ such extinct birds as Aepyozmls.‘-3 Such relatively huge Zygotes are of

1 A general account of the processes of gametogenesis and fertilization has already been given in Vol. I. and they are not further dealt with in this volume. I

2 Assuming that the Zygote of Aeyyormis bore the same ratio in size to its protective envelopes as does that of the Ostrich it would measure about 160 mm. in diameter.

VOL. II B 2 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

interest as being in bulk the largest single cells known in either the

Animal or the Vegetable Kingdom. The adjoining list based upon data obtained by various observers

-—M‘Intosh and Masterman, Bashford Dean, Boulenger, Budgett, Bles, Semon, Salensky and others~—-brings out in more detail the differences in the size of the Zygote amongst the lower Vertebrates.

APPROXIMATE DIAMETER 01*‘ EGG (lN M1LLlMETRES) OF VARIOUS FISHES AND AMPHIBIANS

Amph/io:1:u.s' -1. Botlms ma.'r'i'nm.s 1. Petro1nyr:on 1. Pleummectes platessa. 1-8. Bdellosfnmu. 14 -— 29 x 7 - 10-5. P. 7m7('rocephalw.s 1-3. P'I"ist't'm‘u,s melnxnustomus 16. P. limamla -75. A(:¢mth'ias 35 - 40. Salsa, 'vu.l_qm"z's 1-2.

J apanese L.-imnid '1 (1 )oflein) 220. C'l'u.pea hctreugus -9 — 1. Torpedo m:¢:ll(I.fa 20 — 25. 0. sprattws -1. P0lypf.m'ns 1-1. (7m'wt0du.s 2-8.

A ¢'v'penser 2 —— 2-8. I"r0t0pfM'u.s 3-5 — 4. I.ep1Idosteu.s 3. ' Lep'i¢l0s'i7-en 6-5 ~ 7. A'rmTa, 2-5 —- 3 x 2 -—- 2-5. Axolotl 2.

Boccus lalmuv 1-4. Salmncmtlra. wmculosa 4. M wllus .su/r'rm1.la:t'u.s -9. T'rz'(o'n 1-8.

Cottus scorgnlus 1-75. Nectums 6.

Tr'igla gumardus 1-5. Hypogeophols altcmans 4 -— 5. Agonus catwphractws 1-8. - H. 'rostrams 7 — 8. Tmchxinus vipera 1-3. ' Xenopus laemls 1-5. Scomber soombrus ]-2. Pipu; 6 —- 7.

’ Gobius mrinutus I — 1-4. Alytes obstemfcans 3 - 5. Oyclopterus lumpus 2-5. Pelobates fuscus 1-5. Anarrhichas lupus 5-5 — 6. Bufo lent1'g7I'nosu.s 1.

Pholis guomellus 1-7. Hyla. goeldz"i 4.

Garlus collcwis 1-4. Nototrema fissvipes 10.

0. aeglefimos 1-4. Paludvlcola fwscomacmlata 1. G. m'reus 1-l. .Engg.s-towwr, ovale 1-25. Motella mustela -7. Oomufer salomom's 5. Brosmz'us brosme 1-3. Rhacophorus re1Inwcmltz"i 3. Ammodytes lcmceolatus -76. Rana. temporarta 2. Hippoglossus mtlgaris 3 — 3-8. R. opisthodon 6 — 10.

Within the limits of a single genus different species may show marked differences in the size of their eggs, (2.9. the Teleostean fish Arius austmlis has eggs a little over 3 mm. in diameter (Semen) while in the case of A. boekii they measure over 10 mm. in diameter and in A. commersom"i as much as 18 mm.

Even within the limits of a single species quite measurable, though less conspicuous, differences in size exist between the eggs of different females, and the same holds also, though to a far less extent, for the individual eggs laid by a single female.

The differences in size which have just been alluded to are correlated with the fact that the egg of the Vertebrate carries in its cytoplasm a less or greater amount of reserve food-material or yolk The presence of a readily available supply of food within the egg carries with it the immense .adva‘i1tage of freeing the young I YOLK V 3

animal, during the early stages of its development, from the need of having to fend for itself. And, correlated with this, the necessity of developing more or less complicated adaptive fea- tures to fit it for survival as an independent free- living creature in these early stages is avoided. The yolk consists occa- sionally of but more usually of rounded or { sub - 0 s ‘ highly nutritious material. The yolk granules are frequently of a character- istic co r salmon colour or «rreenis and these impart their colour to the egg as a whole. vvhere however the yolk A.-—Section through egg of A7Ii—p/I.io.-:.'u...s'. becomes very finely subdi- After Cerfontaine, 1906.) i

vided W8 filld, 3.3 ill the (53.89 The small crosses mark the position of apical and abapical

0 ' col 1. d 1 roun poles. Near the apical pole is seen the second polar body t on e g ass g d adherent to the surface of the egg. The egg- and sperm-

into Powder: that the char‘ nuclei have not yet fused, and are seen in close proximityito acteristic colour is replaced one another.

by white. ' ‘his fine- subdivision of th yolk with its acconi- ‘s paiiying White colour is commonly found in parts of the egg { where metabolism is particularly active, for example those p_ortions in which active growth or cell division is about to take place, the fine subdivision making the yolk readily assimilable and so available for 1ngt',a_ B.———-Section through egg of 1'...-',,-',».'» . .-..~.-, showing more marked

- _ tendency for the yolk granules to concentrate towards b°11° needS- Where the abapical pole (Teloleeithal condition).

the yolk is com- y . l

, _ . n, lint-leIi.~. 1;, pigmi-iii. pa-I'al':1V61y 3131311 111 'l‘lu~ yolk gi‘:inule.s' :m- lll|ll(':I.lI-Il in lmth li;_:un_-s by dark dots. amount, as in Am- philomus (Fig. 1, A), it may be distributed nearly equally through- out the egg substanoe; in other words there is an 3 approach


Fin. 1. 4 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

to the isolecithal condition: but as a rule in the Vertebrate the yolk is large in amount and is concentrated towards the lower or abapical pole of the egg, the protoplasm towards the upper or apical pole being comparatively poor in yolk (Telolecithal condition).

This segregation of the dead yolk and the living protoplasm towards opposite poles of the egg is well seen in the relatively huge egg of the bird where the protoplasm is concentrated in a germinal disc containing practically no yolk and forming a cap at the apical pole of an enormous mass of yolk practically free from protoplasm.

' It has already been indicated that the egg may have a character- istic coloration due to the colour of the yolk. Such yolk coloration may be looked upon as accidental and without any special biological significance in itself. Many eggs on the other hand especially amongst the Ganoid fishes and the Amphibians are given a dark colour by the presence within them of brownish-black pigments belonging to the melanin group. Such pigment appears to be of definite biological significance, providing as it does an opaque coat which protects the living protoplasm from the harmful influence of light. Eggs in which it occurs develop, as a rule, under conditions where they are exposed to intense daylight. The eggs of ordinary Frogs and Toads for example which are surrounded by clear trans- parent jelly have a well-developed pigment coat. On the other hand in the case of Frogs and Toads whose eggs are surrounded by light- proof foam (see Chapter VIII.) or are deposited in burrows under- ground‘ they are commonly without pigment.

In all probability this deposition of melanin pigment in the superficial protoplasm of the egg (normally in its w.ppe'r portion) is to be interpreted as having been originally a direct reaction to the influence of light, the metabolism being so affected as to bring about the formation of this particular iron-containing excretory pigment.

It may be objected that the pigment is produced before the egg is laid (eg. the (lommon Frog) and therefore before it is exposed to the action of light, but as a matter of fact the body-wall of the adult is by no means opaque to light rays and even while still in the ovary the eggs are exposed to the influence of faint light. If we may take it, as seems probable, that the influence of natural selection has gradually developed in such cases the particular type of sensitiveness to light which leads to the formation of melanin, on account of its protective value, then there is nothing surprising in the developing of this sensitiveness at earlier and earlier periods until at last it has resulted in the pigmentation of the still intra-ovarian egg in response to the feeble light rays which penetrate the body-wall.

The other possible explanation of this precocious pigment forma- tion is that the production of the pigment though originally taking place as a direct reaction to light in the laid egg, has become so engrained in the constitution of the species that it now comes about even in the absence of the original stimulus. The objection to this explanation is that it postulates the inheritance of an “acquired I SEGMENTATION 5

character,” and that is unfortunately not justified by our knowledge so far as it goes at present.

SEGMENTATION

The first important steps in the evolution of the unicellular Zygote into the multicellular adult are seen in the» process of Segmentation which is, in fact, a process of mitotic cell division showing special peculiarities in difl'eren_t groups of the Vertebrata. During this process there appear in succession on the surface of the egg grooves which gradually deepen and eventually divide the egg incompletely or completely into dis- tinct segments or Blastomeres. Before entering into the details of this process it will be convenient to describe it in outline and define the various technical terms used in its description.

The first phase of segmentation is commonly marked by the appear- ance of a superficial groove which may conveniently be designated by the letter a, passing through both the upper and lower poles of the egg. Such a groove or furrow is termed meridional, as it marks a great circle on the surface of the egg corresponding to a meridian of longltlide on 3’ terresbrlal globe’ Fm. ‘2.——-Diagram to illustrate technical The Slllgle nucleus Of the 80139 terms used in describing the process of meanwhile divides by mitosis -— a segmentation- dallghter nucleus passing into each am, apical pole; ab.p, abapical pole; c, equa- hernisphere. From the known facts §‘i’(‘;:f‘;1.f,}1‘l‘I‘_’;‘(’)““’,‘, '1 3 ’"” ‘""" of fertilization we have reason to ' A‘ ' believe that the Zygote nucleus contains exactly equivalent amounts of chromatin from each of the two parents. In the process of mitosis this maternal and paternal chromatin is again shared equally between the two daughter nuclei. L

The first meridional furrow gradually deepens so that the egg becomes completely divided into two blastomeres or segments each representing a hemisphere of the Zygote. A second meridional furrow (/3) now appears in a plane perpendicular to that of the first and by the deepening of this the egg becomes divided into four equal blastomeres. s

The next furrow to appear may be one running round the equator of the egg (equatorial). In eggs, however, which are not absolutely isolecithal-—~and this holds for all the lower Vertebrates-— 6 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the third furrow appears, not at the equator but at a level nearer the apical pole, and is termed a latitudinal furrow, corresponding as it does with a parallel of latitude upon a terrestrial globe. The distance of this furrow from the equator, its degree of latitude so to speak, is roughly proportional to the degree of telolecithality of the particular egg, suggesting that the volume of living protoplasm may be roughly equivalent in amount upon the two sides of the division plane to which this furrow gives rise.

When this third division is completed the egg consists of eight blastomcres, the four on the apical side of the division plane being smaller (micromeres) than those on the abapical side (macromeres).

The next furrows to appear are two in number and in the simplest condition they are meridional, bisectin g the angles between the two first furrows. More frequently however these furrows instead of traversing the pole of the egg are discontinuous at this point and each is displaced somewhat so as to join the first or second meridional furrow at a less or greater distance away from the pole. To such a furrow we apply the term vertical (Fig. 2, '12., cf. also Figs. 14 and 16, C).

It is as a rule noticeable that meridional or vertical furrows tend to become apparent first in their portions nearest to the upper or apical pole of the egg, their lower ends gradually extending down- wards towards the abapical pole. This phenomenon appears to be due to the retarding influence of the dead and inert yolk. The proportion of this to the living protoplasm becomes greater and greater as the distance from the apical pole is greater, and in correlation with this the retarding effect becomes more and more pronounced.

After segmentation has reached the stage indicated its further progress tends to become irregular. New furrows make their appearance—~latitudina1, and vertical or meridiona1—and the surface of the egg takes on the appearance of a mosaic-work, while its substance becomes cleaved or split apart into corresponding blaste- meres as the superficial furrows gradually deepen into slits.

At somewhere about this period there begins a new type of mitotic division in which the individual blastomere becomes split in a direction parallel to a plane tangential to its outer surface, so that it divides into an outer blastomere visible in surface view and an inner one concealed in the interior of the egg.

With the further progress of segmentation the blastomeres divide over and over again, so that eventually the egg is converted into a very large number of small cellular elements. The rapidity with which the cells divide bears a rough inverse relation to the richness ‘of their contents in yolk. Dead inert yolk tends to cause the cell to lag behind in the process of division, and the result of this in telolecithal eggs is that the difference in size between inieromeres _and macromeres becomes more and more marked as segmentation goes on—-—the lower and more richly yolked segments 1 SEGMENTATION 7

tending to lag, in their mitotic division, more and more behind the less yolky upper elements. This inequality is found at its maximum in the large eggs of Elasmobranchs, Reptiles, and Birds, where the main mass of the egg has its proportion of protoplasm reduced so nearly to vanishing point that it does not divide at all. It is only a small portion of the egg in the neighbourhood of the apical pole that is rich enough in protoplasm to carry out the process of segmentation into separate cells. This is known as the germinal disc or, later on, when it has segmented into a mass of cells, blastoderm. An egg of such a type, showing partial or incomplete segmentation, is termed meroblastic in contrast with the more primitive holoblastic type in which the egg segments as a whole.

The blastomeres into which the egg divides being composed of protoplasm——a somewhat viscous fluid——-tend under the physical laws of surface tension to assume a spherical shape except when flattened by pressure against their neighbours. There thus exist normally chinks between the blastomeres filled with wateryfluid.

As the process of segmentation proceeds this intercellular fluid increases 'in amount and the process normally culminates in the stage known as the blastula. The blastula consists of a more or less spherical mass of cells surrounding a relatively considerable volume of fluid which is for the most part no longer distributed in small chinks but collected together into a large space——the blastocoele or segmentation cavity.

In the simplest case, that of Amphiowus, the wall of the blastula is composed of a single layer of cells—the cells towards one pole being larger and containing fine granules of yolk or food material. In holoblastic Vertebrates above Fishes it is however, as a rule, no longer composed of a single layer, the roof of the segmentation cavity being frequently composed of two layers while the floor is composed of a thick mass of large heavily yolk-laden cells.

The details of the segmentation process may now be followed out as it occurs in the various types of lower Vertebrates.

AMPHIOXUS

Amphioxus is, of all the lower Vertebrates, that in which developmental processes are least interfered with by the presence of yolk, and for this reason the phenomena shown during its seg- mentation must form the basis for the comparative study of the corresponding phenomena in the Vertebrata in general.

The process of segmentation in Amp}:/iowus was described first in two works which are now amongst the classics of morphological science: the first by A. Kowalevsky (1867) and the second by B. Hatschek (1881).

The process begins (Fig. 3) with the appearance of a depression of the surface in the region of the apical pole. This depression takes an elongated groove-like form and extends outwards at each -8 l<‘..\-all-’._l=€\r_"(_) "H" (f)l<‘ 'llll'|C l.()V\’|<ll{ \~’F.l{.'|‘l*}l-‘.lcl.-'\PICS cu.



F" end until linal .‘ ‘it forms a wide Ill(:1‘.i(.llull:.l.l vulle_y encircling the entire egg (}'_4"ir_f. 3, This valley g1‘aLl11ally deepens dividing the egg into two halves. Finally after about 5 minutes from the commencetm-nt of the process the protoplasmic bridge connecting

the two lmlves srmps across and the egg is now completely divided

Fro. 3.»--Illustrating the process ofsegI'n'1cntat.i«)n1 o: (_After Hnt.~'«_-.h(-.k, 188].)

'I'}'w::pi(e:11 pole is a‘.»n\'u- in 4-.:u-h Iigm-r-.. The serum! pnlm ljmcly is sc-en in proximity to it.

into two blastomeres, each of which assumes a spherical shape in response to surface tension. The two hlastomeres become slightly flattened where they are in contact 71.6. in the plane of the first meridional furrow (Fig. 3, C). The future course of development shows that this plane corresponds to the sagittal plane of the embryo (Cerfontaine, 1906): in other words the two blastomeres represent the right and left halves of the developing individual. I SEGMENTATION 9

After a resting period of about an hour a second meridional furrow develops in a manner similar to the first and in a plane perpendicular to the plane of the first furrow. This gradually deepens and each hemisphere becomes divided into two blastomeres, each of which as before assumes a spherical shape and then becomes flattened o11t slightly against the other. Of the four blastomeres which are now present two, shown by subsequent development to be anterodorsal in position, are according to Cerfontaine normally smaller than the other two. .

The two meridional furrows (a. and /3) are followed after an interval of about a quarter of an hour by a latitudinal furrow slightly above the equator and this divides each of the four segments into two. The egg now consists of eight blastomeres———four smaller micromeres on the apical side of the latitudinal division plane, and


Fm. 4.—-—Apioal view of Am_phio.\'us eggs at the oiglnt-l)lastome1'c stage. (After E. B. Wilson, 1893.)

A, “Radial” type; B, “Spiral” type; and C, “ Bilateral ” typo.

four larger macromeres upon its abapical side. Each microinere lies, according to Hatschek, exactly over the corresponding macromere so that the apical side of the egg as seen from above looks like A in Fig. 4.

Wilson (1893), followed by Samassa (1898), has however drawn attention to the fact that in a considerable proportion of cases the cap of four micromeres is, as seen from above, rotated in a clockwise direction through an angle varying from 0° to 45° (Fig. 4, B) thus conforming to Wilson’s “spiral” type of segmentation or cleavage. Again in a still smaller percentage of eggs at this stage the blaste- meres are arranged according to Wilson’s “bilateral” type (Fig. 4, C) the eight blastomeres being arranged symmetrically on each side of the first division-plane but either two or all four macromeres being displaced outwards somewhat from this plane.

Fourth division. After another short interval (less than a quarter of an hour) a new set of furrows appear bisecting each of the already existing blastomeres so that the embryo comes to consist of sixteen blastomeres arranged in two tiers, eight micronieres above and eight macromeres below (Fig. 3, F). Hatschek described this fourth set of furrows as being meridional (Fig. 3, F) while according to Cerfontaine (1906) the division planes are when first 10 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

indicated meridional in position but become displaced somew-hat so as to be in the case of the micromeres perpendicular to the first (sagittal) division-plane or in that of the macromeres slightly oblique.

Fifth division. Each blastomere divides again} the smaller blastomeres towards the apical pole dividing rather earlier than, the others, and the result is that there are now thirty-two blastomeres in all, arranged in eight meridional rows of four cells each, the cell at the lower (abapical) end being decidedly larger than the others. Between these four large elements a wide opening is present (Fig. 3, G) leading into a space which made its appearance as a little chink between the blastomeres of the four-cell stage but which has since then increased greatly in size. This space in the interior of the egg is the blastoeoele or segmentation cavity.

From this period onwards the segmentation process becomes less regular. There has already shown itself a tendency for the larger blastomeres towards the lower pole to lag behind somewhat. And the arrangement of the blastomeres becomes less regular as they become smaller and fit more closely together. In particular the bilateral symmetry in the arrangement of the blastomeres which is conspicuous in most of the eggs during the earlier stages (Cerfontaine) ceases to be apparent.

To summarize the later phases of segmentation it may simply be said that the blastomeres go on dividing, the segmentation cavity increases in size, its communication with the exterior closes up, and there is formed eventually a blastula of approximately spherical shape. The wall of the blastula is composed of a single layer of cells those towards the apical pole being smaller and less rich in yolk than those on the opposite side (Fig. 3, I).

RAN A

In the case of the Frog we have an egg in which as compared with that of Amphiowus there is present a much greater proportional amount of yolk and which in consequence serves well to illustrate the nature of the influence of yolk upon segmentation.

The process of segmentation begins with the appearance, in the region of the apical pole, of a small dimple on the surface of the egg which gradually lengthens out to form the first meridional furrow (a). The furrow gradually extends downwards over the surface of the egg (Fig. 5, A) and becomes completed by reaching the lower pole after about an hour and a quarter.” It also extends inwards from the surface and finally bisects the egg into two

hemispheres. The second furrow (,8) is also meridional and is in a plane

1 As there are marked discre ancies between the accounts given by different observers we may take it as pro able that there is considerable variability in the details of segmentation about this stage.

9 See, however, later for caution in reference to the time factor in development. noticed further that the egg as

I SEGMEN TATION 11

peI'pem‘liuul.a1' to that of the first. It appears about three-quarters of an hour after the latter and, like it, extends downwards and inwards so that the egg becomes divided into four approximately equal segments.

The third furrow is latitudinal in position being situated (Fig. 5, C) roughly about 20° above the equator. It extends inwards and the egg is now converted into eight blastomerr-s, four micromeres towards the apical pole and four macromeres towards the lower pole. ‘ ‘

Closer study of these first three cleavages in the ease of the Frog brings out a number of im-. portant points. It will be noticed in Fig. 5 that the circular area of the egg-surface which is free from pigment is placed somewhat eccen- trically so that at one edge it approaches the equator of the egg much more nearly than it does at the opposite edge. It will be

judged by the distribution of pigment is arranged symmetrie- ally about the plane of the first furrow. This furrow seems to cor- respond, under normal conditions, with the sagittal plane of the embryo, and therefore the two hemispheres separated by the first furrow correspond to the right and left halves of the embryo. The study of later stages will bring out the fact tha'.t the point: In the Flu. 5.——lllu.stratiug segnwulationofl~‘rog's boundary of the unpigmented egg_ (Mm sc1,i.m.., 1.s9o,) portion which lies nearest to the equator marks what will become the posterior end of the embryo.

From tlw time of appearance of l".-lll‘. third furrow onwards wide tlilh-I.'m1(:<~.:-i ooour lwtween different Occasionally one may be found in which matters proceed with diagrammatic regularity. Two new 'lIH_']‘l(ll()l1al furrows appeai.- i11tei'seeting the angle between a and ,8 and like the latter tlwy gradually extend downwards, halving each of the existing lvlastonieres and thus giving rise to sixteen blastomeres—-in two tiers of eight, .micro_1nere..s above, macro- meres below. Then a latitudinal furrow e'tpp(’.a.1.‘H dividing the micromeres, and later a similar furrow dividing the macromeres; so that there are new four tiers of eight blastomeres each.

Commonly however there is no such regularity either in the arrangement or in the time of appearance of the furrows. The meridional furrows in particular tend to la‘ roplzuizcd by vertical 12

furrows which intersect u or /3 at some distance froin the poles.

l*}MP.RY(_)L()GY 011‘ THE LOWER V ERTEBRATES

CH.

As

regards the variation in order of development of the various furrows a good idea will be got from Fig. 6. Whatever be the case with the divisions immediately succeeding

D .

Fro. 6. «Illustrating the varia- tion in the order of appear- ance of the first cleavage furrows in Rmm p¢¢lu.s'trz'.s'. (After Jordan and Eyelas- hyincr, 1894.)

The .s4‘qllt*l1('e, in time, of the appearance of the furrows is in. «heated as follows:—-1, ; -3, _.._-; 3, ..... .. ; 5, .

4, -1-mu‘

7

several cells thick (Fig

the eight cell stage, from now onwards there is little regularity. All that can be said is that each individual blastomere goes on dividing over and over again, the length of time elapsing between successive divisions bearing a rough relation to the amount of yolk present in the particular blaste- mere.

Already at the third cleavage the eight blastomeres have a distinct chink——the com- mencing b1astocoele— between their inner ends and as segmentation goes on this space becomes larger. The thirty-two-cell stage is a blastula which in a meridional section (Fig. '7, A) is seen to correspond in its general character with the blastula of A mp/mloazus but to diifer from it in three features: (1) it is of larger size, (2) it is co1nposed.oi' fewer cells and (3) the difference in size between the less richly yolkcd cells towards the apical pole and the more heavily yolked cells towards the opposite pole is more marked.

As development proceeds a farther differ- ence becomes apparent. In the various mitotic divisions during the preceding phases of segmentation the axis of the spindle has been arranged more or less tangentially but now divisions begin to take place in which the spindle axes are arranged radi- ally and the division-planes tangentially. When this happens one of the two result- ing daughter cells is nearer the centre, the other nearer to the surface of the blastula and the effect of repeated divisions of this type is that the blastula-wall loses its original character of being composed only of a single layer of cells and becomes

. '7, B).

ELASMOBRANCHS

The egg of any ordinary Elasmobranch such as a Dogfish, Skate, or Torpedo, illustrates the type of segmentation that takes place I i SEGMENTATION . 13

in an egg in which the proportion of yolk present approaches the maximum. In this case the zygote nucleus commonly undergoes two mitotic divisions before there is any external symptom of segmentation. of the cytoplasm. Usually a single furrow makes its appearance first, incising the surface of the germinal disc but not extending to its periphery (Fig. 8, A). Occasionally a second regular furrow makes its appearance intersecting the first at right angles



' .31 0' ¢‘;“*"'I’~70p' -- «ms--.: ,_\__.',_:_‘..‘_1§T.__..‘_ _


\-

FIG. 7.-—-Vertical (meridional) sections through hlastulae of Frog. (From Morgan,- 1897.) .1 R, commencing invaginatlon ; SG, segmentation cavity.

and it is a curious point that it is sometimes this second furrow

which corresponds to the first nuclear division.

These first two furrows apparently represent the first two meridional furrows of the holoblastic egg though in the Elasmobranch the first to appear may be either or. or ,8. More usually, in place of a second regular furrow developing, irregular branches of the first furrow, or even independent furrows, appear and an arrangement of

somewhat radiating furrows is brought about which gradually becomes converted into a network (Fig. 8, B, C, D).

It should be noticed in regard to these segmentation furrows 14 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

that the first latitudinal furrow cannot be identified in the Elasmo- branch and further that the study of sections shows that the furrows sometimes out into the germinal disc obliquely instead of being perpendicular to the egg surface.

The nuclei of the blastoderm divide synchronously and after four divisions have taken place, when there are sixteen nuclei in place of the original single zygote nucleus, the segmentation furrows (Fig. 8, 0) form a network dividing up the blastoderm into smaller central and larger peripheral blastomeres. These hlastomercs are, however, not completely isolated from one another but are still in continuity

A 3 B‘

- " ‘- 7- ‘*—

‘v \

a '/’F'I_Lr.§{ \ K“

1' \ I \ I -

I '. I‘ I , I i I

s‘ ‘ J \ ’ ’

' ‘- __ .r


Fm. 8.--Surface views of the blastoderm of Elasmobranehs illustrating the process of segmentation. (After Ruckert, 1899.) .

A, 13, U, Torpedo; B*, 1’rist'£m'us; I), E, .\'cyllium.. Fig. 13* shows an abortive segmentation which may often be observed transitorily round the margin of the germinal disc [Merocyte segments of ltuckert].

at their bases, and, in the case of the peripheral blastomeres, at their outer ends.

Up to the fifth mitotic division the axes of the mitotic spindles have been approximately parallel to the surface but now blastomeres begin to divide with their spindle axes perpendicular to the surface so that a set of superficial segments becomes separated off. Beneath these fluid accumulates intercellularly and a segmentation cavity arises (Fig. 9, B).

During the sixth division some of the blastomeres forming the floor of the segmentation cavity become. separated off from the 1 6 SEGMENTATION 15

underlying, unseglnented, yolk (Fig. 9, C) and in surface View the

blastoderm assumes the appearance shown in Fig 8, D. Up to and including the seventh division mitosis takes place-

?‘K'g£? ». -4 r u "

.u -- I-rvpln n ‘r ._,’ ‘iI\~“_' F n V“ "I

FIG. 9.——Vertical sections through Elasmobranch blastoderms illustrating the process of segmentation. (After Riickert, 1899_.)

A, U, D, E, Torpedo; B, Scyllium; F, G, P-ristiums. E, F, and G are sagittal sections with posterior edge of blastoderm to the right.

practically synchronously throughout the blastoderm. In Torpedo Riickert (1899) found that even in the ninth division the majority of the nuclei still divided synchronously and that in some eggs the same was the case with the tenth division but in any case approxi- 16 EMBRYOLOGY OF THE LOWER VERTEBRATES C11.

mately about this period individual nuclei lag behind others and the regular rhythm becomes lost.

This rhythm of nuclear division is of interest in relation to the size of the individual blastomeres. It is often noticeable in an Elasmobranch blastoderm that the blastomeres are somewhat smaller in what is shown by later development to be its posterior half 'i.('. the half next the side on which the embryonic rudiment makes its appearance later. It would be natural to suppose that the smaller size of the blastomeres is due to their having gone through a greater number of divisions but this explanation is rendered less satisfac- tory by the synchronism of the mitotic divisions. Apparently the inequality is at least to some extent due to the zygote nucleus, and, later on, the first segmentation furrows, being not quite central in position in the germinal disc but situated slightly towards its posterior edge (Riickert).

The stage up to which mitosis remains synchronous varies amongst individuals of one species and an fortioml amongst those of difierent species ‘and genera. Thus in Prristvlurus it is, commonly, regular only up to the fifth mitosis according to Riickert.

While segmentation has been proceeding, important changes have been taking place in the segmentation cavity. About the time of the seventh division the rounded inner blastomeres fill up most of the cavity so that it becomes reduced to chinks between the individual blastomeres. These chinks are filled with fluid secreted by the egg substance, and in the yolk beneath the blastoderm the activity of this process of secretion is indicated by the appearance of fluid vacuoles.

As development goes on the amount of fluid increases greatly and about the tenth division it begins to collect especially between the blastoderm and the yolk, forming the “germ cavity” of Riickert (Fig. 9, D, E, F). This cavity is best marked towards the posterior side of the blastoderm and in ground-plan is crescentic in shape. It varies greatly in its degree of development in different individuals.

Whether it is advisable to use a separate name for this cavity is very questionable. When a broad View is taken of the relations of blastomeres and segmentation cavity in the Elasmobranchs these seem to be similar in kind to those which hold iii the case of the Lung fishes. In these fishes, as will be shown later, the blasto- meres which originally formed the floor of the segmentation cavity become later on shifted in position towards its roof but the resultant change in the topographical relations and form of the segmentation cavity would clearly afford no valid reason for giving it a new name.

The Yolk Syncytium.——- The layer of substance immediately underlying the blastoderm and segmcntation- or‘ germ-cavity is distinguished from the main mass of yolk upon which it in turn rests by the finer grained character of its yolk granules, and by its I SEGMENTATION 17

greater richness in protoplasm. This layer shows no division into cells and is therefore termed the yolk-syncytium‘ (II. Virchow: R1'ickert’s term “merocytes ” is synonymous). '1‘l1e marginal portion round the edge of the blastoderm is sometimes termed the germ-wall.

Functionally the yolk-syncytium is apparently concerned especi- ally With the digestion and assimilation of the yolk. Scattered about in it are nuclei, often of enormous size and irregular form. (Joncerning the origin and fate of these nuclei much discussion has raged and the matter cannot yet be regarded as satisfac- torily settled. The question is complicated by the fact that, as shown by Ruckert (1890), polyspermy appears to be a normal occurrence in Elasmobranehs. In addition to the ,single micro- gamete which takes part in the formation of the zygote-nucleus a variable number of extra spermatozoa make their way into the egg and give rise to accessory sperm-nuclei. Where such sperm-nuclei are situated in the coarse yolk they apparently soon degenerate but when, on the other hand, they are within the protoplasm of the germ-disc they remain during the early stages of development in a living and apparently healthy condition, even undergoing mitosis synchronously with the nuclei derived from the zygote-nucleus up to the fourth or even fifth or sixth division in the case of ’1'07'ped0.

The importance of this fact should be noted in connexion with our ideas of the reciprocal physiological relations of nucleus and cytoplasm. It is fully recognized that the nucleus governs and controls cell metabolism: it is not always so fully recognized that conversely the cytoplasm exerts an important influence over the nucleus. Clearly the fact that the accessory sperm-nuclei “keep step" in their mitotic divisions with the embryonic nuclei must be due to some influence exerted on the former nuclei through the cyto- plasm. It should, in fact, never be forgotten that cytoplasm and nucleus are merely locally specialized portions of the same common living substance or protoplasm. ,

At first the accessory sperm-nuclei are clearly distinguishable in the germinal disc from the true embryonic nuclei by their smaller size and reduced (haploid) number of chromosomes. After the zygote-nucleus has undergone two mitoses however—or even before the second mitosis——the accessory sperm-nuclei wander—-or become transported by cytoplasmic movements--outside the limits of the germ~disc. They continue their mitotic rhythm for a time so that, for example, at the 8-nuclear stage of the blastoderm they may be seen in groups of eight lying in the yolk-syncytium. During early stages of segmentation numerous such obviously accessory sperm- nuclei may be seen in the syncytium but as time goes on the nuclei

1 Although Haeckel originally defined the term syncytium (Die 1i'all‘schu.°dmone, Bd. I. p. 161) as a protoplasmic mass formed by the fusion of origwinally separate cells the word has come into such general use for a multinucleate mass of protoplasm which

shows no subdivision into cells, whatever its origin may have been, that there seems no serious objection to the use of the term yolk-syncytium as suggested by Virehow.

VOL. II C 18 EMBRY()L()GrY OF THE LOWER VERTEBRATES on.

of the syncytium are seen to be of a different character. They are now of enormous size and of peculiar lobed appearance. The lobing becomes more complex as time goes on and appears to be due to incomplete and irregular attempts at amitotic division.

The discussions, alluded to above, have centred round the mode of origin of these highly characteristic giant nuclei. Balfour, who first described them (1874), did not express any opinion as to their origin. liickert in his first paper (1885) on Elasmobranch develop- ment looked on them simply as specialized embryonic nuclei and gave the masses of protoplasm in which they are embedded the name “ merocytes.” Latterly however Riickert, after his discovery of polyspermy in Elasmobranchs, has taken the view that the yolk- nuclei are really the accessory sperm-nuclei before alluded to which have altered their character in correlation with the altered environ- Inent in which they find themselves after leaving the germinal disc.

In spite of Riickert’s more recent observations and conclusions, and in spite of their being supported by Samassa, Beard and others,

A B C

Fm. 10. --Views of the segmenting germinal disc of Bdelloslmna stouti. (After Bashford Dean, 1899.)

it must, I think, be admitted that the sperm-origin of the yolk-nuclei is by no means demonstrated. And all general considerations are in favour of Ri'1ckert’s earlier view being the correct one, namely that the nuclei of the yolk-syncytium are genetically of the same order as the ordinary embryonic nuclei. Such general considerations render it extremely improbable that accessory gamete nuclei should really play an important physiological part in the developing embryo: it is far more probable that such nuclei simply persist for a time, undergo mitosis a few times and then degenerate and disappear.

The variations in the process of segmentation are well illustrated by the three cases just described and it will be convenient now to summarize the general characteristics of the process in the various remaining groups.

LAMPREYs.——In the Lamprey the phenomena of segmentation agree closely with those observed in the frog and need not be further described.

MYXINOIDs.—In the Myxinoids the somewhat sausage-shaped egg is heavily yolked and possesses a germinal disc situated close to one pole. A few segmentation stages of Bdellostoma (Fig. 10) I SEGMENTATION , 19

have been described by liashford Dean (1899) and as might be expected the segmentation is meroblastic. Apparently the first two furrows (a and B) -have the normal meridional arrangement the specimen figured by Dean (Fig. 10, A) showing a displacement at the intersection of the two furrows. These latter do not reach the edge of the germinal disc. The third set of furrows (Fig. 10, B) appear to be vertical and in the next stage figured (Fig. 10, C) the furrows have become joined up to form an irregular network which still barely reaches the edge of the blastoderm. CROSSOPTERYGIANs.—-——Our knowledge rests entirely on -the observations of Budgett (Graham Kerr, 1907). These, necessarily fragmentary, observations sufiice to show that the process of segmentation is of great interest. In the earliest stage observed, but not figured, by Budgett the egg was “ segmenting in four equal

FIG. 1'l.—-Segmentation and gastrulation in I’ul;a;p1!erus. (Drawings by Budgett. Graham Kerr, 1907.)

A, represents a view of the apical pole: the remaining figures are side views.

portions, the constrictions being deeper than in the frog.” A second egg (Fig. 11, A and B) is in the eight-blastomere stage. The blastomeres are practically equal in size and it may be inferred with considerable probability that in Pol;/pm-us two meridional furrows are succeeded by a latitudinal one which is very nearly equatorial.

The nearness of the latitudinal furrow to the equator is remarkable

in view of the fact that the egg of Polypterus, as shown by the study of sections (Fig. 1, B, p. 3), is not by any means nearly isolecithal.

ACTINOPTERYGIANS.-—The typical Teleost is characterized by the fact; that its richly yolkecl "eggs show a more complete segregation of protoplasm and yolk than do those of any other Vertebrata. In correiation with this the segmentation is here the most markedly meroblastic in character. These featuresosuggest that in the ancestral Teleost the yolk was large in quantity and that the egg as a whole was of great size. Amongst present-day Teleosts however it is only, comparatively speaking, a few forms mostly inhabiting fresh water, 20 EMBRYOLOGY OF THE LOWER VERTEBRAT IS on.

which produce eggs of very large size 6.{]. (£2,/1/r,ncu'0/ms nilotricus (Budgett, 1901; Assheton, 1907) where they measure about 10 mm. in diameter, or the Salmon or Trout where they measure from 4 to 5 mm. '

'1‘he majority of fishes produce eggs in enormous numbers, amounting in some cases to several millions, and correlated with this the size of the individual egg has become much reduced. The average diameter of a Teleostean egg may be taken as about 1 mm. In an egg of this size segmentation of so markedly meroblastic a character would be puzzling except on the hypothesis that the meroblastic condition had arisen in ancestral forms in which the eggs were much larger.

The larger part of the egg consists of a spherical mass of practically pure yolk. On the surface of this is a thin layer of protoplasm containing droplets of oil, and this layer of protoplasm is more or less distinctly thickened in the region of the apical pole to form a germinal disc in which is contained the nucleus. Irregular prolongations of the superficial protoplasm may sometimes, especially in immature eggs, be traced inwards into the substance of the yolk.

A characteristic feature of many teleosts is the tendency for the yolk to assume a liquid form. This is particularly marked in many pelagic eggs where it is not merely liquefied but runs together at the time of spawning or of fertilization to form a sphere of glassy transparency. There may further be, interspersed amongst the ordinary yolk, droplets of oily looking fluid often with a distinctive colour. These may unite into a few droplets or into a single larger drop forming a conspicuous, often coloured, sphere in the midst of the ordinary_ yolk. The colour and size of such drops frequently afford an easy means of recognizing the species to which a particular egg belongs. They may also have a characteristic position and may be surrounded by a special condensation of protoplasm or, on the other hand, they may simply float freely in the main mass of fluid yolk. Although these droplets may, as already indicated, exhibit peculiarities characteristic of particular species they do not seem to give indications of genetic affinity in regard to genera or larger groups: nor do they show any definite relation to the conditions, pelagic or otherwise, under which the egg develops (Prince, 1886).

The yolk of teleosts is also characterized by a diminution of its specific gravity which causes the egg to assume a reversed position with the apical pole below, and which further, in the case of a vast number of marine fishes, causes the egg as a whole to float freely suspended in the sea water.

Seeing that the Teleostei as a group is above all characterized by specialization for a swimming existence, independent of a solid substratum, we are perhaps justified in assurning that the freely floating pelagic mode of development above mentioned was originally present throughout the group. The demersal type of development, where the eggs are deposited on the "solid substratum, would then I SEGMENTATION 21

be regarded as a secondary reversion to, rather than a persistence of, a pre-teleostean habit. Possibly the reversed position of the egg is to be regarded as a means of protecting its more sensitive apical portion from injury by contact with the surface film of the water in which it floats.

When fertilization takes place the most conspicuous immediate result is the onset of a gradual concentration of the protoplasm in the germinal disc-—-the disc becoming at the same time more heaped up, its vertical diameter in- creasing and its horizontal diminishing.

The segmentation of the germinal disc in A teleostean fishes is usually of a very regular and characteristic kind. It is illustrated as seen in surface View by Fig. 12. The germinal disc lengthens out into an elliptical shape.

The first furrow to appear (A) is meridional B and occupies the shorter diameter of the ellipse.

The second furrow is also meridional and in a plane perpendicular to that of the first. The third and fourth sets of furrows (B, C, D) are vertical and they become arranged so as to be practically parallel to the first and second, with C the result that the hlastoderni as seen in surface

view assumes a very characteristic arrange- ment of sixteen segrnen ts arranged in four rows

(Fig. 12, D).

The internal phenomena of segmentation D may he (lescribed from what occurs in the Trout (Kopsch, 1911). In the first place it has to be noted that the early furrows do not extend Fm 1.-_,_ _Segmentati°n right through the substance of the germinal in the blastodermofa disc but leave a continuous basal stratum of teleosteall fish (3% protoplasm next the. yolk. The blastoderm assumes a two-1ayered condition by the 3rd and wnson, 1391,) 4th furrows curving round in their deeper portions so as to intersect the preceding division-planes which were throughoutperpendicular to the surface (Fig. 13, B). Up to the 16- cell stage all the segments remain connected by broad protoplasmic bridges apart from the continuous basal layer of protoplasm which connects the deepest cells together.

In the 32-cell stage (Fig. 13, C) the cells of the superficial layer have become completely isolated while the deep cells are still connected together. With the next division the blastoderm becomes three layered, the cells of the intermediate layer being derived some from the superficial, some from the deep layer, as isshown by the evidence of broad bridges of protoplasm which persist here and there between sister cells.

With the next division (128-cell stage, Fig. 13, D) the four-layer 22 EMBRYOLOGY or THE LOWER VERTEBRATES _ on.

condition is reached, the cells of the basal layer being still connected by a thick continuous stratum of protoplasm. By this time it is found that the nuclear divisions of the basal layer are clearly lagging behind those of the other layers. As segmentation proceeds further the continuous basal sheet of protoplasm decreases relatively in thickness. For a time (Fig. 13, E) bulgings of its upper surface indicate that it is giving off cells into the overlying layer but as the thinning process goes on these become less and less numerous. H. Virchow distinguished three zones in the basal sheet of protoplasm-——marginal, intermediate and central, although the latter

Fm. .‘.:’..—-\’m'ii«_-ui sq-.(-t'.ioiis' through the blastoderin of a Tcleost (iS'a..lm.ofa.rz'o) illustrating the prov-ess of Seglnelltntion. (After Kopsch, 191].)

A, and of second division. Section perpendicular to plane of first furrow which is therefore seen cut across. B, commencement of fourth division. Plane of section as in A. The division surfaces

of the third division are seen to curve inwards so as to meet the first division surface. As 9. result the latter has become distorted and no longer forms a plane. 0, middle of sixi.ii division. D, be- ginning of eighth division. E, beginning of ti-nth division. F, 62-hour l)last;0(,lt-rln. (The dark portion at the top oi‘ Fig. .13 represents the free S1li'f:u-v bounding the second furrow: the dark tone at the lower edge of (.'.ii('ll ligurc represents yolk.)

is not quite central but situated rather towards the posterior or embryonic edge of the blastoderm. The intermediate zone is marked off from the others by the fact that the thinning process has there progressed farther.

Up to about the twelfth division the nuclei all through the blastoderm di.vide practically synchronously except those ‘of the basal layer which as already indicated lag behind. Soon after this however (from about the 41st hour——Kopsch) the divisions become irregular. '

The basal layer becomes the yolk-syncytium: the cell limits visible on its upper side liieeome obliterated and it becomes more and more flattened out. Altlmugli its nuclei l.lli('lGl?g(i repeated mitosis I SEGMENTATION ' 23

there is no longer any budding off of cells, the nuclei simply lying within the substance of the syneytium.

As they increase in number nuclei from the central and marginal regions spread into the intermediate zone, which up to now contained very few nuclei, while others pass outwards into the peripheral protoplasm (Peribla.st;—-Agassiz and Whitman, 1885) lying outside the limits of the blastoderm.

Towards the end of the second day the syncytial nuclei begin to increase markedly in size and they begin to undergo abnormal multipolar mitoses. During the third day they complete the assumption of these peculiarities which are characteristic of the nuclei of a yolk-syncytium——enormous size,curiously lobed appearance, and the tendency for the lobes to become nipped off irregularly so as to give rise to groups of small nuclei.

During these later stages of segmentation the hlastoderm becomes flattened somewhat and instead of bulging out over its attached base all round, its surface passes into the extrablastodermic surface by a slope very much as it did before segmentation began (Fig. 13, I4‘).

ACTINOPTERYGIAN GANOIDS.-—-The ganoid fishes are of special emhryologieal importance because, so far as actinopterygians are concerned, they appear to be the least modified descendants of those ancestral forms from which the Teleostean fishes have been evolved. Study of their developmental phenomena is desirable in order to see to what extent they throw light upon the peculiarities of development which characterize the Teleostean fishes. It will be necessary therefore to review the segmentation processes so far as they are known in each of the three types———the Sturgeon, Amia and Lcpiclosteus.

The only Sturgeons of which anything is known regarding their early development are the common sturgeons of the genus Acipenser. Poly/odon, I’sep/tmws and Scaplmlrhynchus are so far completely unknown, though it is highly desirable that their development should be investigated.

In both Acripenscr Irutltenus (Salensky, 1878) and A. sturio (Bashford Dean, 1895) the segmentation (Fig. 14, A) is of the same general type. The unsegmented egg measures about 2 mm. in diameter in the Sterlet (A. ruthen-Ms), about 2'8 mm. in the Sturgeon (A. sturlio). The lower part of the egg contains coarse yolk granules while in the region of the apical pole it is richer in protoplasm and the yolk is more finely granular. The first furrow (a) is meridional, appearing first at theapical pole and gradually spreading downwards and at the same time cutting more deeply into the yolk. The second furrow (,8) is similar and at right angles to the first. The third set of furrows seem to be typically vertical (A 2) but they show

much variation and may be practically meridional or may show a.

tendency to be latitudinal. The next set of furrows again vary between vertical and latitudinal and from now onwards there is no apparent regularity in the segmenting of the various blastomeres. There 24 EMBRYOLOGY OF THE LOVVER VERTEBRATES CH.

eventually results a blastula (A 5) the upper portion of which, forming rather less than a hemisphere, is composed of micromeres while the lower part is composed of lame, richly yolked, macromeres.

In Amxia the “ oval ”-shaped thgg n’..s=,;..~«.en~.~.a shout 2'5-2‘. mm. hy

about 2-26 mm. In the region of the apical pole which lies at the end of the long axis is s cap-slnapmil portion richer in protop_lasm—- an approach in fact to a germinal disc-——while the 1'cS’t of the egg is rich in dark gr:-1.yi_sh brown yolk. ‘

The segmentation (Fig. 14, B) begins, about an hour and a, half I SEGM ENTATION 25

after fertilization, with the successive appearance, at the apical pole, of two meridional furrows (u. and ,8) which gradually sweep down- wards to the opposite pole of the egg. Before they reach it, four vertical furrows make their appearance, commencing at a point on furrow (1. not far from the pole and gradually extending downwards over the lower part of the egg (Fig. 14, ll 1).

Before these vertical furrows reach the lower pole a new furrow ~—-latitudinal———deve1ops a short distance from the apical pole, marking oil’ a polar group of eight micromeres (Fig. 14, B 2). At the next division these divide into a superficial and a deep seglnent (the former being separate——-the latter continuous with’ the yolky mass beneath) while the macromeres divide by vertical furrows.

Next an irregular latitudinal furrow develops below the previously existing one, by which a new micromerc is segmented off from the upper end of each macromere.

liegnldostezts.-———Tl1e ellipsoidal or “oval” egg measures about 3'5 mm. by 3'2 mm. and has a cap of protoplasm with fine grained yolk at its apical pole.

Segmentation (Fig. 14, U) in its early stage is like that of A7n'i0r, except that the furrows are more sluggish in spreading downwards over the egg-surface. They never in fact reach much beyond the equator; in other words, in the case of Lepz'zI0steu.s~, the lower hemisphere of the egg does not normally segment at all. The egg therefore has advanced beyond the condition seen in A7nz'a, and has become meroblastic.

In the later stages of segmentation the region of the upper pole is occupied by a lentieular mass of blastomeres which may be termed the blastoderm, and this is bounded at its lower edge and over its lower surface by a set of elements which remain in continuity with the yolk. Later on the divisions between these elements tend to disappear and their place becomes occupied by a “ yolk-syncytium ” containing numerous nuclei.

To summarize then, we have exemplified by the three ganoids Acipenser, A.’ln’t(t and Lepidosteus, three steps in evolutionary change, associated with an increasing degree of telolecithality, from the holoblastic type of egg met with in Lampreys or Amphibians or Orossopterygians to the meroblastic type as it exists in modern Teleosts.

LUNG-FISHES.——Tl1e early stages of segmentation have been observed in two out of the three still existing lung-fishes--C’emtodus (Semon, 1893) and Lepz'dos'i7'en (Graham Kerr, 1900).

In the case of Oeratodus the egg measures about 3 mm. in diameter and is pigmented in the neighbourhood of the apical pole. The first two furrows (Fig. 15, A 2 and 3) are meridional and at right angles to one another. Each appears first at the apical pole and extends downwards with varying rapidity. The third set of furrows are vertical and make their appearance usually before the second meridional furrow (B) has reached the lower pole. The egg 26 EMBRYOLOGY or THE LOWER VERTEBRATES on.

becomes thus divided into eight practically equal blas- tomeres.

A latitudinal furrow then develops about 715° above the equator, so that the egg now consists of eight micromeres round the apical pole and eight macromeres.

After this stage seg- mentation usually becomes irregular» although some- times two additional latitu- dinal furrows make their appearance in succession so that the egg consists of four tiers each of eight blaste- ineres. Eventually, as seg- mentation proceeds, a blas- tula is formed of the type shown "in Fig. 15, A 7.

The segmentation cavity first appears about the time of the fourth cleavage as a small chink. It rapidly expands and in the blastula figured (Fig. 15, A 7) it is of large size.

In Lepidosiren (Fig.

'15, B) the egg measures usually between 6'5 and 7 mm. in diameter. It is free from pigment in correlation with the fact that it de- velops in a burrow Sl1mil.ml

'l'1‘OIl1 the action of light. In the region of the apical pole is a whitish cap in which the yolk is in very minute particles while else- where i.t is in large coarse« granules.

The first two furrows (Fig. 15-, B 2 and 3) are meri- dional and at right angles to one another. The third

, set of furrows (Fig. 1.5, B 4) are vertical tlioiigli ocea- I SEGMENTATION 27

sionally one or other of them may become latitudinal. The various meridional and vertical furrows gradually extend downwards towards the lower pole of the egg in the order of their appearance, and during the earlier stages the lower hemisphere of the egg possesses only such furrows (Fig. 15, 'B 6).

As the blastomeres go on segmenting there is produced eventually a blastula with an upper hemisphere of small cells which appear white because of the finely subdivided condition of their yolk and a lower hemisphere of larger more yolky elemcn ts (Fig. 15, B 7).

Already at the stage when the egg is divided into four segments a space develops between the blastomeres. As segmentation goes on the micromeres tend to round themselves ofl', leaving wide chinks between containing fluid. By the blastula stage the fluid has collected together into a spacious segmentation cavity which is visible in the whole egg as a dark shadow in its upper hemisphere. At first the cavity is rounded and is roofed in by a single layer of cells but later it spreads out, takes a planoconvex form and its roof comes to be composed of two layers of closely apposed cells.

AMPIIIBIA.——The Amphibia are in the matter of segmentation the most interesting and important group of the vertebrata, for in no other group does there exist so much variety in the proportional amount of yolk present in the egg. Much work still remains to be done in regard to this group in the way of detailed study of the process of segmentation in its relation to the amount and concentration of the yolk.

As already indicated the extent of the influence which the yolk exerts in retarding the living activities of the protoplasm, such as growth and division, bears a rough relation to its proportional amount. As regards the majority of cases this may be said to vary directly with the size of the egg. The largest eggs are as a general rule the most richly yolked. But the rule is by no means an invari- able one that the influence on the segmentation is directly pro- portional to the total amount of yolk in the egg as a whole. For a smaller egg, containing a smaller amount of yolk, may yet have that yolk more concentrated in one region so as to produce there a more intense retarding influence——as is the case naturally in many small Teleostean eggs or as may be demonstrated experimentally by con- centrating the yolk artificially through the action of centrifugal force. 0. Hertwig was able by centrifugalizing frogs’ eggs and so causing the yolk to become concentrated in the abapical hemisphere, to bring about a complete cessation of cleavage in that hemisphere so that the egg thus assumed a meroblastic character.

The variety in the size of the egg within the limits of the group Amphibia has already been indicated by the table on page 2. The process of segmentation agrees in the main with what has been described for the frog but there is much variation in detail. The variations have to do both with the position of the furrows and with their appearance in point of time. One gets a good idea of the 28 EMBRYOLOGY OF-THE LOWER VERTEBRATES CII.

general tendency of variation by studying numerous eggs of a single species, for example in the case of Rana pal'ustr75s Jordan and ' Eycleshymer (1894) found amongst other variations in the mode of appearance of the first furrows, those illustrated in Fig. 6 (p. 12). And similar difi“ercnces occur between the eggs of different species.

‘.1

A B C

l"|H. 16. -Variations in topographical relations of early segmentation l'urrows in the egg of [tuna h'))I])(r/'41,!‘/(I. (A. B after Morgan, 1897 ; C after Jenkinson, 1913.)

Tlw llguie in each (-use I'('])['0S(‘lltS a. \ new of the apical pole of the 1-gg,

As regards difference in position of the furrows two of the coninionest variations are the following. At the four-hlastomerc stage two blastomeres may be pressed outwards from the apical pole as in Fig. 16, A. Again meridional furrows may be replaced hy vertical furrows as in Fig. 16, B and 0.

As regards variations in time these are chiefly associated with the retarding of segmentation in the lower yolk-laden segments. This reaches its maximum, so far as Ainphihians are concerned, in the Gy1nnophiona,where segmentation spreads so slowly into the lower


Flo. 17.——Vertical section through apical portion of egg of lclat/z_2/oplus at an advanced stage of segmentation. (After 1’. and F. Sarasiu.)

parts of the egg that during what are ordinarily called the segmenta- tion stages the yolk remains completely uncleaved. It would in fact be concluded from an inspection of these stages alone that the egg is a meroblastic one. Examination of later (gastrulation) stages however shows that the yolk does eventually segment although tardily.

Upon the whole it seems to be the‘ case that the Urodele egg 1 SEGMENTATION 29

segments more slowly-—during at least the first stages—~than does that of the Anura. It may be said also that, on the whole, eggs with a large mass of yolk show a tendency for the first latitudinal furrows to be nearer the apical pole, so that the micromeres which they cut off are relatively smaller. Also it seems to be the case that in the lower, more yolky, parts of the egg the latitudinal furrows are retarded to a particularly great extent, so that in such heavily yolked eggs there is frequently visible a preponderance of vertical and meridional furrows in the lower parts of the egg.

ELASMoBRANUH11.——Of the more typically meroblastic vertebrates the Elasmobranchs call for little in the way of further remarks. The general features of their segmentation have already been described (p. 12).

The eggs of all Elasmobranchs hitherto investigated are of large size and undergo a meroblastic segmentation. Up to the present time no Elasmobranch has been discovered in which the eggs are small and holoblastic, though it is quite possible that such forms exist. It need hardly be said that if they do the study of their embryology will be of extraordinary importance as it Will be of the greatest help in enabling us to disentangle those developmental phenomena of Elasmobranchs which are primitive from those which are merely secondary modifications due to the accumulation of yolk.

SAUROPSlDA.—-The Sauropsida, like the Elasmobranchs, possess large and richly yolked eggs with a meroblastic segmentation, but the process of segregation of yolk and protoplasm has not been carried to such an extreme as in Elasmobranchs, not to mention Teleosts. A germinal disc is present but this still contains a con- siderable amount of yolk and at its periphery passes by much more gradual transitions into the main mass of yolk. Further in the more primitive Reptiles the blastoderm frequently occupies a much larger proportion of the whole egg than it does in the Elasmobranch.

The general features of segmentation resemble those of Elasmo- branchs though the earliest phases depart in many cases less than they do in Elasmobranehs from what is seen in holoblastic eggs. Thus the process may commence with the appearance of a meridional furrow followed by a second at right angles to it and then by two pairs of vertical furrows very much as in an actinopterygian ganoid (Fig. 14, B and C).

This is seen most clearly in the less specialized egg of Reptiles. Even in the Reptile however the process is liable to become irregular at an early stage by the reduction of particular furrows or their irregular orientation. In the Birds (Patterson, 1910) the irregularity is still more marked and even the third set of furrows may no longer be clearly recognizable.

As in the case of other bulky and heavily yolked eggs poly- spermy appears to occur normally and an abortive accessory segmen- tation may make its appearance round the accessory sperm-nuclei. As in the Elasmobraneh (Fig. 8, B”) this is only a transient 30 EMBRYOLOGY or THE LOWER VERTEBRATES on.

phenomenon the accessory furrows flattening out and disappearing as the accessory sperm-nuclei degenerate.

Again as in the Elasmobranch a yolk-syncytium is developed beneath and around the segmented portion of the blastoderm.

A marked difference between the Sauropsidan ‘ and the Elasmo- branch type of egg at a fairly advanced stage of segmentation becomes apparent on comparing them with the corresponding stages of eggs of a less markedly telolecithal character (e.g. Fig. 14). It is seen that the blastoderm in an Elasmobranch such as that shown in Fig. 8 I), E corresponds to the mass of micromeres of the holoblastic egg, while in the Sauropsidan it corresponds to the mass of micromeres together with the apical ends of the large macromeres.

This is really an expression of the fact that in the Sauropsidan the germinal disc extends outwards into the main yolk, and shades oil’ gradually into it. The result is that the segmentation process in the outer portion of the blastodcrm is delayed by the presence of yolk precisely in the same way as in the lower portion of the hole- blastic egg.

GASTRULATION

The segmentation process is in the more primitive Vertebrates, as in many other groups of the Metazoa, succeeded by a process of gastrulation, in which the blastula becomes converted into a gastrula 'i.e. a type of embryo consisting of the two primary cell-layers, ectoderm and endoderm, enclosing a cavity, the archenteron, which corresponds morphologically with the coelenteron of the Coelenterate and which opens freely to the exterior.

While the process of gastrulation is fairly clear in the most primitive vertebrates it becomes less and less so in the more highly modified members of the group until finally in the Amniota it becomes completely obscured. To facilitate the understanding of the modifications which the process of gastrulation undergoes it will be well to study it first as it occurs in three of the more primitive Vertebrates namely Amp}:/iomus, Poly/pterus, and Lepidosvlren.

(1) AMPHIOXUs.———The blastula of A1n10h'i0ams is composed of a single layer of cells, those towards the apical pole being smaller, those on the opposite side being larger and containing in their cyto- plasm larger and more numerous granules of yolk. The process of gastrulation is ushered in by the large-celled portion of the blastula- wall becoming flattened as shown in Fig. 18 A. The portion of the flattened area which, as shown by later stages, is anterior in position develops a slight depression (Fig. 18, B) which gradually deepens and at the same time spreads backwards (Fig. 18, C, D, E, F, G) until the large-celled portion of the embryo. is completely invaginated within the small-celled portion and the two-layered gastrula stage is attained. _

‘ See Chapter X., Segmentation of'Fow1’s egg, F, G, H. } ASTRU LAT .10 N 31

Had the process been merely as stated the result would have been a gastrula with a very wide mouth. But along with the

process of involution there takes place an active growth of the lip or rim of the gastrula. This growth is especially active anteriorly as is shown by the fact that mitotic figures are most numerous in this region and become less and less frequent towards the posterior part of the rim. The result is that the original mouth of the gas-

trula —— the proto-'

stoma.-—becomes gradually en- croached upon by the gastrular lip—— the encroachment being most marked anteriorly so that the opening becomes not merely dimin- ished in size but also appears to shift its position towards what will become the posterior end ‘of the embryo. It will be noticed thatwhat really happens is not a process of shifting of the opening as a whole, but rather the persistence of the binder portion of the opening while its anterior portion has disap-

FIG. 18.—-«Illustrating the process of gastrulation in .-1 mph-z'o:w.s as described by Cerfontaine. The second polar body marks

the neighbourhood of the apical pole. The individual figures are viewed from what is seen later on to be the left side of the Amphioxns, the dorsal side being above and the head end pointing towarils the left side of the page.

peared. Such a remnant of the original protostoma may conveni-

ently be known by the special term blastopore. Expressed somewhat differently-—the cavity of the gastrula has 32 EMBRYOLOGY 014‘ THE LOWER VERTEBRATES CH.

become roofed in by a process of overgrowth on the part of its lip. The most conspicuous factor in this process consists of backgrowth of the anterior portion of the lip while the growth of the lateral portions inwards towards the mesial plane (so as to narrow the opening from side to side) is less and less active the greater the distance from the anterior end, until finally, in the extreme posterior portion of the lip, such growth as takes place is relatively inconspicuous.

In the process of gastrulation in Awipltiowus there are then two distinct processes at work ( 1) a process of invagination or involution of the large-celled portion of the wall of the blastula and (2) a process of overgrowth, most pronounced in the case of that portion of the gastrular lip which is originally anterior. By the agency of (1) there are established the two primary cell—layers——-ectoderm and endoderm while by the agency of (2) there is formed the dorsal wall of the embryo with its potential later developments such as central nervous system and notoehord.

It will be noticed that the originally anterior portion of the gastrular rim, wl1en it has completed its backgrowtb, lies above, dorsal to, the now greatly diminished gastrular opening or blaste- pore. Consequently it comes to be spoken of as the dorsal lip of the blastopore.

Although, strictly speaking, the terms endoderm and ectoderm are expressive of topographical relation a11d their use is permissible only after the one layer has become at least partially invaginated within the other, yet it should be carefully borne in mind that these two primary layers have already become distinctly differentiated from one another during the blastula stage, long before the process of invagination begins. One might indeed go farther and say that endodermal characteristics, e._r]. richness in yolk, have already made themselves apparent in the abapical portion of the egg even before segmentation begins.

This fact is of far-reaching importance as we shall find in other Vertebrates that the histological characteristics of ectoderm and endoderm become apparent not merely before the actual process of gastrulation takes place but, it may be, completely independently of that process. '

(2) PoLYP'1‘EnUs.—-ln 1’ol3/pteras an early stage in the process of gastrulation (Fig. 19, B) shows a well-marked groove encircling the egg a short distance on the abapical side of the equator. This groove marks the line along which involution of the egg-surface is taking place and its adapical lip represents the lip of the gastrula. It is to be concluded from a still earlier stage observed and drawn by Budgett (Fig. 19, A) that the involution groove appears first in the region corresponding to the anterior lip of the gastrula of Amjolz/iozzms and gradually becomes extended at its two ends until complete. This is of importance as betraying a tendency for the invaginative activity to be accentuated in this portion of the gastrular lip and diminished elsewhere. - hemisphere. As a

I GA STRU L A'l‘ ION 33

The fact that the yolk portion of the hlastnla consists not of a single layer of cells as in Am,phi0w'u,s but of a solid bulky mass forming a large proportion of the whole hlastula, renders it physically impossible for the yolk hemisphere to he involutcd bodily in to the interior of the apical

consequence we find in Polypterus that the process of in- volution is replaced to a greater extent than in A71Lp}m'ox~zz.s by overgrowth, the gastrular lip g'I‘0W- ing over the mass of yolk-cells as seen in Fig. 19, C, D and E. As this process of overgrowth con- tinues the project- ing yo1k—plug-—the mass of yolk-cells not yet enclosed -—gradua1ly ‘dimin- ishes in size and eventually disap- pears completely in the new narrow blastopore. As yet material isnot avail- able to show defin- itely‘ whether the overgrowth is more active in what corre- sponds to the an- terior portion of tlie gastrular lip of

Amp}:/iowus but the ,

bili is in FIG. 19. -------lllnsl.)‘:1l.ing prm.-ess of g:ist.1-11l:1tio11 in Poly/ptm--zas. proba 1t-‘y - - (I“i}_r.<. A, U, E, F al'l.er <lra\\'ing.:s by Budgett.) favour 01 this being i

the case and the _r/.1, gastrular lip; ;;.,», yell:-}'»lu;_-'. The egg is in em-h case \'iewml fi ‘ - from the ltffli side and has the dorsal si«.l«.- .-chm-e. The ori;,-in.-Il apirul gures ale Omen‘ pole is directed downwards and towards the left side of the page as in

tated 011 the assump- the preceding figure ol‘ A"-mph.iu.:__u.-.

tion that this is so. The two features to be specially noted in the gastrulation of

l’0l3/ptems as compared with that of Amphz'oa:-us are (1) the

accentuation of the process of overgrowth and the reduction of the

process of involution and (2) the tendency, in early stages -at least, VOL. II D 34 EMBRYOLOGY or THE LOWER VERTEBRATES on.

for the invaginative activity to be diminished along the region corresponding to the posterior part of the gastrular rim of Amphioasus.

(3) LEPIDOSIREN.-——-The first sign of gastrulation is afforded by the appearance, at short distance to the abapical side of the equator, of a latitudinally arranged row of small dimples or depressions of the surface which soon become joined up to form a continuous in- vaginationgroove. This (Fig. 20, A and B) may extend through about one—third of the circumference of the e g but in contrast with what happens in Polypterus the groove, instead of increasing in length, becomes shorter, flattening out and disappearing at its two ends. ' ‘

The finalstage is seen in Fig. 20, E, where the gastrular lip is


FIG. 20. Illustrating the process of gastrulation in Le}nfdos'zIre'7c.

Fig. A is a side view, the egg being OI'icntab6d so as to correspond with the ligures of .-lm.pIn‘oxu.- and 1’olypte~rus, the largecclled yolky abapicai portion of the egg being above and towards the right hand. Figs. B to E are views looking directly at the gastrular rim (dorsal lip of the blastopore), or in the case of E directly at the completed l)last()p0I‘e. Consequently, as the gastrular rim is during these phases of development not stationary, the views 13 to E are not orientated _morphologica1ly in exactly the same way.

short, and curved into a crescent, forming the dorsal boundary of the blastopore. ' b

At. this stage the large yolk-cells with their conspicuous salmon colour have been completely covered in by small cells-——-a condition that has been brought about through the agency of two distinct factors (1) the process of overgrowth with which we have already become familiar and (2) a new process to which the name de- lamination is given.

As is shown by the sections drawn in Fig. 21, the yolky or abapical portion of the blastula-wall is in Lepidoszren, as it was in Polypterus, far too bulky to be involuted bodily as was the case in Amplmlozcus. Again the enclosing of the yolky mass by the gastrular lip growing overit as in Poly/pterus is rendered impossible by the fact that the gastrular lip is normally here never completed to form an entire circle. It is, as has been explained, restricted to a com- paratively small linear extent.

This small persisting portion of gastrular lip probably does advance over the surface of the yolk by a process of overgrowth, giving rise in this way, just as in Amplmioxus, to what will become the dorsal wall of the embryo with its central nervous system and notochord. That this is the case seems to be indicated by sagittal sections through eggs cut in celloidin while still contained within I GASTRULATION 35

the egg-shell. In these the gastrular lip has a distinct wedge shape this being apparently impressed upon it as it pushes its way between the egg-shell and the surface of the yolk. Corroborative evidence is afforded by the numerous mitotic figures found throughout the

Fm. 2l.——Sagittal sections illustrating gastrulation in Lepidosirm. _f].l, g‘:1sbrul.'u‘ lip.

thickness of the archenteric roof which indicate that it is undergoing rapid - growth.

It will be gathered, however, from a consideration of Fig. 20 that those parts of the margin of the s1nal1—celled region of the egg’s surface which are not involuted to form a groove, must also advance over the surface of the yolk, for the-.sc parts ol' the margin form at first (Fig. 20, A and B) practically a great circle of the egg, while in subsequent stages (0 and D) they form a curve of gradually diminishing radius.

The method by which the small-celled area extends is shown 36 EMBRYOLOGY OF THE LOWER V ERTEBRATES UH.

clearly in Fig. 22, A, where it is seen that portions of large yolk-cells adjacent’ to the small-celled area hecorne split off as small cells which are added to that area. It is to this splitting-ofl’ process that the

1nm1e.delanaina,tion is applied. It is clear, then, that in the gastriilatioii of Lepidosrirevt three

Fm. 22.-——l’orti0ns of sagittal sections of Lepidosilren egg during early stages of gastrulation.

A, showing process of delamination. The small-celled ectoderm is seen on the left: it is becoming extended by the addition, to its lower edge, of cells split on‘ from the yolk-cells. The latter are recognizable by their larger size and by the larger size of the yolk-granules with which their cytoplasm is laden. B and 0 showing involution of the surface along the invagination-groove.

processes are at Work (1) Involution of the surface———this is conspicu- ous in the first stages (see Fig. 22, B and C), (2) Overgrowth by the gastrular lip—the “dorsal” lip as it is commonly termed‘ from its ultimate position, and (3) Delamination. These same three factors are at work in the gastrulation of the lower Vertebrates in general, and a. clear realizing of their nature is iiecessary to a comprehension I GASTRULATION 37

of the superficially very different gastrulation-phenomena observable in the various groups. .

With regard to the first of these processes, involution of the sur- face, it must be clearly understood that such appearances as that depicted in Fig. 21, B, point indubitably to the occurrence of a true process of invagination or involution of the surface of the egg. It is necessary to emphasize this as some embryologists are sceptical as to the occurrence of true invagination and believe that a. more im- portant part in the formation of the archenteric cavity is played by a mere cleavage, or splitting apart, of the cells which are to form the roof and the floor respectively. Brachet (1903), indeed, goes the length of stating in regard to Lrpidosireot and Protopterus, with only the data published by myself before him, that “ the first trace of the archenteron is due to a cleavage, the result of which is- the formation of a slit” a statement which is certainly not justified.

On the other hand it should also be borne in mind that there is no difiiculty a priorzl in the way of admitting that portions of enteric cavity may come to arise secondarily by a process of splitting in the midst of a solid mass of endoderm or yolk-cells. This type of modi- fication in the embryonic development of organs which were originally formed by invagination or cvagination is one which occurs quite frequently. Numerous examples of it are mentioned in the course of this Volume. '

Before closing this account of the gastrulation of Lcp'i(losz"re7L attention should be drawn to a remarkable and important pheno- menon which has been observed in both L61)z'rI08'i'/'e'It and P7"0t0pte'r71s. During the early stages of gastrulation, while the segmentation cavity is widely patent, the small blastomeres in the neighbourhood of its abapical side are seen, where not distorted by pressure from their neighbours, to be approximately spherical in shape. Elementary physics teaches that this is an indication that they are isolated masscs——that their protoplasmic substance is not continuous from cell to cell. In a later stage (Fig. 21, C) however; the blastomeres in the region of the segmentation cavity do become continuous with their neighbours and form a coarse reticulum traversing the cavity, the fluid contents of which now fill the meshes of the network.

The importance of the phenomenon described lies in the fact that here we have an actual case, clearly demonstrable, of isolated em- bryonic elements fusing to form a syncytial reticulum-—a type of process which may probably, as will be indicated later, play an important part in the development of the Vertebrate nervous system.

GASTRULATION IN THE VARIOUS GROUPS OF ANAMNIA

LAMPREYS.-—-—The Lamprey (Petromg/zen. fium°atz'l'is) shows in its gastrulation (Fig. 23) an intermediate condition between that of Amphiomus and that of the more heavily yolked holoblastic forms. The abapical portion ‘of the blastula is yolk-laden and thickened, 38 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

though not to the extent seen in Polyptems. The segmentation cavity remains, therefore, relatively capaeious so as to permit of a considerable amount of imagination of the yolk mass into it. ‘

The gastrulation process so far as can be judged seems to conslst mainly if not entirely as in Avie];/mioams of invagination and (2) overgrowth, only in this case the relative importance of the former has been lessened and that of the latter increased,

AMPIIIBIA.--It will be convenient to consider first the gastrula- tion-phenomena as seen in the common frog (Ra/rm tempowma), this animal having been more exhaustively studied than has any other Amphibian. '

The first sign of the onset of gastrulation is the appearance of a short latitudinal linear involution (Fig. 24, (1,) of the surface of the blas-





“\.~.(\\\‘.“\";.llll,‘ , .&‘\‘t l'\'ivil'i’I;i;""i . ’ I . ‘W... "


0 ,9 ‘I

. ‘ - u c O I I ‘ ‘ I "

Flo. 23.——-Gastrulation in J’etrom,yzo7t : based on Goette’s figures (1890).

The individual sections are orientated in the same way as the corresponding sections in Figs. 18 and 21.

tula considerably on the abapical side of the equator(ahout 25 °', Kopsch) and just at the boundary-of the large-celled region. It appears on that side of the egg on which the blastula roof is commonly rather thinner than it is elsewhere. This involution groove, as seen in a surface view of the egg, extends laterally and as it does so assumes a crescentic curvature (Fig. 24, b). The extension of the groove in length continues while its radius of curvature diminishes until finally it forms a closed circle (Fig. 24, b, c, d). i

The groove at its first appearance lies close to the boundary between the small cells of the apical region-—---characterized in the frog by their dense black pigment—and the large pale-yellow yolk- cells. During subsequent stages the groove continues to mark the boundary between the two types of cell, so that in the last stage mentioned when the groove forms a complete circle the mass of almost white yolk-cells within it (yolk—plug) stands out in striking contrast with the coal-black cells covering the rest of the egg surface. I GASTRULATION 39

As will be gathered from an inspection of Fig. 24 the gradual covering in of the yolk-cells takes place in an eccentric fashion. On the side opposite to that on which the original involution groove makes its appearance there is comparatively little displacement of the boundary between large cells and small, while on the side where the groove is the displacement is relatively great——l'rom a to (Z in the diagram. Intermediate points of the boundary between large and small cells are displaced more or less according to their greater or less proximity to the point of original involution.

As regards the method by which the yolk-cells become covered

in, it would appear that the “dorsal” lip of the groove advances over the yolk by “ a process of overgrowth, while at those parts of the boundary where there is no invagination-groove the process is one of delamination. The growth of the dorsal lip is clearly indicated .by the outline of the yolk-plug in sagittal sections which indicates distortion by pressure from the dorsal lip.

It will ‘be realized from what has already been said that the outer lip of the circular groove (Fig 247 (1) is simply Fm. 24.--Diagram to illustrate the rim of the gastrula-mouth or prot0— overgrowth by dorsal lip of stoma and that the preceding stages are "“°S‘l°P°"“ i“_”‘° F"°=‘=" i“'‘‘'‘' above all characterized by this rim being M°‘g“"’ 189") i“‘3°mP1"te-. In OW? W013“ the *‘°“i"it3' i,.§r.'.’1°..t1s‘.T§Tsg£i;..i7;Ii,in-i°.333§.‘§3':Z'l.i;.',Z'.3§ concerned in the involution of the gas— of dev(slopme[[t_ trular rim is accentuated at one point (a) while it is suppressed to such an extent elsewhere as only to become apparent at a comparatively late stage when the edge of the small-


°celled region has already spread to a great extent over the yolk-cells

by a process of delamination.

It will also be realized that it is not strictly accurate to speak of the circular area bounded by small cells as representing the gastrula- mouth until it is completely enclosed by the gastrular rim.

The internal changes which accompany the phenomena just described are illustrated by the sagittal sections shown in Fig. 25. In C the portion of the involution groove which first appeared has become much deepened and runs for some distance parallel to the surface as the archenteric cavity.. It is bounded superficially by a completed portion of gastrular wall showing the two primary cell layers, ectoderm and endoderm.

Some of the yolk-cells round the margin of the segmentation cavity are frequently to be seen, though not in the section figured, to be spreading along the inner surface of its roof, towards the point which was the apical pole of the blastula. ' .

In the later stages the overgrowth bv the gastrular lip, 40 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

accompanied, no doubt, by a eer_tain amount of involution though this is difleieult to determine with certainty, has proceeded much

8.6.

Fm. 25.—-Sagittal sections through the egg of Rana tem_p0-7'u.1'1'a illustrating the process of gastrulation. (After J enkinson, 1913.)

act, ectodenn; ent, an-clu-.ntex-ie (.-.a.vity; g.l, gs.st.rula.r lip; s.«-, segmentation calvity; 31.9), yolk-plug. The arrow indicates the original apical pole.

further so that the archenteron is much deeper. The spreading of the yolk-cells along the roof of L_ the segmentation cavity, already I GASTRULATION 41

alluded to, has in these later stages sometimes proceeded so far that that cavity is nearly completely walled in by yolk-cells.

While the archenteric cavity increases in volume the segmenta- tion cavity becomes gradually rcduced. Normally the latter cavity goes on shrinking until it is finally obliterated but according to O. Schultze (1887) a certain proportion of eggs show a variation from the normal which appears to be of importance for the inter- pretation of what happens regularly in certain other groups. As seen in Fig. 25, E, the layer of yolk-cells which separates the archenteron from the segmentation cavity is liable to become extremely thin, and Schultze believes that in certain cases this thin partition breaks down and disappears, so that the archenteric and segmentation cavities are thrown into one. What appears at first sight to be the archenteric cavity of subsequent stages would in such cases be really a complex consisting of the true archenteron fused with the remains of the segmentation cavity.

If these observations are to be depended upon, they are of very special interest. For, if the confluence of arehenteric and segmenta- tion cavity really occurs as an occasional variation in such Amphibians as the Frog, this may be taken as a foreshadowing of the similar phenomenon which has become a normal characteristic of the development of many of the higher Vertebrates.

It must however be borne in mind that there exists a dangerous source of possible errors of observation, which it is difficult to guard against, namely that when an egg of the stage of Fig. 25, E, is trans- ferred from one fluid to another, as in the ordinary technical pro- cesses which precede section-cutting, violent diffusion currents are set up between the fluid in the segmentation cavity on the one hand and that in the archenteron on the other, and these currents must be very liable to cause rupture in the intervening partition, even when in life this is quite continuous.

As gastrulation nears its end the circle formed by the gastrular lip becomes gradually smaller. Finally its lateral edges come together so that it takes the form of a short longitudinally placed slit, the remains of the yolk-plug at the same time passing out of sight. The gastrula is now complete.

As regards the subsequent fate of the slit-like blastopore it may be mentioned that, for the most part, it becomes obliterated by fusion of its two lips. The portions at its two ends, however, remain open as two pores of which the more anterior becomes the neurenteric canal while the posterior becomes, either directly or after temporary obliteration, the anus.

The process of gastrulation in the majority of Anurous and Urodele Amphibians pursues a course similar in its main features to that of the frog. Detailed studies of the process are, however, urgently needed in those Amphibians which have particularly small

eggs and in which therefore gastrulation is less modified by the presence of yolk. 42 - EMBRYOLOGY OF THE LOWER VERTEBRATES CI-I_.

GYMNOPIIIONA.--—Thcrc are certain Amphibians in which the telolccithal condition of the egg is so pronounced as to lead to a condition nearly approaching the meroblastic. Any such forms occurring either amongst the llipnoi or the Aniphihia must necessarily be of great importance owing to the fact that these groups are less far removed than are any other Vertebrates, from the line of descent of the Ainniota and that, in consequence, the study of their develop- ment may be expected to throw light upon features occurring in the meroblastic etr s of Amniotes.

Amongst od;in}iliibians of this type the Gymnophiona alone_have

been subjected to careful study (P. and F. Sarasin,

_. . 1887-1893; Brauer, 1897). The following sum-

mary of the main features in their g'astrulation

' processes is based on Brauer’s description.

_ _, The egg of Hg/pogeophis shows at the period

(‘}  preceding gastrulation a round patch of micro-
 meres, or blastoderm, covering   of the

._ surface of . the egg in the neighbourhood of its

O  apical pole. Gastrulation commences with the
 posterior edge of the blastoderm losing its forward

curvature and becoming curved backwards (Fig.

26), the curved part of the edge becoming

. sharply demarcated by the formation of a slight

- cleft-like invagination of the egg-surface-—-which & is deepest in its centre and shallower towards

its extremities. In front of this invagination the Flu. 26. ——Su_ccessive superficial (ectodermal) cells of the blastoderni

““‘8°* °‘7g““"“1.“”“1’ take on a distinctly columnar form. The edges

in llypogcophzs as _ . . . . . see“, 9;“ (AM. of the blastoderm apart from the line of 1nvagma- Brauer, 1897.) tion are in the meanwhile gradually spreading outwards over the yolk. As shown in Fig. 27, A, the cells (g.l) forming the anterior wall of the invagination are columnar in form, and the fine-grained character of their yolk makes their general appearance resemble that of the ectoderm cells. This is, however,- to be taken, not as meaning that they really are of ectodermal nature but rather merely as an indication of active metabolism associated with active growth. The invagination-groove gradually, by backgrowth and ingrowth of its lateral portions, assumes a more pronounced backward curvature (Fig. 26) taking first the shape of a crescent, later of a horseshoe and finally of a closed ring.

The central part of the groove almost from the beginning increases rapidly in depth so as to form a narrow cavity——the archenteron-—-which extends forwards. The roof of this cavity is formed of cells agreeing in their fine-grained protoplasm with those of the ectoderm, while its floor on the other hand is composed of cells which"in their coarse-grained character resemble rather the yolk-cells.

In front of the archenteroni are the irregular remains of the segmentation cavity and a. communication becomes established I GASTRULATIO-N 43

between the two cavities so that they form a continuous space-—the broader front part of which is derived from the seginentation cavity, the narrower posterior part from the, true a.rchcnte1-on (big. 27, U

Flo. 27.—-—-Sa.gittnl sections illustrating the pl‘()('('.\'H of _L,":1.\'i.l'lIl:1tl0ll in lI_:/,'2u_:/cu)»//:'.s:. (After Bram-r, 18$-97.)

ecl, e-ctmlu-nn ; cur, :m-lucntmir «-:m't_\' ; _«:.I. 3.-;a.<trnl:Lr lip: .s-.c, se;.rmen1.a1.ion cu\'it_\'.

and D). The two sections of the cavity remain for it time clearly distinguishable by the character of the cells which form the roof —-—those of the archenteric portion being composc;-.«l of fine-grained protoplasm like that of the ectodcrm while those of the portion derived from the segnientation cavity are typical yolk-cells. 44 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

At a stage when the involution groove forms a nearly complete circle a sagittal section presents the appearance shown in Fig. 27, C. The ectoderm is thick and columnar posteriorly, but in front and laterally it thins out into a cubical epithelium which has extended over the whole surfa.ce of the egg with the exception of a small area behind the gastrular rim. 111 the roof of the enteric cavity the boundary between the archenteric portion formed by overgrowth (and probably involution) and that formed from yolk-cells is marked by an abrupt change in the char'acter of the cells which at once become less tall and less columnar. Farther in still the yolk-section of the roof shows marked irregularities of its inner surface and its cells assume a more rounded form. The anterior limit of the blastocoelic portion of the enteric cavity is not, as yet, clearly defined.

The last section figured (Fig. 27, 1)} is taken from an egg in which the gastrulation lip forms a complete ring. Jonsequently the section shows a conspicuous yolk-Iilug (3/.1») within the gastrular lip which, it will be noted, has developed a covering of small iine—grained cells over its surface. The inrolling of the gastrular lip visible in the section indicates that the enteric roof is growing actively in length though Brauer does not make it clear to what extent the formation of the arehenteron is due to this and to what ex tent to actual involu- tion. Naturally it would be very dillicult if not impossible to decide this point definitely without experiments on the living egg. The gastrular opening gradually decreases in diameter (the yolk—plug disappearing from view as it does so) and eventually it closes from before backwards. by its lateral lips coming together (Fig. 26); its posterior part however remains open as the anus.

In the foregoing description is given merely a summary of those features in the gastrulation of H3/pngeopimls which appear to be of importance in relation to the corresponding phenomena of the Amniota: amongst these may be specially mentioned the process of constriction of the gastrular opening, and the double origin of the enteric cavity from archenteron and blastoeoele, only its hinder portion being derived from archenteron.

Another important feature not specifically alluded to in the text but which is indicated clearly by. Fig. 26 is that during the process of gastrulation the boundary of the small-celled area is sweeping onwards over the egg’s surface. It does this probably by a process of delamination as in Leyoridosiren. The important point to notice, however, is that the small-celled boundary is not blocked in its ex- tension onwards by the gastrular lip. The yolk-plug becomes covered with small cells and after the ends of the rim have met so as to form a complete circle the small-celled region still spreads onwards, so that the slit~like blastopore of later stages lies well within the margin of the small-celled area. Thus were development modified by the slurring over of the early stages of the invagination-groove so that this only became apparent at the period_when it had assumed the form of a longitudinal slit, it would at the time of its first appearance I GASTRULATION 45

be situated Well within the small-celled area instead of at its hinder margin. The importance of this consideration will become manifest later on in connexion with the interpretation of the developmental phenomena of the Amniota. ' ELASMOBRANCHII.-——The egg of the Elasmobranch at the time immediately preceding gastrulation difliers from the blastula of the ordinary Amphibian or Lung-fish in its much greater size. The small- celled or micromeric apical portion of the blastula is represented here by a relatively small mass of cells-——the blastoderm--in the region of

Fro. 28.-«Sagittal sections through Elasmohru.nel: hlastmlerms ('I'm'1}r’¢/u) illu.~'t.rating the process of gastrulation. (A.flcI‘ Ziegler, 19052.)

,,_], ;_.-a_,-tm];u- lip; ,u_c, ,s;e,«_gu1anta.t'.ion 1-.-n'it_\' : lI.7l, yolk nuclei.

the apical pole while the large-celled portion is represented by the yolk. This latter is composed, practically, of a mass of yolk granules, the protoplasmic matrix being reduced almost to van1shing—point. As in the eggs previously described, the nncromeric portion gradually spreads round and encloses the yolk and here again_we find the same three factors at Work-—involution, overgrowth and delamination. The first step in the gastrulation process consists in the involu- tion of the surface along the posterior edge of the blastoderm. This involution groove spreads outwards on each side until it may ex- tend along 5 to 3; the circumference of the blastoderm. The blaste- derm is meanwhile spreading outwards all round and, as it does so,

the central part of the groove becomes deepened to form a tubular’ 46 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

cavity, the archenteron, which runs forwards from the mid-posterior margin. In the roofing in of this archenteron it is apparently a process of overgrowth which plays the main part-«but along the rest of the blastoderm margin the process of overgrowth appears to die away and its place is taken by delamination very much as was the case in ])ep'£(l()s'zI'rm. This is shown by the fact that the invagination- groove, which, as already remarked, extends outwards on each side for some distance, never deepens to any considerable extent except in its middle part.

In the region in front of the archenterou the deeper or lower layer cells of the blastoderm increase greatly in number and spread forwards so as gradually to fill up the segmentation cavity. The remains of the latter persist longest near the anterior margin and the ex,-torlerm covering the last remnant of the segmentation cavity commonly projects as a small but conspicuous elevation above the general surface of the blastoderm.

’l‘hese lower cells eventually take on a mesenchymatous character for the most part. These lying next the yolk-syneytium however give rise to a definite epithelium, known as the yolk epithelium. Some of them are said to penetrate actually into the yolk where their nuclei assume the characters of the nuclei of the yolk-syncytium. The floor of the archenteron is formed by the yolk epithelium which is continuous round the inner, or anterior, end of the archenteric cavity with the endoderm of its roof.

ACTINOPTERYGII.-—lt is unfortunate that in the more familiar Actinopterygians belonging to the group Teleostei-—— of which it is so easy to obtain developmental material——-the phenomena of gastru- lation .are obscure and their investigation is impeded by technical difficulties in the way of making satisfactory sections. We shall therefore confine ourselves to indicating in a few words the more conspicuous features of the process.

On the whole the features of gastrulation closely resemble those met with in Elasmobranchs—-—a resemblance which however we are not justified in regarding otherwise than as a phenomenon of con- vergence, seeing that the general evidence of morphology points to the ancestors of the Teleosts being much more closely related to the holoblastic Ganoids than to the existing Elasmobranchs. A characteristic feature to he noted is that here, as will be found to be the case in various mammals, the superficial cells of the blastoderm become much flattened and form a thin protective covering layer which takes no part in the development of the embryo.

When gastrulation is commencing the posterior margin of the blastoderm presents in longitudinal vertical sections the appearance of being turned inwards to form the two primary layers. There is no actual patent archenteric cavity though the inflected portion clearly represents the archenteric roof, the floor being apparently represented by the underlying syncytial layer.

The growth in length of the archenteric roof seems to be brought I GASTRU LATION 47

about mainly by a process of overgrowth similar to that met with in other forms.

A point of special interest is that the posterior portion of the arch- enteric roof, in the neighbourhood of what will become later the mesial plane, is without the distinct demarcation between ectoderm and endoderm whichis present elsewhere. This continuity of the two primary cell layers apparently represents what is known in the Amniota as the primitive streak~—a structure of great morphological interest which will be discussed later on (Goronowitseh, 1885 ; Jablonowski, 1898).

While these processes are in progress the margin of the blaste- derm elsewhere is also advancing over the surface of the yolk so as gradually to enclose it. This enclosure of the yolk clearly corre- sponds to what we have seen in other cases but it is ditiicult to be quite certain as to how far it takes place by actual delamination and how far this has been replaced by a secondary independent growth.

. It is only when the exposed surface of yolk becomes reduced to a small round patch that the cell-margin bounding it shows inflection all round so as to justify us in speaking of a blastopore.

In the surviving Ganoid members of the group Actinopterygii we find that the process of gastrulation, as is the case with other characteristics, repeats conditions which are probably to be looked on as ancestral. The gastrulation clearly belongs to the same general, type as that of Lampreys, Amphibians, and Lung-fishes. That of Aczpenser (Salensky, Bashford Dean, 1895) seems more nearly to resemble that of Poly/pterus, and that of Amia. (Bashford Dean, 1896) and more especially Lepwldosteus (Bashford Dean, 1895) to point towards the mode of gastrulation found in the modern Teleosts.

GASTRULATION IN AMNIOTA

In comparing the process of gastrulation in the Amphibians and Lung-fishes with that in Am.p/m.'oa:us or Pul_2/pterus we have seen that there is a tendency for the greater part of the gastrular rim either to become completely obsolete or to be, at least, greatly delayed in its appearance, for example in the frog the greater part of the gastrular rim makes its appearance only in a comparatively late stage in the process of gastrulation.

In the Amniota we find that this tendency has gone further. It is only in the lowest group-—-the Reptilia —~—that an undoubted gastrular lip is clearly recognizable. In the two remaining groups, the Birds and Mammals, there is no convincing evidence that it has not completely disappeared from development.

REPTILES.-——In a Reptilian egg before the commencement of gastrulation the apical portion is covered by a blastoderm consisting of a superficial layer of flattened ectoderm cells and, underneath this, rounded lower layer cells which are separated by interstices containing fluid.

In the centre of the blastoderm (Fig. 29, A) an area, circular 48 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

or elliptical or pear-sliapecls with its narrow -end posterior, becomes distiriguisliable from the rest of the hlastoderm by its slightly greater opacity. '.l.‘he area in question is known. as the embryonic shield (as), and its opacity is due to its ectoderm being thickened, the individual cells having taken on a columnar form.

Either enclosed within or projecting beyond the posterior outline of the embryonic shield (Fig. 29, B) is a small area in which there is

3'

FIG. 29.--~—lllustratiu;: gasti-ulul.ion in the Gecko (I’/Ml;/4/m-/3//us). (_Al.'tcr Will, 1892.)

A, showing complete lll.'l\'ttNli'l'lll with the embryoni(.- .~'«l|l1‘l(l in th-- centre. This is l)()lIn(ll.'d behind by the gasti-ul:u- r-im, prev.noia_ms1y developed in this spc-cimen. B, embryonic shield of specimen at the st.:igl- in which the 2tl'L_'ll4.-lllHl'i(' floor is breaking down. C, embryonic shield at, Int-,¢_-r stage where gas- l.i-ulzir rim is be-nt hack into it A-shape bounding the yolk-plug: l..lu- outline of the mesoderm sheet is seen on each side. 1), embryonic shield showing stage at which tlw «.,;usl1-i1l:ui- lips h:m- mum-. 1'.o;:«-tlicr so as to bound in longitudinal slit. b.¢t, edge of blastodcrm; o-..-, «:nibryuni«.- .~liiuld; _4/J, ;;-nslrular lip; -mes, limit. of Ina-smlcrni.

no layer of columnar ectoderm sharply marked off from the lower cells. This forms the primitive plate (Fig. 31, pp). The boundary of the embryonic shield gradually spreads outwards and the primitive plate comes to be, if it is not. already, enclosed within it.

Within the limits of the primitive plate the surface of the egg now becomes involuted to form a groove bounded anteriorly by a lip

which from its correspondence with what we have seen 1n lower forms, more especially 111 the Gymnoph1ona,'1s clearly to be recog- I GASTRULATION 49

nized as the gastrular lip. This lip gradually shifts backwards and, as it does so, undergoes alterations in shape, which differ some- what in different species and even in different individuals of the same species but which in their main. features are illustrated by Fig. 30. In its later stages the lip becomes bent or curved back- wards so as to have the shape of a 11 or a A (Figs. 29, C, and 30).

Considerable variation occurs between differeiit individuals in the time of the first appearance of the gastrular lip, and in the Gecko 1’lat_7/dactylus Will (1892) observed a correlated variation in shape. Where it appeared relatively early, the involution had the form of an elongated creseentic groove, while in cases where its appearance was delayed the involution formed a shorter and more rounded ‘opening.

As in other cases the central part of the invagiiiatioii groove becomes deepened to form - a cavity which is clearly homologous with the

main part of the archenteric cavity in, say, a ‘Z A 5 ‘ o_ ‘I '. ' ‘ o o frog. 'lhis cavity starts by passing directly ,-/‘K

inwards, perpendicular to the egg-surface, but it soon bends forwards and runs parallel to the surface (Fig. 31, C and 1)). The cavity just [3 mentioned (Fig. 31, 1), am.) being an archenteron the layer of cells lining it corresponds to that O which in the lower forms is called endoderm. _ _ It is therefore misleading to replace this by any F“f’t- ,:‘3°'1,”“‘f““i‘_’°“5_’."° other name : to emphasize the fact that they line :15 g::e,:ui:r the true archenteric cavity it may be advisable faceview. A,(,‘h.elom'a, to speak of the cells in question as the wrchentemfi: (fin-““““_"‘v 1396" .3‘

. . _ . _ . __ . .atg/tlactji/lzts, (Will, endoderm in spite ol the clunisiiiess of the ex- 189.4 pression.

In the nieantime the lower -layer cells immediately underlying the ectoderm assume a flattened form and become joined together by their edges to form a definite epithelium which may conveniently be termed the secondary endoderm (Fig. 31, C and D, end’). The more deeply situated cells underlying the secondary endoderm remain spherical and are separated by wide spaces forming a seg- mentation or subgerininal cavity. 'l‘liese deeper spherical cells have their numbers constantly reinforced by additional cells which are apparently budded off from the underlying yolk-mass.

The floor of the archenteron becomes closely apposed to the secondary endoderm immediately beneath it (Fig. 31, D). The two cell-layers fuse, irregular perforations develop in the membrane formed by their fusion, and the result is that the archenteroii is thrown into communication with the “ subgerininal” cavity (Fig. 31, E). Shreds of the partition persist for some time. but eventually the two spaces form a perfectly continuous cavity just as happened with archenteric and segmentation cavity in the Gymnophiona.

The portion of the primitive plate which is embraced by the 50 EMBRYOLOGY OJ?‘ 'I‘TlE I.()WER VER'l‘EBRA’1‘ES CH.

. horas shoe. l-'»l'll-L}._)U(.l g:lsLru1a.r lip corru.sponds to the yolk-plug of

E ' » '7»

FIG. 31.—-Sngit.t:l..l .s'H'.tio11.~.: t1n'oup;h early .‘~'l:l-;.fL‘..\‘ of I‘./rt(3/aim‘/y/u..<. (At‘tu.1' W ill, 1892.) Ala of the .~.1:n_-.:»- ~'lmwu in |"i_~4_ .1... A ; E is of I luv .~s1,:1;_-,'n- of Fig. '.-'0, B: 1}, C, um! I) :m- ml" int.-rm.-.]i_-,tp

stages.‘ (LS, e.mln-ymnic. shiulll; rut, vc-tuulerwn; uni‘, .~u-urnul:u'_\' --mlm_1:-run‘, no/, :m-lu-.nL«-I-'1: (J:L\'ll}'; 11.1., gash-ular lip; m./:_. Lhickelxed +'.ct;odex‘m which will gi\'e risv later to the central nervous system

(medulla:-_v [n|:(Lu); 12.1), pr-imitzive plate; g/.p, _vulk-plug.

amphihi.'u'1s aml in 801116 cases too (_La._certa~—Wi1l) it becomes com- pletely enclosed, the taps of.‘ the 11o1‘se-shoe Cl11‘V1lIf__I' lnwnrds zmd 1 GASTR ULATION 51

meeting to form a closed ellipse. The yolk-plug di11'ers from that of amphibians merely in its being elliptical in outline instead of circular.

During these changes the anterior or dorsal part of the gastrular lip grows actively backwards over the surface of the yolk-plug, _the portion of yolk-plug which is covered over in this way becoming added to the floor of the archenteron and its superficial cells becoming converted into archenteric endoderm.

The last phase in the closing of the gastrular opening consists in its lateral walls approaching the mesial plane so that the opening assumes the form of a longitudinal slit (Fig. 29, D). l’art of this slit persists for some time as a neurenteric canal-—a communication between the enteric cavity and the cavity of the neural groove or tube (Fig. 32, ())——-while a portion farther back seems to be repre- sented by the anus although in this case the patent opening disappears temporarily so that no absolute continuity can be traced. In the region where the lips have undergone fusion there persists for a time complete continuity between the different cell-layers. The study of sections shows this continuity to be precisely the same as that which occurs in the primitive streak of Birds and Mammals (Fig. 32, B, I), E), and we have thus suggested a clue to the meaning of that otherwise enigmatical structure.

It will have been gathered that the archenteric cavity has become greatly reduced in importance in the Reptile as compared with the more primitive vertebrates. It has become much reduced in relative size,‘ and it soon loses its individuality, becoming merged with the irregular segmentation spaces lying beneath the blastoderm. Correlated with this we can no longer speak of direct conversion of the archenteric cavity into the enteron or alimentary canal, except to a trifling extent. The latter arises, for the most part, as will be shown later, in a quite different manner from the secondary endoderm.

BIRI)S.———In the Reptile, as compared with one of the more pri1nitiv,e anamnia, the main peculiarity of the gastrulation process lies in the fact that the cavity which opens to the exterior by the blastopore is normally of double origin, only its posterior portion being derived from archenteron. Consequently the layer of endoderm which lines it is only to a comparatively small extent derived from the archenteric lining, the much greater anterior part being formed from elements of independent origin.

In the Amniota above Reptiles the replacement of archenteric by secondary endoderm has gone still further, inasmuch as the formation of an archenteron has in them either completely dis- appeared from development, or at the least is reduced to a faint vestige, and the endoderm is therefore entirely secondary.

1 In some forms, such as Lacerta, the archenteric portion of the enteron appears to be for a time much shorter relatively than in others (e.g. Platydactylus) but this is corrected later on by active overgrowth on the part of the dorsal lip (Will, 1895). 52

EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

As regards the Birds, which of the higherAmniotesalone concern us in this_ volume, there is com- plete agreement that they are to be looked on as highly-speciaL ized descendants of Reptiliati ancestors. It follows therefore that their develop— mental phenomena should be considered in relation to the cor- responding pheno- mena in Reptiles.

Leaving out of account certain vague phenomena which have been inter- preted, in the present writer's opinion un- justifiably, as re1nin~ iscences of gastrula- tion processes (see Chapter X.), the for- mation of a gastrular lip seems to have

Fm. 32.——~'l‘rmisverse sec- tions through region of ncurenteric canal of Che- lorzia embryo with about 16 segments. (After Mitsukuri, 1896.)

The mid-dorsal ectoderm has become covered in to form the neural tube (s.c.) as will be de- scribed in Chap. 11. Fig. C shows the neurenteric canal opening upwards through this, while Figs. 13, D and E, taken from sections anterior and pos- terior to the neurenteric open- ing, show the continuity of tissue from eclodcrin to endo- derm which is a characteristic feature of a. primitive streak. ect, ectoderm; end, endodcrm; mes, mesoderm; N, notochord; ne.c, neurenteric canal; 3.0, spinal cord. I GASTRULATION 53

been eliminated entirely from ontogenetic development in Birds. What is conspicuous is a well-marked primitive streak which makes its appearance in the posterior half of the blastoderm along what will be the axial line of the body of the embryo (see Chap. X.). A groove develops along the surface of the streak —-- the primitive groove.

Histologically the primitive streak is, in its early stages, a line of proliferation from the inner surface of the ectoderm, the blasto- derm being composed only of the two primary layers at the time of its appearance. That the ectoderm alone is responsible for the first appearance of the primitive streak, a point difficult to make absolutely certain by ordinary observation, appears to be demonstrated by the study of an abnormal 36-hour embryo Peawit (Vanellus eristtttus) described by Riithig (1907) in which the endoderm was completely absent while ectoderm and primitive streak were quite normal.

An inspection of blastoderms at successive periods in development shows the primitive streak lying always behind the medullary folds (of. Fig. 227, Chap. X.), and it might therefore be readily assumed that the embryonic body develops entirely in front of the primitive streak. That this is not so is clearly shown by experiments (Kopsch, 1902) in which a scar is made with a hot needle about the front end of the primitive streak during an early stage in its development. If the egg is carefully scaled up again it may go on with its development, and in such a case the scar is found later on to be situated not near the hind end of the embryo but well forward in the head region.

What apparently happens is that the primitive streak grows actively in length with the general growth of the blastoderm but that all the while it is becoming correspondingly shortened at its headward end. As a matter of fact its anterior end becomes gradually converted from before backwards into notochord and'the adjoining parts of the mesoderm. The front part, which is under- going this change, loses its connexion with the ectoderm while it becomes on the other hand continuous with the endoderm and is reinforced by proliferation from it: it then forms what is known as the Head process.

The point of special morphological importance to notice about the primitive streak is its continuity with the two primary cell- layers. Throughout the greater part of its length it is continuous with ectoderm, in its front half with both ectoderm and endoderm, and in its forward prolongation-——-the head process--with endo- derm. Correlated with this is the further fact that in some cases (Tern, Goose, Duck, Wagtail, Melopsvlttacus) the tissue of the primitive streak is traversed by a typical neurenteric canal.

Taking these various features into consideration it is impossible to avoid the conclusion that the primitive streak represents the line of coalescence of the gastrular lips just as it actually is in 54 EMBRYOLOGY OF THE LOWER VERTEBRATES cu.

Reptiles, and that the neurcnteric canal represents a persisting portion of a once slit- like gastrula mouth which is otherwise

obliterated.

ORIGIN OF‘ 'l‘lllt' MESODERM

(_ll«‘.NERAl. REl\1Al{KS.--AlT(-Etuly during the process of segmentation the dilltsrentiation of the two primary cell—l-ayers commences--the superlicial cells towards the apical pole dividing more actively, being smaller, and being less laden with food-yolk and thus establishing a character of their own as ectodermal cells. The full establishment of the primary layers however is only consummated during the process of gastrulation when the cctoderm comes by the various processes already described to enclose the remaining cells the (archen teric) endoderm.

The establishment of the two primary layers is followed immediately (indeed the two processes frequently overlap) by the development of the intermediate cell-layer———the mesoderm—-—wln'ch will in the adult form the great mass of the body— all in fact except the epidermis and its derivatives on the one hand and the enteric epithelium and its derivatives on the other.

The problem of the evolutionary history of the mesoderm of Vertebrates is one upon which there is little agreement. Anything of the nature of elaborate and detailed treatment of the subject would be out of place in a textbook of moderate size and a short sketch such asthe following is necessarily coloured by the general

unorpliological views of the wri_ter. While the views set forth in the

following paragraphs seem to the author to fit most satisfactorily the facts so far as these are established beyond reasonable doubt there are other embryologists who would give an account differing consider- ably from that given here.

To the present writer it seems of importance in endeavouring to arrive at reliable general conclusions from the facts of observation to bear in mind particularly the risk of reaching erroneous conclusions through basing arguments upon phenomena observed in the head region or tail region of the embryo. Intense cephalization, «Le. intense structural modification of the anterior region of the body, to form a head, is admittedly one of the fundamental characters of the phylum Vertebrate. In this modification the mesoderm has been deeply involved so that there is always a considerable weight of probability against conditions observed in the head region being primitive. Again the tail region is also intensely modified, as is indicated e.{}. by the transient appearance within it of a vestigial portion of alimentary canal with surrounding body-cavity. Here again then, though not to the same extent as in the head-region, suspicion rests upon the primitiveness of all phenomena of develop- ment peculiar to this region of the body.

It is advisable then, for these reasons, to exercise great caution in I ORIGIN OF THE MESODERM 55

making use of any developmental phenomena except those observed in typical trunk segments as a basis for speculations upon the evolu- tionary origin of the mesoderm.

It has, further, to be borne in mind that observations upon the development of the mesoderm in its early stages have to be made by the method of serial sections, and that in the interpretation of such sections the liability to error becomes greatly increased if the sections are not exactly in one of the three following sets of planes——(1) transverse to the morphological axis, (2) “ horizontal," and (3) parallel to the sagittal plane. This type of technical difficulty is in many Vertebrate embryos most marked in the head and tail regions.

For these reasons it seems safest, in considering generally the ontogenetic development and the probable evolutionary history of the mesoderm, to ignore all observations except those made on typical trunk segments between the level of the otocyst in front and of the anus behind. This will accordingly be done in what follovs s.

It is agreed by the majority of students of Vertebrate embryology that the most nearly primitive condition of the mesoclerm known to occur in the embryos of Vertebrates is that seen i11 Amp]?/ioxus, where it consists for a time of a series cl‘ endodermal pockets, converted later into closed sacs, upon each side of the body (Fig. 34, B).

It appears fully justifiable to conclude that both of the stages mentioned represent ancestral conditions in the evolution of the Vertebrate mesoderm. The excretory organs of the Vertebrate, in the form of paired segnientally arranged tubes, afford in themselves strong evidence that at one time the Vertebrate coelonie was in the form of isolated segmentally arranged chambers.

In the case of Aznplmfoxus the segmented character of the meso- derm persists only dorsally, the ventral portions of the successive segments becoming fused together so as to give rise to a continuous unsegmented splanchnocoele or peritoneal cavity.

In the Craniata the smallest departure from the condition in Amp/mjowas is seen in such comparatively primitive forms as Lam- preys, Crossopterygians and Lung-fishes. In these a solid continuous mesoderm rudiment becomes split off from the endoderm on each side, remaining for some time continuous laterally with the endoderm (Fig. 40, B, C, p. 65). In the outer or lateral part of this mesoderm rudiment the segmentation, which even in Amp}:/ioams was only temporary, never makes its appearance. The dorsal portion does segment but the segment is a solid block of cells in which a cavity only appears later on. It is fairly clear that these mesoderm seg- ments, except for the fact that they are continuous in their ventral portions and that they are at first solid modification of develop- ment which is very common in hollow organs), agree closely with the segments of Amplmlocms and that they are homologous structures merely somewhat modified from the primitive condition met with in Amphioxm.

In endeavouring to institute a mor_e precise comparison of the 56, EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

mesoderm segment in its earliest stage, in the typical Vertebrate, with that of Amplzxioxus, the way is found to be blocked by a second- ary adhesion (or absence of separation!) having come about between the mesoderm segment and the endoderm from which it has arisen. The young mesoderm pouch of Amplvioxus is attached to the endoderm at its base————7I.e. its ventral end. Its homologue in the more typical Vertebrate, on the other hand, is continuous with the endoderm in two different regions, one ventral and one dorsal. This is illustrated by such a diagrammatic section as that shown in Fig. 33, B, in which the solid mass of mesoderm on each side, indi- cated by the medium tone, is continuous with the mass of endoderm or yolk-cells at the points a and b. The question is, which of these two points is to be interpreted as representing the root of the meso— derm pocket in Amphiomzs ? Clearly only one of them can represent

I


FIG. 33‘.-—-Diagram illustrating (B) the origin of mesoderm lfroni endoderm in an Amphibian,

and (Aand (3) the two methods of correlating it with the mode of mesoderm formation in A mphicwus. ‘

n, b, see text; cct, ectmlerm; en-t, enteric cavity ; N, notoeliordal rudiment.

this and the other region of continuity must represent a secondary fusion of mesoderm with endoderm.

. The majority of embryologists, following 0. Hertwig (1882), believe that the dorsally situated region of continuity marked 12 is the primary connexion as is illustrated by Fig. 33, C. On this view the mesoderm segment of the Vertebrate springs from the endoderm at a point about the level of the notochord, it grows downwards on each side of the alimentary canal and eventually its tip meets the tip of its fellow of the other side of the body in the mid-ventral line.

-In this view, again, the .continuity which can sometimes be shown

to exist between mesoderm and endoderm at the point a would be regarded as secondary and without evolutionary significance.

If however due weight be accorded to what is observed in the development of the lower holoblastic vertebrates it seems more reasonable to the present writer to conclude that the more ventrally situated connexion, that marked a, is the primitive one and that the more dorsally situated, 12, is the secondary aequirement (Fig. 33, A). .tion of mesoderm.

I ORIGIN OF THE MESODERM 57

Upon tl1e ;i'ormer hypothesis the extension of the mcsoclerm l.ater~ ally by delamination from the endoderrn, which does certainly occur in some forms (see below), would be an inexplicable mystery.’ On the second hypothesis on the other hand this splittilig off of mesodei-m would be comparable with a gradual deepening of the iLIlgl.G which bounds the mesodermal pocket of Amp/uloanm on its mesial side (see F'g. 34, B). The dorsal attachment is, on this view, to he rega1‘Cle(l as a secondary fusion between mesoderm and (m<‘lo(leI‘Ii1. In the higher vertebrates e is o s l this region becomes the ‘seat of active cell proliferation which plays a great part i.n the produc-

_ After these general remarks we may proceed to con- sider shortly the details of the early development of the mesoderm in a few examples of the lower Vertehrata. AMPHIOXUS.--— In Amqokioxus the development of mesoderm begins with the formation of a longitudinal fold or outpushing of the endoderm on each side of the

mud " dorsal e Fm. 34.——'1‘ransverse sections of young ..47n.p}i.-('0.-rcus illustrating

(Fig. 34, A, mes). the origin of the mesoderm. (Ai'tci'I-1at.s-click, 188.1.) _In thls W3)’ there col, ectodvrni: 4'-nt, enteric cavity; m.p, im-dull.'u‘_\' [|l.‘|U‘I tiles, 18 formed on each mesodi-.rm; N, notocliordal rudiim-ni.-.

side an upwardly-

projecting groove or gutter, the narrow cavity of which is a pro- longation of the archenteric cavity (Fig. 34, B). Constrictions appear now in the wall of this gutter which divide it up into successive segments --—"—’- the constrictions developing in order from the head-end backwards. 'l‘he groove or fold is in this way con- verted into a series of pockets‘ the coelomic or enterocoelic pouches. The cavity of these pouches except in the case of the first two usually becomes for a time practically obliterated by the outer and inner walls coming into contact. Finally the communication between pouch and archenteron beeoliies closed and the pouch 58 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

itself hecomes nipped oil’ from the remainder of the are-henteron (Fig. 34, O). _ The‘origina1 a.rchenteron is now replaced by a. main portion, the

Flu. 35.——Transverse sections through embryos of Lepidos1.'7‘en to illustrate the origin of the mesorlerm.

A, stage 12 ; B and C, st.ug-- 14. act, ectodex-in ; et-rul, endorlerm ; «mt, enteric cavity; mes, mesoderm ; N, notochordul rudiment.

enteron, the wall of whieh——the definitive eiidoclerm-—-will eventu- ally become the lining epithelium of the :_Lli.1nenta,ry canal, and, lying dorsal to this on each side, a series of closed sacs, or practically solid I ORIGIN OF THE MESODERM 59

blocks (their outer and inner walls being in contact). These sacs or blocks are the mesoderm segments and their cavities are the seg- mentally arranged rudiments of the coelome. The subsequent fate of the mesoderm segments will be traced later (Chap. IV.).

Of the lower holoblastic forms amongst the Vertebrata in the stricter sense we will consider lirst Lepidosiren, in which, owing to the large size of the cell-elcmen ts, the details of mcsoderm formation are particularly clear and unmistakable.

The mode of origin of the mesoderm which occurs in Lep'id0s7§rre'n. is illustrated by Fig. 35. The section shown in Fig. 35, A is taken from an egg of the same age as that figured on p. 35 (Fig. 21, Q) in illustration of the disappearance of the segmentation cavity. Im- mediately below the eetoderm is a mass of rounded blastomeres with intervening chinl<s——rc1nnants of the segmentation cavity: towards the mesial plane the blastomercs are more closely packed together. The small blastomeres in question are clearly distinguished by their finely-grained yolk from the large yolk-cells with their coarsely- grained yolk which form the bulk of the egg. The mass of small blastomeres is destined to give rise laterally to the mesoderm and mesially to the notochord. It must clearly be borne in mind that the mass is composed simply of small blastomeres and that it passes at its outer margin without any break into the ordinary yolk-cells.

As development goes on, the mass of small elements becomes compacted together (Fig. 35, B), the chinks between the cells disap- pearing. At the same time the boundary between them and the yolk-cells becomes more definite, so as to delimit more clearly the mesoderm rudiment (mes) from the definitive endoderm.

Fig. 35, C is taken from an egg of the same age but here the mcsoderm rudiment has become limited also on its mesial side by a split which marks it oil‘ from the notochord (N).

At a somewhat later stage, the mesoderm mass on each side becomes divided into segments by splits, transverse to the axis of the body, which make their appearance at regular intervals from before backwards, but it is to be noted that in Lepidosziren (as in all Verte- brates except Amphz'0am.s) this splitting of the mesoderm is confined to its dorsal portions. There is thus produced along each side of the body a series of incomplete mesoderm segments1 which pass at their lower or ventral ends into an unsegmented sheet of “lateral”

1 Such incomplete mesoderm segments as are described above occur in all the typical vertebrates and are known by various names such as mes-ohlastic somites, protovertebrac, myotomes. These names are in various degrees erroneous or mis- leading. The word somite means a complete body segment and it is not allowable to apply it to a. single organ. The name protovertebra dates from the days in which these structures were supposed to be the embryonic vertebrae, which they are now known not to be. Of the three terms mentioned myotome is the least objectionable as at least the greater part of the segmented portions of mesoderm become definite myotomes later on. On the whole however it seems most convenient to retain the ex- pression mesoderm segment, the word segment not being necessarily used in the precisely defined way in which such a purely technical morphological term as “somite ” must be used. 60 EMBRYOLOGY OF ’l‘HE LOWER V ERTEBRATES CH.

mesoderm. This latter gradually spreads ventralwards by delamina- tion from the large yolk-cells and eventually the mesoderm sheets on the two sides become continued into one another ventrally.

As will be noticed there are no coelomic spaces within the mesoderm rudiments at these early stages: they arise secondarily later on.

If we review the abovc—dcscribed stages in the early development of the mesoderm segment in Le])'id0s'ire'n., in which, as already indi- cated, the large size of the cell-elements ensures unusual freedom from the danger of errors of observation, we see that the last described stage is clearly in agreement with the hypothesis that it is a repetition of the stage in Amplzxiowus when the mesoderm existed in the form of a series of enterocoelic pouches on each side. The only conspicuous dill'crence is that, whereas in A7n.};/I/ioasas these were actual pouches, here they are solid blocks of cells in which a cavity only makes its appearance at a later stage of development. That this difference is in no Way a serious one will become apparent to the reader as he realizes that it is one of the commonest modifica- tions of developmental phenomena, when yolk is abundant, that primitivcly hollow organs develop in the embryo from solid rudi- ments and only form their cavity secondarily.

It may be accepted then with coulidence that the solid mesoderm segments of Lepidosiren at the stage indicated, continuous ventrally with the endoderm, represent the enterocoelie pouches of Amp}mJo:2cu..~: modified in correlation with the abundance of yolk.

The first stages in the development of the mesoderm of Lexp’id0- siren are obviously very dilibrent from what are found in Amph'ioa:us and the differences here also we may justifiably attribute to the immense thickening of the cndodermal wall of the archenteron corre- lated with the storing up of a large amount of yolk in its cells.

In the other groups of holoblastic vertebrates the main features in the early development of the mesoderm agree with those just described for Lep'idosi'ren. In all of them the archenteron is pro- vided with a thick wall of heavily yolked endoderm cells, those forming the roof or dorsal part of the wall being smaller and pro- vided with finer yolk-granules. Out of this smaller-celled mass the mesoderm segments become carved by the development of splits very much in the same way as in Lepidosiren (cf. Fig. 40, B—Petrom3/zon).

Amongst these groups the Amphibia call. for a little further consideration. .

In the frog a split develops on each side which separates the roof of the archenteric cavity into two layers, an inner layer, one cell thick, of definitive endoderm and an outer, two cells thick for the most part, the mesoderm. This split is seen in Fig. 36 which repre- sents a section, transverse to the axis of the archenteron, through an egg with large yolk-plug. The split in this section terminates below at about the level of the floor of the arehenteric cavity while above it stops short of the level of the notochord. I ()Rl(a‘rl.' N 01*‘ 'l‘llE .\Iul*}f_\_‘(Q).l)l4}l*i,M 61

A little later a split at its dorsal end C.lmn:_-u‘ea.tcs the mesoderm rudiment from the notoehord. The mesoderm rudiment, forming now a broad band on each side of the embryo, becomes divided into segments by splits which cut it across and a condition is reached corresponding closely with that already described for Legivlrlostren where the mesoderm consists of a series of solid segments on well side continuous ventrally with the mass of yolk cells forniing the main part of the endoderm.

As in Lepwldosiren the ventral unsegmented part of the Inesodcrm becomes prolonged ventrally by the extension downwards of the split between it and the endo- ' derm. In other words the mesoderm extends ven- trally by a process of de- lamination from the endo- derm.

In the anterior part of the body the sheet of meso- derm becomes split off completely from the yolky endoderm before it quite reaches the mid - ventral line so that the sheets be- longing to the two sides are discontinuous ventrally but in the binder region the two splits meet ven- trally so as to give rise to a sheet of mesoderm con- tinuous across the middle Fm. 36,-—'.l.‘ransverse section tlIl'ou;_~,'h an t‘llll)l'_\'U or hne. - Under Ordlinary c1¥._ ougm ol th lm~.Ul(lllJ. cumstances the Iuesodemn e't ectoderm° and Pll'l0(lPl"ll ‘ m Ill ode ' sheets in the anterior " ‘ i ’ ’ i N: ,‘m0c1;W',j cs’ °s ""‘ region grow ventrally and eventually fuse with one another (as will be described later) while in the posterior region this fusion is anticipated by the two lateral rudirnents being continuous from the beginning.

So far everything seems fairly simple, but it now remains to allude to certain peculiarities which have done much to obscure the clear understanding of the hietliod of 11u'.s'o(‘le1"'In_ formation and which are especially important for the proper emnprehension of the flrst forma- tion of_n1esode1'm in the nneroblastic vertebrates.

The peculiarities in question are to be seen in the hinder part of the trunk region. In this region the split which separates off meso- derm from endoderm remains _for a time incomplete at a point just external to the notocliord. Ear-.11 svgiiieiit therefore remains for a time continuous with the endoderm at this point. The level oi" these junctions of mesoderm and endoderm is marked by a longitudinal 62 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

groove of the inner surface of the wall of the archenteron so that where the junction exists the archenteric cavity may be said to pro- ject slightly into it. The cells at this point develop pigment in their protoplasm (Fig. 37, B); they frequently show mitotic figures and appear to be actively proliferating, cells being added at this point to the mesoderm.

'J.‘he peculiarities which have just been described, and which occur in various amphibians, have important hearings in two different directions. In the first place they form an important part of the basis for O. Hertwig’s hypothesis of mesoderm formation in the Vertcbrata, the junctions, which have just been described, between endoderm and mesoderm being interpreted by him as representing the original stalks of the mesoderm segments as they occur in Am- pltioxus. As already indicated there do not appear to the writer to

Fm. 37.—-Transverse sections through embryos of (A) T‘I‘l'tu-IL and (B) Rana teonpziramia showing continuity of endodcrm and mesoderm on each side of the notochord. (After 0. Hertwig, 1882 and 1883.) -'

end, emloderni; m.p, medullary plate ; mes, mesoderm; N, notochordal rudiment.

be sufficient reasons for regarding these connexions as primitive rather than those more ventrally situated. The balance of probability appears rather to favour the view that of the two connexions it is the ventral one which is the persistent original one, and that it is the dorsal which is to be interpreted as "due to secondary- fusion. .

The second bearing is at least equally important. It rests on the occurrence of active cell-proliferation on each side of the noto- chordal rudiment. For in some of the meroblastic vertebrates (Amniota)-—~correlated with the more and more complete segregation of yolk from protop1asm——this zone of proliferation becomes ap- parently the main source of the mesoderm.

ELASMOBRANCHII.-—-In the Elasmobranch, while there are still traces of formation of mesoderm by a process of delamination from the main mass of endoderm or yolk (Fig. 38, A), a more con- spicuous mode of formation is provided by active proliferation of I ORIGIN OF THE MESODERM 63

the endoderni cells along the inner and outer edges of the sheet of

mesoderm.

In early stages and in the anterior part of the enihryo this pro- liferation process may alone he in evidence, so that in place ofa l)r0ad continuous sheet of inesoderm there are found two narrow strips—~ one (Fig. 38, 0, mos’) arising from the e1l(lOCl.€I‘l11 just external to the notochordal region and the other ('mre-23”) arising from the endoderm

FIG. 38.-—-Three transverse sections through an embryo of [’rz7sti-m'u.s- (Stage B, Balfour), illustrating the origin of the mesoderm. (After C. Rah], 1889.)

Section A, through the posterior half of the mnln-yo; B, through the iniddlc ; (3, through the anterior half. eat, ectoderm ; and, endoderm ; mes, I11(?SOfleI'I1l ; 3/.n, yolk nuclei.

peripherally. The two strips are known respectively as the axial (Riickert; or Gastral, Rabl) and the peripheral mesoderin (Riickert; or Peristomal mesoderm, Rabl).

Much discussion has centred round this double origin of the mesoderm and attempts have been made to distinguish axial and peripheral inesoderm in holoblastic forms including even Amplmloasus, thus infringing one of the chief canons of embryological science- that developmental phenomena in the higher forms are to be ex- plained by those of the lower and not vice versa. 64 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

In the formation of axial mesoderm we recognize a zone of fusion of niesoderiii and endoderm accompanied by proliferation of mesoderm entirely analogous with that which occurs in Amphibians but which had not yet made its appearance in lower holoblastic forms.

Whether'it is justifiable to regard the outer zone of mesoderm formation in the Elasinobranch as equivalent to the region of de- lamination (a process which necessarily involves cell-proliferation) is doubtful. lt is indeed doubtful to what extent there is justificatioii for drawing any morphological distinction between axial and peri- pheral inesoderin, seeing that the two regions of proliferation are on the protostonia hypothesis morphologically closely related to one another (see Chap. lX.).

If we look at the matter from the point of view of physiology rather than of morphology we may probably recognize in the active formation of axial iiiesoderin an expression of the general tendency in the iiieroblastic egg for all processes of growth and cell prolifera-


Fiu. 39.—--'l‘ransverse section through the hlastoderin of a siiake(T7~opi¢i0n.otus) illustrating the origin of the niesodei-in. (After 0. llertwig, 1901).)

act, ectodei-iii ; mil, Bn(l()Il¢*1'lIl : mes, inesoderin.

tion to become concentrated towards the mesial plane dorsally and to slacken off peripherally and ventrally.

l‘tEPTII.ri«:s.——-According to the View taken in this book the inesc- derni in the holoblastic Uraniates at one period spread outwards by a process of delaiiiination from the yolk-laden endoderm. _ _

In the Amphibians we have seen that a new source of addition to the mesoderm had made its appearance in the form of a zone of pro- liferation on each side of the Il()t0(3llO1'(l,ll1 which region cells are hudded off into the inesoderiii. _ ' O

In the Reptiles--—adinitted1y descendants from Amphibian-like ancestors-—-—in correlation with the concentration of developmental activity towards the mid-dorsal line brought about by the accumula- tion of the yolk ventrally, this parachordal source of mesoderm has become predomiiiant while the lateral source has become greatly

reduced. In Fig. 39 is represented the typical mode of mesoderm forma-

tion as seen in a transverse section through the trunk region of a reptilian embryo. The mesoderm is seen to be spreading o_ut.a.s a wing of cells towards either side from the iiotochordal or primitive streak region between the two primary cel1—1ayers.

BIRDS.---ln the Birds also the method of first mesoderm formation 1 ORIGIN or THE MESODERM 65

appears to be closely comparable with that of Reptiles and Amphib- ians. Here, at the time when the mesoderm begins to make its appearance, the position of‘ the notochord is occupied by the primitive streak. The mesoderm fornis a loose sheet of irregularly shaped. cells spreading out on each side and added to from two distinct sources: on its inner side by proliferation from the primitive streak and on its outer side by delamination from the endoderm of the germ wall. It will facilitate comprehension of the evolutionary changes which

0 FIG. 40.——Sen1i~¢li:1.gramma1.ic transverse sections through the embryos of various vertebrates to illllstmte the origin cl‘ the inesmlerin.

A, .~lmph.-i.mus; ll, 1'c!rumy:nn. : 1‘, Imp-idosire-n; D, Amphihism; I11, l‘lln.-nmbrum:h. ect, (‘.(_'.tO(l6l‘ll1; end, emlomnnz vent, 1-nt'nrlc x-:u'it_\'; mes, mesoderm; N, inotoclmnl; n.r_, neural rluliments. '|‘l'n-. .~nnall crosses _in«_li«_-nt_«~ regions in which active extension of the nxesoderm is taking place.

the writer beli.eves to have taken place in the mode of development of the mesoderm within the phylum Vertebrata if the main steps are summarized in a diagram. In Fig. 40, A shows the primitive condi- tion where the mesoderm segments are in the form of enterocoelic pockets (Ampholoxus). In B, with increasing amount of yolk, the hollow pocket is represented by a. solid block in which the cavity will develop secondarily (Petromyzon). In C the condition is similar but the dorsal portion of the embryonic body is more flattened out, the bulk of the yolky endoderm .over which it is spread having VOL. II F 66 EMBRYOLOGY OF TIIE LOWER VERTEBRATES CH.

become greater (Lung-fish). In D the secondary continuity of the mesoderm with the endoderm just oiitside the notochord is present and proliferation of mesoderin cells has commenced in this region (Amphibia). Finally, in E, with the very great increase in the bulk of the yolk, the dorsal part of the embryo is still more flattened out, and the addition to the mesoderm by proliferation of‘ endoderm cells into it close to the notoehord has now become conspicuous (Elasmo-

branch). THE MEH l+JN(“lTYMl«1 °

The fate of the mesoderm whose origin has just been traced is to give rise directly to the peritoneal epithelium which lines the body cavity and covers the organs lying within it, and also to the muscular system. lndireetly it, however, also plays a great part in the forma- tion of what is known as the meseuchymc.

Whereas for a time the Vertebrate body is composed of compact masses or layers of cells, it is a general characteristic that, as develop- ment goes on, individual cells detach themselves and wander away through the body, multiplying by fission accompanied by mitosis, and behaving‘ in fact very much as if they were independent organisms. In the course of the many generations of these cells which arise during the process of individual development, they become divided into various strains which show marked differentiation for the per- formance of difierent functions.

Some retain a relatively primitive amoeboid form and undertake such functions aslthe transport of food material, the absorption of moribund tissues in regions where shrinkage in volume or atrophy is taking place, and the ingestion and destruction of attacking organ- isms such as. disease germs. Some, their protoplasm laden with insoluble excretory products as a result of their active metabolism, wander towards the light and settle down near the surface of the body as pigment cells or chromatophores which serve on the one hand to protect the underlying tissues from the light and upon the other to give distinctive coloration to the animal. Others again settle down in an abundant jelly-like intercellular matrix to form connect- ive or packing tissue, which in turn shows evolution in various directions in accordance more particularly with different developments of the intercellular matrix. Of special importance are these types in which the matrix becomes hard and rigid so as to form skeletal tissues such as bone and cartilage.

Another important strain of these cells is characterized by the fluidity of the matrix and the independence of the individual cells which float in it. This liquid connective tissue forms the blood which, pumped through an elaborate system of vessels, serves on the one hand for the transport of food and oxygen to the tissues, and on the other for carrying away the waste products of metabolism to the special excretory organs the duty of which is finally to remove these harmful substances. ' I MESEN CH YME 67

The sum of these amoeboid cells, which proceed along the various evolutionary paths above indicated, were, by O. Hertwig, given the name Mesenchyme—to distinguish them from the mesothelium, or mesoderm in the restricted sense, in which the cells remain in the form of continuous layers or epithelia.

The original mesenchyme cells arise by emigration from the pre-existing cell layers. Possibly all three layers give rise to mesenchyme cells. It is the mesoderm however which does so most conspicuously. In an Elasmobranch embryo, for example, active budding off of mesenchyme cells is seen over large areas of the

somatic mesoderm and similarly from the inner surface of the splanchnic mesoderm. Most active of all is the production of mesen- ehyme cells from the splanchnic mesoderm near the lower end of the mesoderm segment, where the proliferating mesenchyme cells may form a conspicuous mass projecting towards the mesial plane and termed the sclerotome.‘ The special consideration of the sclerotome and of the mesenchyme in general will come most conveniently after the other derivatives of the mesoderm (Chaps. IV., V., VI.).

1 The use of the word sclerotome in this restricted sense has come to be practically universal in embryological literature and is therefore followed in this volume. The word was invented by Goodsir and defined by him, at the British Association meeting in 1856, as meaning a segment of the supportin tissue or framework (whether “ fibrous” or cartilaginous or osseous) in a segmcnte animal.

LITERATURE

Agassiz and Whitman. Mem. Mus. Comp. '/lool., xiv, 1885 and 1889. Assheton. The Work of John Samuel Budgett. Cambridge, 1907. Balfour. (.,!t1a;'lt.1.3Io1%rri. Micr. Sci., N .S. XIV, 1874.

Brachet. Are . io ., xix, 1903.

Brauer. Zool. Jahrbiicher (Anat. Al)t.), x, 1897.

Budgett. Trans. Zool. Soc. London, xv, 1901.

Cerfontaine. Arch. (le Biol., xxii, 1906.

Dean, Bashford. Journ. Morph., xi, 1895.

Dean, Bashford. Quart. Journ. Micr. Sci., xxxviii, 1896.

Dean, Bashford. Kupllcrs Fcstschrift, 1899.

Goette. Ahhamll. zur Entwickelungsgesch. d. Tiere, v, 1890. Goodsir. Edinbiggh }Ne}vlPll1ilosophical Journal, v, 1857. Goronowitsch. or )1. a 11'). x, 1885.

Hatschek. Arb. zool. Inst. Univ. Wien, iv, 1881.

Hortwig, O. Jenaische Zeitschrift, xv and xvi, 1882 and 1883. Hertwig, O. Handbuch d. Entwieklungslehre, Jena, 1906. Jablonowski. Anat. Anzeiger, xiv, 1898.

Jenkinson. Vertebrate Embryology. Oxford, 1913.

Jordan and Bycleshymer. Journ. M01'pl'l., ix, 1894.

Kerr, Graham. Phil. Trans., B, excii, 1900.

Kerr, Graham. The Work of John Samuel Budgett. Cambridge, 1907. Kopsch. Internet. Monatsschrift f. Anat. u. Phys., xix, 1902. Kopsch. Arch. mikr. Anat., lxxviii, 1911.

Kowalevsky. Mém. Acad. Sci. St. Pétersbourg, Ser. 7, xi, 1867. Mitsukuri. Journ. Cell. Sci. Tokyo, x, 1896. . Morgan. The Development of the Frog's Egg. New York, 1897. Patterson. Journ. Morph., xxi, 1910.

Prince. Ann. ltfiag. Fat. Hist., Fifth Series, xviii, 1886.

Rabi, C. Morp . Ja1rb., xv, 1889.

Rothig. Arch. mikr. Anat., lxx, 1907. 68 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. I

Rilokort. SB. Gas. Morph.'Phys4. Miinch., 1885.

Rfickort. SB. Goa. Morph. Phys. Miiuch., 1890.

Rfickort. Kupffers Festschrift, 1899.

Balensky. Memoirs of the Society of Naturalists of Kasan, vii, 1t. 3, 1878. (In Russian. A French translation of the more important parts will e found in Arch. do Biologic, ii, 1881.)

8a.ma.asa.. Arch. Entwickclnngsmecluinik, vii, 1898.

Barasin, P. and 1-‘. Ergcbnisse naturwiss. Forsch. auf Ceylon. Wiesbaden, 1887 1893.

Schultze, O. lidllikcrs Festschrift. Leipzig, 1887.

Bchultze, 0. Arch. mikr. Anat., IV, 1899.

Schwink. Die Entwicklung dos mittleren Keimblattes und dcr Chorda. dorsalis dcr Amphibien. Munich, 1889.

Semon. Zoologische Forschungsreiseu, i. Jena, 1893.

Will. Zoolog. Jalu'bi1cher(Ana.t.), vi, 1892.

Will. SB. Burl. Almd., 1895.

Wilson, E. B. Journ. Morph., viii, 1893.

Wilson, H. V. Bull. U.S. Fish-Commisesiun, ix, 1891.

Ziegler. Entwiukelungsgcsch. (1. niedercn Wirbeltiere, Jena, 1902.

CHAPTER II THE SKIN AND ITS DERIVATIVES

THE skin of the vertebrate consists of the epidermis—the persistent and less or more modified ectoderm—resting upon the superficial layer of mesenchy1ne—the dermis—which in the higher forms becomes strengthened by the formation of numerous tough inter- lacing fibres.

In studying the development of the skin in the various types of vertebrate we find that the ectoderm undergoes characteristic modifications to fit it for the carrying out of special functions. In the fishes it becomes converted into a highly glandular mechanism concerned with the production of slippery mucus for the diminution of what the naval architect calls “skin-friction,” in other words the friction between the surface of the body and the water in contact with it. Local or general specializations of this glandular apparatus lead to the development of cement organs by the secretion of which the young animal is able to attach itself to solid supports, to the production of digestive ferments by which the eggshell is softened or, in the case of the portion of ectoderm which lines the buccal cavity, the digestion of the food initiated, or to the production of poisonous defensive or offensive secretions. In the case of the terrestrial amphibians the glandular apparatus serves to keep the skin moist, while in the Birds it develops arrangements for oiling the feathers.

Again the ectoderm develops important protective functions. It becomes hardened and toughened to give mechanical protection: it becomes more or less loaded with opaque pigment to prevent the penetration of light rays, while in those highest vertebrates, in which, correlated with intensely active metabolism, the body is kept at a higher temperature than its surroundings, the superficial horny layer becomes as it were frayed out into a fluffy coating of feathers or hair which with its entangled air retards loss of heat from the surface of the body.

Finally the ectoderm forms the great mechanism for the reception of impressions from the external world. It develops sensory cells which may become crowded together to form organs of special sense

69 70 EMBRYOLOGY or THE LOWER VERTEBRATES on.

while from its deeper layers arise tl1e main portions of the central nervous system.

THE EPIl)ERMIS

The cctoderm covering the surface of the embryo becomes converted, normally, into the epidermis of the fully developed individual. Very usually the embryonic ectoderm consists of two layers of cells, the lower layer composed of actively living cells, the superficial of flattened plate-like protective cells. This outer layer has been termed by Krause the periderm: its superficial protoplasm is commonly hardened to form a cuticle in the strict sense of the term. Normally it plays no active part in development a11d is shed at an early period.

The deep layer of the ectoderm on the other hand is active. Its cells multiply so that it becomes several layers thick: the outer layers become cornified to form the horny stratum of the epidermis while the deeper cells, composed of active living protoplasm, form the stratum of Malpighi.

The outer layer of ectoderm cells may be for a time ciliated. -This is well seen in young Amphibian embryos (Assheton, 1896). In Rana. tempommltv, the 6-mm. embryo possesses ciliated cells scattered thickly over its surface, the movement of the cilia being such as to drive a current of water tailwards over the surface of the embryo‘. When the external gills develop, a specially strong ciliary current sweeps backwards over them and it is noteworthy that this current passes over the olfactory organ on route to the external gills so that the olfactory organ possibly plays an important part in testing the quality of the water going to the respiratory organs. The ciliary -apparatus is sufliciently powerful at the stage in question to cause an embryo of this stage when laid on the bottom of a. flat glass vessel to slide along at the rate of a millimetre in from four to seven seconds. As development proceeds the ciliation becomes less and less prominent and in a 20-mm. tadpole it has almost disappeared except on the surface of the tail which remains richly ciliated until the time of metamorphosis. This persistence of the tail cilia is doubtless correlated with the fact that the skin of the tail plays an important part in the process of respiration.

HORNY DEVELOPMENTS OF THE EPIDERMIS

8ca.1es.——In many terrestrial Vertebrates the horny layer of the epidermis becomes so thickened and hardened as to become practically rigid. In such cases the flexibility of the skin as a whole is retained by the thickened areas of epidermis being separated from one another by lines along which thickening does not take place. The thickened portions now form epidermal scales of the type seen in Reptiles. They may take the form of simple rounded projecting bosses or tubercles as in Chameleons, or they may be flattened II p THE SKIN AN D ITS DEItl.VA,’l.‘l\’il*3S 7].

horny plates arranged edge to edge-——as in Uhelonians or as ml the ventral side of the body in Crocodiles or the dorsal surface of the head in Snakes and Lizards—or, finally, they may overlap like slates on a roof as is the case on the bodies of Lizards and Snakes. Occasionally, as in certain Lizards, individual scales may become greatly thickened and assume a conical spike-like form.

The individual scale arises in development (F ig.,4l) as a slight elevation of the surface beneath which the dermal connective tissue is somewhat concentrated. The epidermis covering the projection

L develops a well-marked cuticle. As development goes on the

epidermis increases much in thickness and the cells of the outer layers become entirely cornified so ‘as to form a horny plate or scale ——~supported by the underlying tough condensed portion of the dermis.

It will be borne in mind that su_ch typical reptilian scales have

to be sharply distingu'ished from the morphologically quite different

FIG. 41.-—Early stage in the development of the scale of a :s'|ml{v .-is seen In a lungitutlmal


section perpendicular‘ to the sm‘face of the skin.

scales developed in the dermis in fishes, The ordinary reptilian scales serve mainly to protect the body from mechanical violence and from desiccation.

Feathers.-——-In the homoiothermic Birds, where the body is kept at a constant temperature usually higher than that of the surround- ing atmosphere, the scales havebecome for the most ‘part replaced by fluffy feathers which W/ith the air entangled in their interstices form an admirable non-conducting envelope to retard the loss of heat by radiation, or convection, from the surface of the body.

The rudiment of the feather begins (Fig. 42, A) as a slight thickening of the epidermis resting upon somewhat condensed dermis. The rudiment in fact differs little from that of a normal scale. The rudiment comes to project backwards (B) and then in- creases in length (0), projecting freely tailwards while its now relatively narrow base of attachment becomes sunk below the general surface into a pit or follicle.

The rudiment now consists of a core of dermis surrounded by thick epidermis. The epidermis becomes incised along its axial surface by deep longitudinal grooves which divide its deeper portions into longitudinally arranged masses (Fig. 42, D, b), the rudimentary barbs, While leaving the superficial portion as a continuous sheath (sh.). The grooves in question do not reach to the base of the rudi- 72 EMl’»RYOl'.()GY' 011‘ THE LOWER VEl'-t'.l‘EBRA.'sly‘ES UH.

ment—~ the uuineised basal portion forrni.ng the quill of the feather. The horny sheath becomes strongly cornified and then lgreaks open and the 1ongitu(.linal thickenings of the epidermis, now also strongly coruilied, break away from the sparse cornified dermal tissue of the axis and form the fluffy barbs of the clown feather. _ _ In the basal quill portion of the feather the epidermis immedi-

1"n:. -12. |l_l11st.1-:1tin:_q the ah-.ve1opme.nt of feathers. (After Davies, 1889.)

A, B, ('3, lm1_«.fil.udiIml so-.(;l.i0ll.*~'-2 I), E. F, tr:ms\'t-rsr- .\-«-(-tions (I), E, «lown 1'e=.:1t.lu-1': 1!‘, flight feather)-

I U, lu1'1giI_4uli1n::l Sl'.(?l.l()lI llIl‘uIl_<_,"ll lmrh rudiment sl1o\\'in:.,-' «le\'u'-lopi1'1;,j' b:u‘lmles: H, lo11_r_-;itudin:1lsection 1..ln'ou;;'h l_ms«- of fa-ullu-r. b, hurh; bb, h:n'l)Hlo.': 4'. lmrny svln‘-:'I: c.r. l:n,\‘m' of I-ylin:_lI‘i¢.-:11 4-pitlwlium; _/‘, I‘:-utln-r rmlinu-nt : 3/_. ;.u-nniiml re-gion ; p, pulp: 4,, quill; I'_, r.-u.-hi~<: sh, slu-nil).

ately covering the outer end of the axial dermal tissue or pulp forms a thin strongly cornified superficial layer which separates off as a septum cutting across the cavity of the quill. This process being repeated periodically gives rise to a series of horny caps fitting one over the other, in the interior of the quill (Fig. 42, H, 0).

The flat feathers, found as contour featliers arranged in patches

over the general surface and as Remiges and Rectrices in the wings II THE SKIN AND ITS DERIVATIVES 73

and tail, originate from the basal portions of down feathers which undergo a great increase in length. The basal part of the rudiment in this case increases much in diameter. The epidermis here again becomes incised on its inner surface to form barb rudiments. These however are much more numerous (Fig. 42, F) than in the typical down feather and, further, instead of being arranged Strictly longi- tudinally they are arranged somewhat spirally, starting from a con- tinuous epidermal thickening which runs along the outer side of the feather rudiment. This thickening is the rudimentary rachis or shaft and the barb rudiments run from it spirally round the feather rudiment until their tips meet along its inner side.

The feather is thus in early stages curled into a cylindrical form round the central dermis or pulp-—the whole being enclosed in a continuous sheath which disintegrates sooner or later setting free the elastic barbs and allowing ‘them to flatten out to form the vexillum or vane.

As is well known the barbs are united together in the fully- developed feather into a functionally continuous web, through the agency of the barbules which project from the two sides of the barbs much as the barbs do from the rachis. The mode of origin of the barbules is seen in a longitudinal radial section through a barb such as that shown in Fig. 42, G, where the outer portion of the barb rudiment is seen to be splitting up into barbules (121)) while its inner portion remains continuous to form the definitive barb (I2).

Traced downwards, towards the base of the feather, the raehis increases in width so as to extend round the whole periphery of the feather rudiment. Its outer layer assumes a translucent character and forms the cylindrical gculll (calamus), the basal end of which becomes somewhat narrowed, bounding the umbilicus, the opening through which the dermal pulp extends up into the interior of the quill. The pulp of the feather undergoes a gradual shrinkage leaving behind it the series of cornified caps (H, 0) formed on its apical surface as already mentioned and which eventually lie loose within the quill.

The lips of the umbilicus are continued (Fig. 42, H) into a deep rim of uncornified epidermis (g). This with the dermal papilla pro- jecting into the feather base remains inactive until the period of moulting when it springs into activity, grows rapidly, and becomes converted into a new feather which pushes the old one out and takes its place.

The scales which frequently occur upon the legs and feet of birds are probably not, as might at first sight be supposed, to be looked upon as having persisted from the Reptilian condition. They fre- quently bear feathers in the young condition and are probably secondary developments replacing an earlier feathery covering. I

In view of the convincing evidence offered by comparative anatomy and palaeontology we are compelled to believe that Birds have been evolved out of Reptile-like ancestors. Accepting this 74 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

view and having regard further to the fact that Reptiles are typi- cally covered with a coating of scales, we may safely also accept the view that feathers are to be looked upon as highly specialized and modified scales. -

While the mode of development of the feather fully substantiates this hypothesis, perhaps the most interesting point that emerges

FIG. 43.———lllu_strating the neonychium or claw-pad in the developing Bird. (From Agar, 1909.)

A, median longitudinal section through the claw of a chick of 19 days‘ incubation. B, claw of a chick taken in the act of hatching. Thu nconychium is seen beginning to break away from the rest of the claw. C, sa-ction similar to A, but from a chick 12 hours :1fLcr hatching. 4.-.32, claw-plate; as, sole of claw ; n, claw-pad (nuonychiuni).

from its study is that the successive sets of feathers———the down feathers of the nestling, and the annual or other sets of feathers in the adult——are not to be looked on, as has been customary, as suc- cessive series of independent individual feathers. On the contrary the down. feather and the definitive feathers, which succeed it in the series of inoults, are all simply portions of a single greatly elongated and basally growing structure--——the first down feather being its tip, and the succeeding feathers being successive portions of it. The moult consists not in the shedding of the whole feather but merely in the breaking off of its projecting portion.

Claws, which make their first appearance in Anura (Xenopus), II THE SKIN AND ITS DERIVATIVES 75

arise as special developments of the horny layer ensheathing the tip of the digit. To produce and retain a sharp edge or point by differ- ential wear the claw is normally of denser consistency and harder on the dorsal side and laterally, forming the “ claw-plate ” (Boas) (Fig. 43, C, (3.19), while on the ventral side it forms the softer “sole” of the claw (Boas) (Fig. 4.3., C, c.s). A -

Neonychia or Claw-pads.—-To the embryo of an Amniotic Verte- brate, enclosed within its delicate membranes, the possession of sharp claws on the digits would obviously be a source of considerable danger during the later stages of development when the embryo moves its limbs, because of the liability of such structures to tear the foetal membranes. This danger is obviated by a beautiful adaptive arrangement which has been described by Agar (1909).

In the embryo, the concavity on the lower side of the claw is completely filled up by a soft rounded pad or cushion (Fig. 43, A, 71.) formed by a thickening of the horny layer of the epidermis superficial to the sole of the claw. Agar has given the name N eonychium to this structure. In addition to mammals, which do not concern us here, Agar has studied these claw-pads i_n the Fowl and in the Lizard Tejus

and there can be no doubt that the expanded

claw-tips observed by Rathke (1866), Voeltzkow ““.‘ T“"""'“i*‘='~'l" 1T°"“,”"” ' lunh of ill! eml)1'_y0 bro-

(1899) and Goeldi (1900) in Crocodilian (Fig.44) (,(,,m._. ,,1,,,m, W, ,,,.,,,t;,_., embryos are the same structures and it seems after twinm-itiuu, show-


probable that they will be foundto occur in ‘"11 the """"']““’ ‘“’°“>" -

. . . _'.l '1. Al't.1'V>-lt.'_.k w, claw-bearing Amniotic Vertebrates generally. ‘1§§,l5,_)( 9 W I 0

The neonychia are purely foetal structures which become detached soon after hatching (Fig. 43, B and C) leaving behind the functional claw.

Jaws and Oral Combs of Anuran larvae. -——Amongst the most interesting developments of the horny layer are the jaws and oral combs of frog tadpoles. The buccal opening is bounded by an upper and lower horny jaw, and external to and roughly parallel with these are rows of little horny denticles which form the oral combs and are used for fraying out the food. The number and arrange- ment of these rows of denticles-—“ upper labial” and “lower labial” ——-differs in different Anura and 1.11:,-.5’ afford useful characters for the identification of tadpoles (see Boulenger, 1897).

The horny jaw is composed simply of a row of denticles so closely apposed as to be in contact. The terminal fl11'1(_+li0nal_ portion of each dentiele is seen in longitudinal section (l<‘ig. 4:3, A and B) to be com- posed of a series of hollow cones of lmrd horny materi:1.l which closely ensheath ‘one another. The tenninal cone as it undergoes wear and tear eventually drops oil’, its function being taken over by the cone which it previously ensheathed. 76 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

These cones form simply the terminal members of a series vyhich extends inwards in the form of a curved column nearly to the mner surface of the epidermis. Only the terminal members. are strongly cornified, the other members showing less and less cormfication until at a little distance down the series the cone is seen to be composed of unmodified protoplasm containing at one side, near its base, 21.

Flu. 45.—-Jllnstrating; the development of the larval teeth of Tadpoles

A, H, C. l*‘ulmI'icolu j'usvum.aculum ; I), Ihmu tcm.;mr¢Lriu_.. (I) at'tv1-(‘mtzeit, 1889.)

nucleus. The cone is in fact simply a cornified cell. Traced towards the base of the column the cells are seen to be composed of more granular protoplasm and to have not yet developed a hollow, while the extreme base of the column is formed by an initial cell of com- paratively small size and flattened shape.

The whole column is seen, then, to be composed of a sequence of

conical teeth forming a replacement series, each tooth being a single cornified cell. II THE SKIN AND ITS DERIVATIVES 77

The jaw, composed of a closely set row of such columns, is sup- ported by the neighbouring parts of the epidermis which also under- go a certain amount of cornification. Thus just internal to the jaw is a cushion—like mass of large slightly cornitied cells which forms an efficient backing to it (cf. Fig. 45, A and B) while external to the jaw the surface of the epidermis is composed A ,3: of flattened much I cornified cells (Fig. T. _ _ " 45, A). ' . '

The oral combs consist of a pallisade— like arrangement of similar denticles which however in this case are not in contact. Fig. 45, C, shows a longitudinal section through the posterior labial comb of Palac- dvlcolot. Here again we see a succession- column of epidermal cells commencing with a small initial cell near the inner sur- face of the epidermis. From the initial cell outwards the cells increase in size,become gradually cornified and each one fits closely into the base of the next one which be- comes more and more deeply excavated as the tip is approached.

Two conspicuous differences distinguish the denticle of the A‘__u_“‘_1_.BWN_M_ I ]_ g oral O0]-nb from of , }l)€-l})lll£,l; is-, llL111utio1liil-siiiiuz2I.3i11:’);:31;);.’ll:;li::::m:'"MI the jaw: (1) instead I A I of being regularly conical in shape it is claw-shaped with serrated edges (Fig. 45, D) the tip being recurved, and (2) the hollow base of the cornified cell is not entirely occupied by its successor in the series: it also accommodates an indifferent cell of the epidermis (supporting cell of Gutzeit) which bulges into it.

I have described the development of these interesting structures

Fm. 46.——Vertica1 section through lingual spine of Pet1'o7nJ/sort. (After \-‘Varrcn, 1902.) 78

EMBRYOLOG-Y OF THE L()WiER VERTEBRATES

Cll.

as they occur in a. South American tadpole (.Pal'u.d'icola.)‘ but the description [its quite well the mode of development as it occurs in

_|<‘I<:. ccuu-mt — organ sagittul M.-«_'.t.ir'n1|~.

47.--——|ll11sll':'ltlng the «lu.vulopIm_-..nl.. of the

of Ln/"M/_o.s'ira'n as S0011 in

A, 8t&lg'I' 132-}; ll. .‘-l.'l_'_;¢‘ ‘_'."i; (,'. .~l.'1_:.'~‘ til : I). Sim.-;v-. 35. In Atlle rualim:-nl. ml‘ the 4-1-lllvnl -4-I-;_;:iI1 is .\‘I'o'H tn) lw :i tliickcning or llw «lo-up I.-nya-r of tho 0‘(5l(_)Il|‘l'lll: in I; anal (3 th«- !~‘.|l|H'l'lll‘l.'|l l.'l_\'t‘l‘ has IllS:l1)]M':lI‘I‘(l m‘--r tho- t.lii«-lu-nu-«I _-.-,1.-uulului-:u.-:u ; in I) t-lu- m-;.-,:m is <:ommvn-

ring 1o.-In-iv:-l and i-1'o\\-«ls of pli:igo(-._\'l«.-s :m- 1-ollI'ct.-ml in its nu.-igliliuiurlmod.

Tadpoles generally (Keiffer, G utzeit), the differences be tween different species and genera, though of systematic import- ance, being differences in detail. such as sl1ape and arrangement of the individual teeth of the comb. ' “Teeth” of Cyclostomes.-- The horny teeth of cyclosto- niatous fishes, though they would naturally fall to be treated in the next chapter, situated as they are within the buccal cavity, may con- veniently be considered n0W owing to their resemblance—- on a much larger scale and with multiccllular structure—-—to the horny denticles of the tadpole. The tooth-like spines of the eyclostome are cones of highly cornified epidermal cells. Each tooth develops in the substance of the epidermis (Fig. 46, A) being strikingly like a hair- rudiment during early stages. Successional spines develop be- neath the bases of the func- tional ones as shown in Fio‘. 46. GLANDULAR DEVELOPMENTS 01? THE ErInEuM1s.—In the Anamnia it is usually the case that scattered cells of the epi- dermis take on a glandular function and serve to form a slimy secretion which amongst other functions serves to dimin- ish the “ skin-friction ” which is the main resistance to movement through Water. Such unicellular glands may become collected

together to form multicellular glands. Of these the most conspicuous examples are found, outside the Mammalia, in the Lung-fishes and Amphibians--where they form the flask-glands and the cement—organs.

1 Probably P. fusconmculata according to Boulenger. ll ’.l‘l--H4} SK'll_\T AND ITS DERIVATIVES 79

‘The flask-glands of Lung-fishes and Amphibians develop in the ilirst instance as solid local proliferations of the deep layers of the epidermis which grow down into the subjaeent connective tissue of the dermis and form a lumen by secondary excavation. The fully developed flask-gland is ensheathed in a coat of smooth muscle-fibres and it is an interesting fact that these are believed to be developed from the cctodermal cells of the gland—rudiment.

Cement-organs of apparently ectodermal origin occur in two out of the three surviving lung-fishes-——P7'0toptea‘-as and Lep'£¢7los1Ir(m—- and form conspicuous features during late embryonic and larval life (see Fig. 200, F, Chap. VII.). ' -

In an embryo of Lepz'dos7S~re'n three days before hatching the cement-organ forms a crescentic structure stretching across the mid-

FIG. 48.——Embryos and larvae of Bufo ':m,_7_qu..n'.s tn sllmv the cement-organ upon the ventral surface. (After 'l‘hiele, 1887.)

ventral line with its concavity forwards, just behind the position in which the mouth will appear later. About stages 32-34 the organ is at its maximum development and forms a large prominently pro- jecting structure ventrally below the opercular region. Towards the end of larval life the cement-organ commences to atrophy, the process being furthered by its invasion by crowds of phagocytes, and in a short time the organ has completely disappeared.

The cement-organ is a derivative of the deep layer of the epi- dermis. It commences as a slight thickening of this layer (Fig? 47, A) the cells assuming a tall columnar form. These columnar cells become the secretory cells while the superficial layer of the ectoderm breaks down over them so as to expose their outer ends (Fig. 47, B and C). It is to be noted that there is no trace whatever of any connexion of this cement-organ with the endoderm: it is ontogenetically a purely ectodermal structure (see, however, p. 181).

Cement-organs of very similar appearance are found in the larvae 80 EMBRYOLOGY OF THE'LOWER VERTEBRATES CH.

of many Anura. In this case they appear very precociously in development, being indeed in some cases the first definite organs to become visible on the surface of the embryo. Fig. 48 illustrates the development of the cement-organ in the common toad (Bufo vulgaris) from the time of its first appearance up to the time of its atrophy. The organ is seen to be at first crescentic as in Lepidosdren then to become V-shaped and finally to become paired by the atrophy of its median portion.

When at the height of its development, the cement-organ shows characteristic differences in form and position in diiferent species of Anura and is consequently of use in identifying the species. of Tadpoles.

The general appearance of the Anuran cement—organ as observed in sections is illustrated by Fig. 49. The glandular layer is com- monly said to belong to the superficial layer of the ectoderm but this does not seem by any means certain.

Pigment of the

Skin.-——One of the con-

|<‘:«:;‘..1u. '.‘lul'l.l()ll l.ln'ougln .tl1egccn1e1l’t-organ of a l*‘rog spicuous features ofthe hulpnla (Rum! /4-.-m-,mu-u.m1.) 8 mm. 111 length. (From Skin in the majority of

ASSMOII’ 1896') _ I , _ _ _ Vertebrates is the fact (.'.‘0, cc-nit-int-t):::::t:Lt}:n‘Lfgt’ .h:.1.}.:r-r’l‘l(‘-‘1(:‘n£nl::‘yI:-lg:al vctmlu-1-m, is coloured

the deposition within

it of excretory matter in the form of pigments. This is of physio- logical importance to the organism in two different ways, firstly in that it gives to the particular species its characteristic coloration, and secondly it serves to protect the underlying living tissues from

.. the harmful influence of light rays...

A certain amount of pigment may be formed within the proto- plasm of the ectoderm cells. For example in frog tadpoles of about an inch in length, numerous fine granules of melanin are crowded together near the surface of the outer layer of ectoderm cells, just beneath the cuticular superficial layer.

But by far the most important part of the pigmentary system of Vertebrates consists of mesenchyme cells with pigment-laden cyto- plasm which are positively heliotropic during life and creeping by the extrusion of slender pseudopodia, like those of Foraminifera, crowd together immediately beneath the ectoderm and form there a l.ight-proof layer, some of them even wandering into the substance of the ectoderm between i.ts constituent cells}

The chromatophores, during the process of development, commonly become specialized in different directions so that in the fully developed

1 The interpretation of the branched clirmnatophores as mesenchymatous in origin appears to the author to accord best with observation but it should be mentioned that some regard them as modified ectoderm cells, as_,for instance Winklcr (1910). II THE SK_IN AND ITS DERIVATIVES 81

condition several distinct types may be recognized. Thus in Lepido- siren the most abundant type of chromatophore is characterized by the stout projections of the cell-body which carry the finer pseudo- podia and by the somewhat brownish pigment granules. A less abundant type has long slender, less richly branched and often vari- cose pseudopodia with dense black and opaque pigment granules. Still another type of chromatophorc has its protoplasm charged with bright yellow pigment.

The melanin pigments are probably to be looked upon as waste products of cell metabolism. They are iron-containing pigments and during at least the later periods of development their produr tion is commonly associated with the breaking down of that other great iron-containing pigment——haeinoglobin.

Their production is also related to the degree of activity of the cell metabolism. Thus, in the male Lepidosiren, at the close of the breeding season, when the moribund remains of the richly vascular respiratory projections of the hind limb are being devoured by crowds of voracious phagocytes, there takes place an active formation of melanin.

Again melanin apparently tends particularly to be produced when the cell metabolism of comparatively unspecialized cells is interfered with by the prolonged action of light-rays. Thus as already indicated the layer of protoplasin in the egg which is turned towards the light frequently develops melanin granules. Again in developing eyes it commonly holds that comparatively unspecialized mesenchyme cells wandering into the zone of exposure to the light deposit melanin granules in their cytoplasm.

Cells then which become chromatophorcs may be regarded as cells which are specially sensitive to light-stimulus and whose meta- bolism is liable to be so modified thereby as to produce pigment.

Although it is reasonable to suppose that melanin-formation is primarily related to the influence of light it must not be forgotten that, as indicated in the preceding chapter, the actual laying down of pigment in the case of species where it has become a specific character may take place under circumstances in which the light- stimulus is incomparably more feeble than that which probably originally brought about pigmentation in the course of phylogenetic evolution, as e.g. in the case of the ovarian egg of the frog. That pigment -formation during individual development still remains linked up with exposure to light is shown by the frequency of the unpigmented condition in Anuran larvae which develop in water rendered opaque by fine clay held in suspension (Wenig, 1913).

To illustrate the dependence of pigment-formation upon light during individual development the case of young flatfish (Pleuron- ectidae) is sometimes quoted, where the shading of the upper and the illumination of the lower surface during development brings about a reversal of the ordinary colouring (Cunningham, 1893). It is possible however that this reversal of colouring is due merely to the strongly

VOL. II G 82 EMBRYOLOGY or THE LOWER VERTEBRATES on.

heliotropic tendencies of the chromatophores which lead them to migrate actively towards the illuminated side and there to remain. The chromatophores of Vertebrates often display their sensitive- ness to light very markedly by movement reactions. Such are well seen in the young stages of many fishes and Amphibians. In the young Lepidosiren for example the chromatophores duri.ng the day have their pseudopodia extended in all directions and their bodies flattened out into a plate-like form so that they constitute a light- prnof coat giving a rich

purplish-black effect. At

dusk the pseudopodia become slowly withdrawn so that a few hours after darkness has set in the chromatophores have shrunk into minute spheres so wide apart as to have no influence on the general colouring. The young fish is then practically colourless except for the large yellow chrornatophores here and there which remain expanded. FIG. 50.---Section through ('.pl(l(‘.l‘llllS of Lepvldoszlreln Dunng the course of 1;”-V.-u.._ development in many fishes, A. lixml nmlvr comlilions of «lull tlayliglnt; B, under ainurous Amphlblans’ and a’ crmtliliunsul'<l;n'l<m-sstlIn'in;.:l.l1enig'lIt. ('l‘lIv chroma- few RBIJUICS Such as the

tophores in the dermis, nrhich crowd together under the chalneleon’ the compara-

inner surface of the epidermis are not shown in A.) - - . ,- , 7», cll1'()1ll:ll'.I)])h0I'(‘-', av, l_)loo(l-vessel. tlvely Sllnple reactions to

light such as have just been indicated develop into reactions of a much more complex type in which the central nervous system is involved. Research into the development of these more complex reactions is highly desirable for at present little is known regarding them. »

NERVOUS SYSTEM

The nervous system, which has to do with the receiving of, and the reacting towards, impressions from the outer world, appears to have arisen in evolution, as might have been expected, from the outer layer or ectoderm. The first steps in the evolution of the Vertebrate nervous system are not within the scope of direct obser- vation but the view is probably correct that it arose from a diffuse subepidermal network or plexus of the type still persisting in some of the more primitive invertebrates. 111 the development of the Vertebrate embryo the main parts of the nervous system may stillphe seen to take their origin from the ectoderm. II NERVOUS SYSTEM 83

ORIGIN or THE CENTRAL NERVOUS SYsTEM.——The first obvious trace of the central nervous system consists of a thickening of the ectoderm of the dorsal surface of the embryo. This thickening extends forwards from the anus, or from a point slightly behind the anus, to the head region, and is termed the medullary plate. The thickening of the medullary plate is due primarily to the ectoderm cells, or where two layers are present, to the deep-layer cells of the ectoderm taking on a tall columnar form.

There is also growth in area of the mcdullary plate and this, in conjunction with the binding down of the medullary plate along the line of the notochord and primitive streak, causes it to become curved from side to side so as to form a gutter or groove~—the medullary groove. The, usually conspicuous, lips of this groove are known as the medullary folds.

As the medullary plate keeps on increasing in width it bulges downwards and laterally into the surrounding mesenchyme and assumes the form of a longitudinally placed tube with a slit along its dorsal wall representing the original opening of the groove.

Finally the lips of this slit grow towards one another and under- go fusion, so as completely to close in the neural tube, which now separates off from the ectoderm of the outer surface. The closing in of the neural tube commonly commences in the binder head region and spreads from this point forwards and backwards.

An interesting modification of this normal mode of origin of the neural tube is found in the case of Lampreys, Lepidosiren and many of the tclcostomatous fishes. In this modification of the typical process the increase in bulk of the medullary plate leads to its growing downwards into the underlying tissue as a solid keel. In the middle of this at first solid rudiment a cavity appears secondarily either by the development of a fine intercellular split or by the cells along the axis breaking down. The cavity so formed gradually dilates and eventually there is a neural tube agreeing with that of normal forms.

The neural tube which has originated in the way described is the rudiment of the central nervous system, its anterior portion becoming relatively enlarged to form the brain while the remainder forms the spinal cord. .

The central nervous system during the period of its development gradually attains to a condition of the greatest complexity and all that will be attempted here is to give an outline sketch of the more conspicuous changes which take place in its general form and in the arrangement of its parts without going into minute detail.

SPINAL CORD.—'l‘he spinal cord remains throughout life in the form of a tube the lumen of the tube becoming relatively insignifi- cant while the walls become greatly thickened especially laterally. The relatively small size of the lumen (central canal) is not, as a rule, due merely to its retaining its embryonic dimensions while the Walls of the tube are growing in thickness. On the contrary actual 84 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

occlusion of part of the lumen takes place in the great majority of the lower Vertebrates. The side Walls of the tube approach one another so as to convert the rounded lumen into a vertical slit and finally they come into contact and fuse so as completely to obliterate the cavity except in its ventral portion which remains open as the definitive central canal.

In the ease of the Bird-—in which the process has been worked out in detail (see ltamcin y Uajal, 1909) the. increase in thickness of the wall of the neural tube is due primarily to the cells composing it taking on a tall columnar form-—-the individual cell extending right from the central canal to the outer surface. The cell-body becomes very attenuated, with a marked dilatation containing the nucleus. The nuclei become necessarily situated at difl'erent levels and this in an ordinary transverse section obscures the fact that the wall is still composed only of a single layer of cells.

With subsequent development the cells become dill'erentiated into those which are actually nervous and those which remain relatively indifferent and fulfil a mainly supporting function. The latter con- tinue for a considerable period to traverse the whole thickness of the wall. They increase greatly in length: their form becomes more and more attenuated the greater part of their length becoming prac- tically filamentous with small irregular projections and varicosities, while_the portion nearer the central canal, in the course of which the nucleus is embedded, remains somewhat stouter.

The presence of such supporting cells traversing the Whole thick- ness of the wall is only temporary: in later stages they are replaced by the greatly branched neuroglia cells. While many authors have taken the view that these latter are to be regarded as immigrant mesenchyme cells——-a View that has weighty general considerations in its favour——-Ramon y Cajal and others have adduced strong evi- dence to show that they are simply the original supporting cells which have withdrawn, or lost, their internal and external portions and assumed a complicated branched form.

In addition to the comparatively indifferent supporting cells which have just been mentioned there are present in the wall of the neural tube the numerous elements which are destined to become actual neurones or nerve-cells. Such embryonic nerve-cells have been termed by His neuroblasts in contradistinction to the non- nervous elements or spongioblasts.

At first isodiametrie these cells like their neighbours take on a tall columnar shape stretching throughout the thickness of the wall: their terminal portions become more and more attenuated and they present a spindle-like (bipolar) appearance. Later their shape becomes pearlike the stalk being prolonged into a nerve fibre (neurite, axon) while finally the development of branched projec- tions (dendrites) brings about the definitive multipolar condition.

These developing neurones lie ‘in the spaces between the in- different cells and from an early stage (third day in the case of the II NERVOUS SYSTEM 85

fowl embryo) the use of appropriate methods reveals the presence of neurofibrils in their protoplasm. The tail-like prolongation of the neurone which forms the neurite or axon is still believed by the great majority of workers to arise as an actual outgrowth of the cell body as was taught by His. Others regard the appearances upon which this belief is based as being probably deceptive, as will be explained later.

The longitudinal axons of the spinal cord are concentrated in its outer layers forming the “ white substance ” of the early anatomists. This makes its appearance as a rule in the more primitive Vertebrates as a continuous layer and in the higher forms as a sharply separated dorsal and ventral portion upon each side.

The enclosure of the axons in the insulating medullary sheaths commences only within a few days of the end of incubation, in the case of the bird, and at similarly advanced stages of development in other Vertebrates. The sheath is generally believed to be secreted by the protoplasm of the axon. Its formation tends to take place approximately synchronously in all the axons belonging to a particular group. This fact, in conjunction with the use of specific stains for the insulating substance, facilitates the mapping out of the various groups of axons.

The spinal cord, like the rest of the central nervous system, becomes invaded during ontogeny by immigrant mesenchymatous tissue. This provides the capillary network which traverses the nervous tissue, in addition doubtless to many other elements of a less conspicuous kind.

A curious detail which is noticed in studying sections of develop- ing spinal cord (or brain) is that the active cell-multiplication is confined to the layer next the central cavity, in other words to what was originally the superficial region of the ectoderm. This is in striking contrast with the general cetoderm of the surface of the body where cell-multiplication is confined to the deep (Malpighian) layer.

BRAIN.—The anterior portion of the neural tube becomes en- larged and dilated to form the brain and this gradually becomes so modelled as to present the various regions seen in the brain of the adult. The general course of this process will first be sketched as it occurs in Lepidosrren one of the lower gnathostomatous Vertebrates in which the egg is holoblastic.

DIFFERENTIATION or THE MAIN REGIONS or THE BRAIN IN LEPIDosIREN.—-The brain rudiment becomes apparent as a slight enlargement of the neural tube. The first sign of differentiation is

- the appearance of a constriction marking off the primitive fore-brain

or cerebrum (archencephalon of Kupffer) from the primitive hind- brain or rhombencephalon. As development goes on this boundary becomes specially marked ventrally where the floor of the brain bulges upwards into the cavity as a transverse fold ‘ (see Figs. 52 and 53, f).

‘ This fold may in other vertebrates make its appearance before the medullary groove is covered in. This is shown clearly in Polypterus-—Fig. 80, B, p. 146. 86 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The side wall of the hind-brain now comes to project outwards as a prominent knob while the side wall of the fore-brain also bulges out—

Fm. 51.——Brain of young IA‘-}ll(l()h'll'(‘ll as seen from the (lm"S{1l side.

A, .s't:i;.:t'* 3l+: B. :-:t.:|_«.,»'u- :55--— ; ('.'-, .~'i:|;,-‘u 38; I"), zulult. c.II, cerebral lu=.mi.~;plw1-c; n.u, lll'Hl':Il :n-(-h; _p£n, pin:-:|l lnNl,\-': ,-h., rlmmIn-In-4-pllillnn; Hml, Llmlaniencephalon; t.o, tectum optic-um. I, II, «-I«-., mmuiul nu-rws. [l<‘i«,gs. A and 13 um nun-u hi;_-'hl_\' umusnified than C, and Fig. C than D. "I

wards—the bulging in this case. he_ing- the rudiment. of the cerebral hemisphere (Fig. 51, A, c.I1).

The portion of the primitive fore~brain lying just in front of the transverse fold of the brain-floor is— the infundibulum. Farther forwards the inner surface of the brain-floor forms a transverse into tmlamencevhalic and mesencevhalic Orbions


II I BRA IN -. _ 87

groove hounded behind and in front by a slightly projecting ridge-~ the rudiments of the optic chiasina. and of the anterior commissure respectively (Fig. 53, ch, a.c).

About stage 31 a little pocket-like diverticulum of the roof of the primitive fore-brain makes its appearance (Fin. 53, D, pin). This is the pineal body and -its appearance is of topographical importance as serving to demarcate the primitive fore-brain roof


FIG. 5'3.-—~Brai11 of young Lepidosiren as seen from the left side.

A, stage 20.; B, .s'l..:|_-.40. 31 ; (3, sl.:tge 35- ; D, stage 39. c.H, cerebral lu!lui.s'plm1'n; f, primitive fold of lu'uin-llnnr; -in_/; in1'muIil_mlmn; o.h_, ulf.-u-tury bulb; o.t, olfactory tubercle; pin, pineal body; (,I¢_.u(._ |.huhum-I|c«-pl1:1l(m; I.u, tact.-"um upI:'v'21.:n.

The lateral bulgings of the fore-brain have become more pro- minent and now project forwards beyond the limit of the rest of the fore-brain. In the mesial plane between the two hemispheres there projects upwards and forwards a little pocket of the anterior wall of the fore-brain. This is the rudiment of an organ of unknown significance———the paraphysis (Fig. 53, D, par). .

Soon after the appearance of the pineal body the roof of the primitive fore-brain becomes divided into a posterior portion belong- ing to the mesencephalon and an anterior portion belonging to the thalamencephalon (Fig. 51, B, 5.0, and thal). As development goes 88 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

” ' aw-'5;-:~' /«.v~;«=.w- //9/Ia;/'/7/W3/%,I%/fw

‘FIG. 53.——Camera tracings of sagittal sections through the brain of Lepidosircn, at successive periods of development.

Figs. A to F urn drawn nndvr tho szuno magnification, Fig. 0 under :1 lower magnification. A, stage 25; B, stage 28; 0, stage 29+; D, stage 31; E, stage 35; F, stage 88; G, adult in second year. a.c, anterior cominiszeuro; oer, cerebellum ; c.H, cerebral hemisphere; ch, optic chiasma; f, primitive fold of brain-floor; h.g, ganglion liabenulao; h.c, lmbe-nular (superior) commissuro; Lg, infnndibular gland ; inf, infundibulum ; par, purapliysis ; pin, pineal body; p.c, postorior commissuro ; 1.1», choroid plexus of lateral ventricle: (.0, teolum opticum ; * originally anterior mug of brain-floor. Structures occurring not in the sagittal plmw but in sections parallel to but some distanco from it, arv shaded with oblique lines. II N ERVO US SYSTEM 89

on the constriction between thalamencephalon and mesencephalon becomes more marked. The roof of the former remains thin and membranous, forming the cushion-like dorsal sac upon which the pineal body rests. 'l‘he .roof of the mesencephalon becomes slightly thickened on each side of the mesial plane forming the tectum opticum but correlated with the small size of the eyes in Lepidosiren the thickening never becomes so great as to produce projecting optic lobes such as are formed in most Vertebrates.

In the hind-brain region the greater part of the roof, covering in the fourth ventricle, becomes thin and membranous. Across the anterior boundary of the hind-brain the roof does not undergo this secondary process of thinning but persists as a transverse thickened band—-the rudiment of the cerebellum.

SUBSEQUENT DEVELOPMENT OF TIIE BRAIN REGIONS

RHOMBENCEPHALON or Hind-Brain.——The hind-brain, correlated perhaps with the fact that it contains nerve—centres of supreme importance to life, develops precociously and reaches a relatively enormous size during early stages (Fig. 51, A, rh). The bulging inwards which marks its anterior limit is doubtless to be regarded as an expression of the active growth in length of its floor during these early stages.

During later stages of development it forms a conspicuous pro- jecting restiform body on each side reaching forwards nearly to the anterior limit of the inesencephalon but this becomes again less and less prominent as the adult condition is approached. The cerebellum retains through life its primitive condition as a simple transverse thickening of the hind-brain roof.

lV.lESENCEPHALON.-——Tl1e roof, as already indicated, becomes thick- ened somewhat on each side (tectum opticum) but not to such an extent as to bulge outwards and form optic lobes. Close to its anterior limit a conspicuous bridge of transversely-running nerve- fibres makes its appearance at a late stage of development. This is the posterior commissure—-an important brain landmark (Fig. 53, Gr, 19.0).

THALAMENCEPHALON.———Tl1e side wall of the thalamencephalon becomes greatly thickened to form the optic thalamus which bounds on each side the slit-like third ventricle. The roof becomes for the most part thin and membranous forming the dorsal sac. On either side of the pineal body however it becomes greatly thickened to form the habenular ganglion. As these ganglia develop a bridge of transverse nerve-fibres makes its appearance uniting them-—the superior or, better, habenular commissure.

The pineal body as development goes on enlarges somewhat and assumes a carrot shape. Its lumen becomes obliterated posteriorly so that it no longer opens into the third ventricle. The anterior isolated part of the cavity becomes eventually almost filled with 90 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

granular material produced by the breaking down of the epithelial linin .

Tghe paraphysis forms for a time a conspicuous tube passing upwards and forwards in the space between the two hemispheres and ending blindly. In later stages of development it undergoes a relative reduction in size, and becomes irregularly twisted and mixed up with the choroid plexus of the ventricles.

On either side of the paraphysis and just dorsal and posterior to its base, the wall of the brain becomes involuted into the third ventricle, the iuvoluted portion being thin and membranous and enclosing an ingrowth of blood-vessels. This vascular ingrowth represents a structure which in most Vertebrates is continuous across the mesial plane with its fellow so as to form an unpaired structure the velum transversum. This is regarded by most writers on the brain as an important landmark i11 brain topography.

On the floor of the thalamencephalon the optic chiasma and the anterior connnissure form prominent bulgings into the ventricle. Each develops nerve-fibres in its substance, connected in the one case with the organs of vision and in the other with the cerebral hemi- spheres, especially those portions devoted to the sense of smell.

In front of the optic chiasma lies a deep optic recess which is prolonged outwards by an outgrowth of the side wall of the brain, the optic outgrowth, which gives rise to a great part of the eye and will be gbscribed later. Behind the chiasma is the infundibulum, the tip of which at a late stage in development (about stage 38) sprouts out into narrow tubular diverticula. These increase in length, wind hither and thither, and partially penetrate into the substance of the pituitary body which lies immediately beneath. The epithelium of these tubular diverticula assumes a glandular appearance and together they constitute the “ infundibular gland ”-— often called the “nervous portion of the pituitary body.”

The series of sagittal sections in Fig. 53 is of interest from its bearing upon a question which has excited some discussion, namely as to what point in the fully developed brain of the vertebrate corresponds to the morphologically anterior end of the brain rudi- ment in earlier stages of development. It has been held by many morphologists, such as V011 Baer, His, Sedgwick, that the tip of the infundibulum represents the anterior end of the primitive brain, the present condition having been brought about by the anterior portion of the brain becoming bent upon itself into a retort shape. As will be seen by an inspection of the figures the brain of Lepidosiren lends no support to this idea. On the contrary the tip of the infundibulum clearly corresponds to a point close to the letter A of Fig. 53, A. On the other hand, equally clearly the anterior tip (*) of the brain- floor of an early stage such as that shown in Fig. 53, B is represented in the adult by a point well up on the anterior wall of the thalamen- cephalon (lamina terminalis) and just ventral to the root of the paraphysis. - II HEMISPHERES 91

CEREBRAL HEMIsPHEREs.— The hemispheres arise as bulgings of the side wall of the fore-brain. As development goes on they increase in size and grow first dorsalwards and later on forwards until in the adult they are relatively very large. This increase in size is associ- ated with a corresponding growth in the thickness of the wall of the hemisphere——except at its hinder end next the thalamencephalon. Here the inner wall of the hemisphere facing the thalamencephalon remains relatively thin.

About stage 35 a small rounded portion of this thin part of the hemisphere wall bulges into its cavity—the lateral ventricle. This ingrowth contains a vascular loop and is the rudiment of the choroid plexus of the hemisphere or lateral plexus. The plexus grows rapidly into the ventricular cavity, forming an irregular crumpled lamina which in the adult attains to great size and complexity traversing thelwhole lateral ventricle (Fig. 53, F and G, 1.)»). No doubt this, by diffusion between the blood in its vessels and the fluid in the lateral ventricle, helps to provide for the nutritive and respiratory needs of the hemisphere wall.

During the later development of the hemisphere its walls become differentiated into regions in the manner described by Elliot Smith (1908). Most important from the point of view of general vertebrate morphology is the fact that a distinct cortex is developed in the form of a layer of ganglion-cells traversing the roof of the hemisphere parallel to its surface, and at about one-third of the distance from the surface to the ventricular cavity. This cortex extends on the one hand just on to the mesial face of the hemisphere and on the other to a point rather more than one-third of the distance from dorsal to ventral edge on the outer face of the hemisphere.

Of this cortical formation, which constitutes the archipallium, the mesial portion corresponds to the hippocampus of higher verte- brates, and the outer portion to the pyriform lobe. The neo- pallium which in the higher forms becomes interposed between these does not appear yet to have become distinctly recognizable in Lepidosiren.

Less important from the point of view of general morphology but more conspicuous in their structural expression are certain changes which take place in relation to the olfactory apparatus.

The portion of hemisphere wall to which the first cranial nerve is attached——the olfactory bulb—is at first simply part of the lateral wall of the hemisphere but as development proceeds it is found to take the form of a sort of cap lying on the dorsal side or roof of the hemisphere at about the middle of its length as viewed from above. This change in position is brought about by an enormous hyper- trophy of the portion of the ventral wall of the hemisphere which lies in front of the optic ehiasma—the olfactory tubercle.

Later on, from stage 38, the portion of hemisphere roof lying posterior to the olfactory bulb undergoes active growth in length with the result that the bulb is gradually carried forwards and 92 EMBRYOLOGY OF THE LOWER V ERTEBRATES CH.

eventually comes to lie right at the anterior end of the hemisphere (Fig. 52, I), 0.7)). At the same time the bulb comes to form a definite hollow projection of the brain surface immediately dorsal to the still greatly enlarged olfactory tubercle (0.6).

l)1l+‘l+‘EREN'1‘IA'1‘ION or THE BRAIN REGIONS IN ACANTHIAS. The development of the brain of Elasmobranchs has been worked out by Kupii'er (1906) for Acu/at/1/ias and his account has been made use of ill writing the following short smnmary.

Figures of the early stages of the medullary plate as seen in surface view are given in Chap. XI. The medullary plate projects forwards from the posterior boundary of the blastoderm and is raised


well above the general surface. As it increases in length its lateral-

edges become raised up so that the portion on each side slopes inwards and dowmvards into a kind of valley. Each hall‘ of the mcdnllary plate extends back into one of the “caudal lobes" which with growth in length come to project freely beyond the edge of the blastoderm.

Another result of the increase in length is that the anterior end of the medullary plate comes to project freely forwards over the blastoderm forming a head-fold. Each side of the mednllary plate arches inwards towards the mesial plane and the whole becomes converted into a neural tube in a perfectly normal fashion.

As in the case of Le1)'i(l0si'I'cn, the first sign of dif'f'erentiation of the brain into its parts is a division into primitive fore-brain (Arch- encephalfii) and hind-brain. The demarcation is again most distinct ventrally where the brain-floor bulges into the ventricle (Fig. 54, B) as a prominent fold. Later on this fold spreads upwards on each side to the dorsal surface forming the rhombo-mesencephalic fissure which marks oil’ the mid-brain from the hind-brain. It is only at a later stage in development that the mesencephalon becomes marked off by a constriction from the anterior portion of the archencephalon which forms the thalamencephalon.

I t is of interest to compare sagittal sections through the brain of the Elasmobranch with the corresponding sections already described for the holoblastic lung-fish. N egleeting small differences in detail there is seen to be a striking difference between the two brains-most marked in the middle stages figured—in relation to the longitudinal axis. In Fig. 54, C the Elasmobranch brain is seen to be as a whole strongly curved in a ventral direction: it shows a high degree of “cerebral fle.1u11'e.” The corresponding stage of the Dipnoan brain is on the other hand almost straight, the superficial appearance of curvature being due mainly to the prominent fold of its floor which projects up into the. cavity at the level of the mid-brain.

This cerel_»ral fiexure, which is especially conspicuous not only in the brain of the Elasmobranch but also in the other types of Brain (Mammalian and Avian) that were. first investigated development- ally, has played a large part in discussions on brain morphology. Thus the idea, already alluded to, that the tip of the infundibulum is the morphologically anterior end of the brain rests upon it. II BRAIN 93

That this idea is unsound. seems clearly to be indicated by such series of sections as that shown in Fig. 53. As a matter of fact it

FIG. 54.———Sagittal sections through the brain of A mnthms. (After Kupffer.)

A, 3-8 mm. embryo; B, 7-8 mm. ; C, 10 mm.; 1'), ‘.27 mm. ; E, 70 mm. cm-, cerelwllmn; ch, optic ehiasma; eat, external (‘.Ct()d(‘.I'1Yl; h.c, habenular commissnre; inf, infnndihulmn; M, cavity of mesen- cephalon; N, notochord; 'n..p, anterior neuropore; p.c, posterior COIIlII1iSSlll‘(‘.; pin, pineal organi; Rh, cavity of rhombencephulon ; t.o, tecmm opticmn; v, velum tmnsvversum. ; v.IV, fourth \'t‘llt.I‘i(:l¢’*.

seems probable that Very pronounced flexure of‘ the brain, such as is seen in the developing Elasmobranch, is to be regarded as a secondary 94 EMBRYOLOGY OF THE LOWER VERTEBRATES cu.

result of the heavily yolked condition of the egg. As a result of the concentration of yolk towards the ventral side in such heavily yolked Vertebrates the processes of growth are retarded upon that side. But it is clear that retardation of growth in length on the ventral side as compared with the dorsal would bring about a flexure towards the ventral side. That the cerebral flexure is due rather to such a general cause than to any inherent peculiarity in the brain itself is supported by the fact that the notochord is also strongly flexed (see Fig. 54, 0).

As a consequence of these considerations, we are inclined to take the view that the phenomenon of cerebral flexure is of much less fundamental morphological significance than is commonly supposed.

Comparing the later stages figured for Acantluias with those of Lepidoszrcn, it will be seen that the brain shows the same elements although these differ in their relative size and other features in the two cases. Thus the cerebellum of the sharks——correlated with the active and complex movements of these fishes——becon1es much more developed. It grows greatly in anteroposterior extent and that causes it to bulge outwards as shown in Fig. 54, E (oer).

The pineal body is slender and elongated in form: the velum forms a conspicuous infolding of the thalamencephalic roof continuous across the mesial plane.

The wall of the anterior portion of the primitive fore-brain under- goes a fairly uniform increase in thickness throughout with the excep- tion of a transverse band just in front of the velum which becomes thin and membranous. This portion of the brain increases somewhat in transverse diameter so that it is broad in shape as seen from above, but there is no definite bulging in its side wall to form a distinct hemisphere. The material that would ordinarily go to form the hemispheres remains here in the thickness of the wall.

The olfactory bulb arises as a slight projection from the side wall of the fore-brain, but as development proceeds and the olfactory organ becomes removed from the brain by the interposition of ni_esenchyme the olfactory bulb remains in contact with the olfactory "organ, its attachment to the brain becoming drawn out into a more or less elongated stalk the olfactory peduncle or olfactory tract.

The salient features in the establishment of the topography of the Vertebrate brain have been illustrated in outline in the sketch which has just been given. It would be beyond the scope of this work to make any attempt to fill in the picture in detail but it is necessary to recall a few points which are of interest to morphologists apart from specialists in neurology.

It should in the first place be borne clearly in mind that the brain——like indeed the whole of the nervous system (see below, p. 118) is to be looked upon as a fundamentally continuous structure. The parts which compose the adult brain—-—medulla, cerebellum, mesen- cephalon and so on—are not to be regarded as constituent units which go to build up the complete brain, but rather as specialized portions of a once homogeneous whole. The process of specialization 11 . . BRAIN 95

has probably been linked up more particularly with the processes of localization or centralization of particular functions in particular brain regions. When this has come about, increase in the number of ganglion-cells devoted to the particular function will cause an increase in bulk of that portion of the brain in which they are situated and it will assume definite characteristics of its own.

The first step in the development of such a brain region consists in the mere thickening of the brain wall but with still greater increase in the number of cellular elements involved mere increase in thickness becomes insufficient for their accommodation and an increase in area comes about in addition. This necessarily causes a bulging of the particular part of the brain wall and some of -the most characteristic differences between the brains of different types of Vertebrate depend upon whether the bulging takes place outwards or inwards.

Thus in the majority of Vertebrates the cerebellum bulges out- wards as has been indicated in the case of Acantl:/ias. In Teleostean fishes on the other hand this is the case with only the hinder part of the cerebellum: its anterior portion in these fishes bulges downwards and forwards underneath the roof of the mesencephalon forming the well-known valvula cerebelli. In the more primitive ganoid fishes on the other hand such as Pol;/pterus (Graham Kerr, 1907) the hind portion of the cerebellum also grows inwards, so as to form a structure projecting back into the fourth ventricle in just the same fashion as the valvula cerebelli projects forwards.

A somewhat similar difference appears to be present in the case of the hemispheres. These originate in most subdivisions of the Vertebrata as paired bulgings of the wall of the primitive fore-brain, and the present writer agrees with Studniéka (1896) in feeling com- pelled to accept on this ground the view taught by many of the older morphologists such as Von Baer, Reichert and Goethe that the hemi- spheres are to be looked on as fundamentally paired structures, rather than the view, more fashionable of recent years, which regards the portion of the primitive brain lying in front of the velum and optic recess as forming together with the hemisphere region an unpaired complex (Telencephalon-—His). The more complete knowledge that we now possess regarding the develop- ment of the brain in the more primitive Vertebrates with holo- blastic eggs, seems to the writer to make it clear that the reasons which have led to a departure from the older view can no longer be regarded as adequate. We take it then that the hemispheres are fundamentally paired projections of the side wall of the primitive fore-brain. Physiologically they are probably to be regarded as portions of the brain wall which have become specially enlarged in relation with the sense of smell, just as are the optic outgrowths in relation with the sense of sight.

N ow whereas in the majority of Vertebrates the hemispheres bulge outwards, in the more primitive Teleostomes (e.g. Polypterus, 96 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Graham Kerr, 1907) they bulge inwards. In the typical Teleosts what apparently corresponds to the hemisphere forms simply a solid mass projecting into the cavity of the fore-brain, the structure which is usually and probably erroneously spoken of as the corpzts st'riat'u'm in these fishes.

A part of the brain which is of very special morphological interest is the thalamencephalon——which persists with singularly little change throughout the series of Vertebrates. Even in .A')IljJ]b’ll0.’l7’Ll»S sagittal sections through the front end of the neural tube present appearances which vividly suggest the thalamencephalon of the more typical Vertebrates (Kuplier) and raise the question whether—as is probable enough on other gro1mds——-the head region in Amplmlowws is degener- ate and once possessed a brain.

Amongst the structures connected with the thalamencephalon special interest a.ttaches to the pineal body.‘ So far this has been alluded to merely as a comparatively simple diverticulum of the thalamencephalic roof. in the majority of Vertebrates it remains comparatively uncomplicated and its main function appears to be that of forming a peculiar internal secretion.

In two sets of Vertebrates-——-the Lampreys on the one hand, and Sphenoclon and many Lizards on the other—-there becomes developed in relation to it an organ possessing a close resemblance to an eye, of the “camera” type, possessing a retina and in some cases a lens. The organ appears to be functional as the tissues overlying it a1'e commonly free from pigment and its retinal cells on exposure to light show a change of position in their pigment granules similar. to what is commonly found in visual organs. Though functional it does not follow that the organ serves for the detection of what We call light: it may be that its sensitiveness is rather towards radiant energy of other Wave-lengths than that included within the range of the visible spectrum.

There is a general tendency amongst those who have carried out researches upon the pineal eye to regard the eyelike condition as a relatively archaic condition ot‘ the pineal organ~—-a tendency which is encouraged by the evidence of palaeontology that certain ancient Tetrapods of the palaeozoic and mesozoic periods possessed a highly developed pineal organ——-the skulls of these animals possessing a relatively huge parietal foramen, corresponding with the foramen in the root‘ of the skull of modern lizards in which the pineal eye lies

embedded. The evidenceof embryology indicates that the most archaic con-

dition of the pineal organ was a simple diverticulum of the brain roof projecting towards the skin on the dorsal surface of the head. There is no clue whatever as to the original meaning of this diverti- culum. But we do know from the study of invertebrates that where tissue rich in nerve-elements comes to be exposed to light there is

1 An admirable account of the structure and development of this region of the brain by Studnieka. will be found in Oppel (1905). II . PINEAL ORGAN 97

frequently shown a well-marked tendency to the evolution of eye-like structure. In Molluscs for example we find eyes developing on the edge of the mantle (Pccten), round the tips of the siphons (Uardium sp.), on the dorsal surface of the body (independently in 0'/titan and Oncvlrlvluum)-—and similar instances might be quoted from other groups.

Bearing such facts in mind one is compelled to acknowledge the possibility, if not probability, of such a projecting piece of nervous tissue as the pineal diverticulum, lying close under the surface of the head on its dorsal side, in the position where light stimulus would be most pronounced, developing secondamly in some cases into an organ of the nature of an eye.

Discovered by Leydig (187 2), its structure investigated by Spencer (1886) and other workers, the development of the pineal eye has formed the subject of a number of excellent researches. It will be convenient to take as an example that of the common lizards of the genus Lacerta ( N ovikofi”, 1910).

The first indication of the organ appears in embryos of about 3 mm. in length in the form of a thickening of the thalameneephalic roof, in the region of the mesial plane, and divided by a transverse furrow ‘on its outer surface into a smaller anterior and a larger posterior portion. This thickened part of the brain roof comes to bulge outwards and forms a prominent projection (Fig. 55, A) the groove dividing it externally into anterior and posterior portions being still visible though less distinct.

The projecting pocket now grows forwards parallel to, and in close contact with, the brain roof (Fig. 55, B), its forwardly projecting portion becoming constricted off from the rest. The constriction in question deepens and the anterior portion (parapvlneal body) becomes nipped off to form a completely closed vesicle (Fig. 55, C)—-the rudi- ment of the eye. As the external ectoderm recedes from the brain roof, with the increase in the amount of mcsenchyme between the two, the parapineal vesicle remains close to the ectoderm and con- sequently retreats from the brain surface (Fig. 55, D).

The eye is now seen to be connected with the brain wall by a distinct optic nerve which, in full accordance with the view taken in this book with regard to nerve-trunks in general, is merely a primary bridge which already existed (N ovikoff) at a time when the eye vesicle and the brain roof were still in immediate contact and which simply became extended in length as the gap between eye and brain became greater and greater (Fig. 56, pm). N erve-fibres develop in this optic nerve which pass at their cerebral end into the habenular commissure. Transverse sections through a 9-mm. embryo show that the fibres on entering the commissure bend away to the right, passing eventually to the right habenular ganglion. In this connexion with the right habenular ganglion Lacerta. resembles the other lizard Iguana, tubercu- lata (Klinckowstrom, 1894) but curiously differs from Sphenodon where according to Dendy (1899) the connexion is with the left habenular ganglion. ’

VOL. 11 H 93$ EMBRYOLOGY OF THE LOWER VERTEBRATES on.

Meanwhile the wall of the vesicle lying next the outer skin

FIG. 55.———-Sa.gittal sections through the pineal organ of embryos of Lacerm. . (After Novikotf, 1910.)

A, L. m'm‘pam, 3 mm.; B, ditto, 4 mm.; C, L. -muralis, 6 mm.; D, L. mdpura, 9 mm. ect, external ectodorm; 1, lens; p.e, pineal eye; pan, pineal nerve; p.s, pineal stalk; pin, pineal mwgrowth; that, roof of thalamencephalon. 11 i PINEAL ORGAN 99

assumes a lenticular form, its cells becoming much elongated though remaining in a single layer. This lenticular thickening occasionally becomes lost during deve1op1nent—-a fact which may be taken as forming a piece of evidence in favour of the view that the eye, at the present time, is in a retrogressive phase of evolution.

.Those parts of the vesicle wall which do not take part in the formation of the lens undergo histogenetic changes into retinal tissue. The cells undergo differentiation in two directions. The one set become pigment eells—-tall columnar cells which traverse the whole thickness of the retina and have their nuclei towards the basal or outer end and which develop dark melanin granules in their proto- " Ian. plasrn. /7,52.

Interspersed with these are the " percipient cells, shorter in form, their basal ends not reaching the outer ‘surface, and carryi.ng at their inner ends Cllllllll-l7llx’(_‘, structures which project into the cavity of the vesicle. The idea that these projec- tions correspond physiologically to rods appears to be negatived by the fact that they occur also on the inner ends of the cells forniingithe lens.

At their basal ends these cells are continued into nerve-fibres, which form a distinct layer internal to the nuclei of the pigment cells and are Fwut ?5:-piarieala 7W1:‘(Llv?78»f25ffIri}Ih1-,1Sagit- eventually continuous phys1o.log1— c‘;p1::’1"m‘1‘f“ (‘Egg N‘(;‘:,‘i’k0‘t’,') “ ‘“”"“ cally with the fibres of the pineal _ _

seemed a»m0nsSt»&n€1 in the ..::;:;.i.:;?.:%::':‘:'.f:i 

l10lgl'll)()1lI'll()0d Of, blllfl lllj)I'0l1S liliyel‘ 70.3, pineal stalk ; pm‘, paraphysis. ganglion-cells are present: they are about the first definite elements to become recognizable during the histogenesis of the retina and appear first close to the point of attach- ment of the optic nerve.

The cavity of the pineal eye is kept distended by a clear substance,

the vitreous body, and this is colonized by a certain number of cells (see Fig. 55, C) which are most probably to be regarded as i1n1ni- grant mesenchyine cells. M In Sphenodon, the sole survivor of the only other existing group of Reptiles in which a pineal eye is present, the development of the organ according to Dendy (1899), who has worked it out in detail, agrees with that of Lacerta in its main features.

‘In the Lampreys, also, somewhat eye-like developments occur in the pineal region. In theadult two vesicles--—a dorsal (“pineal”) and a ventral (“parapineal”)-—~are found overlying the roof of the


/.,-at’ ' 100 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

thalamencephalon. In each of these the lower wall shows histological characteristics of retinal tissue and each is in continuity with the brain-—in the case of the parapineal organ directly and in the case of the pineal by an elongated stalk containing nerve—fibres. '

The parapineal organ lies in some cases (G’eotm3a-——-Dendy, 1907) slightly to the lef't of the pineal and its nerve-fibres have been traced into the left habenular ganglion while those of the pineal organ have been traced to the right habenular ganglion. In neither case does the outer wall of the vesicle show any signs of thickening to form a lens——so that neither organ can form an iinage-—but the overlying tissue is comparatively transparent so that diffuse light stimulus ca11 reach it. '

According to Studnicka the two organs develop as evaginations of the brain roof one (parapineal) in front of the other. The para- pineal evagination soon loses its lumen and becomes solid and it is noteworthy that at first it is continuous on each side with the habenu- lar ganglion of that side. Later on it becomes by differential growth shifted far forwards, away from the region of the habennlar ganglia, and it loses its connexion with the right ganglion while it remains connected by nerve-fibres with the left.

The two questions of special interest which present themselves in regard to the pineal and parapineal organs are (1) were they originally ocular in structure and function and (2) were they paired or unpaired?

(1) It is obvious that the presence of an eye-like pineal or para- pineal organ in certain Reptiles and in Lampreys, and of a large parietal foramen in the skull of various extinct Vertebrates suggests the possibility of these organs having had the form of visual sense organs in the ancestral Vertebrate. Against this however must be set the fact that in all other Vertebrates than those mentioned, including such relatively archaic forms as Elasmobranchs, Cross- opterygians, Dipnoans and Urodeles, there is no trace whatever of eye structure.

It seems highly improbable that a well-developed visual organ once present on the dorsal side of the head in the ancestors of Vertebrates should have disappeared without leaving a trace in all the varied groups, with their very different modes of life, outside the limits of the Lampreys and Reptiles. To the present writer it does not appear that the evidence, so far_ as it exists at present, is anything like convincing that the pineal eye is an ancestral feature _of Vertebrates in general rather than a mere secondary development.

(2) Various recent investigators of the pineal organs are inclined to look upon them as being originally paired structures, the pineal organ in the strict sense being the right-hand member of the pair and the parapineal organ the left. This is perhaps most clearly suggested by the Lampreys in which the .parapineal organ is con- nected by nerve-fibres with the left habenular ganglion and the II N EUROMERY 101

pineal organ with the right (though also with the posterior com- missure). Again in various Vertebrates (Teleostomatous fishes-— Hill, 1894: Amphibians, Birds-—Ca1neron, 1903, 1904) the parapineal organ is in early stages slightly to the left of the pineal organ.

On the whole it does not appear to the present writer that the evidence is suflicient to make the view probable. In the Lampreys the connexion of the parapineal body with only the left habenular ganglion appears, as indicated above, to be secondary : it is originally connected with both right and left. Again, to turn to the lieptilia, the eye is in Spheaodon connected with the left habenular ganglion and in the Lacertilia with the right, although it seems perfectly

.clear from the figures given by Dendy and N ovikolf respectively

that the eye is morphologically the same organ in the two types mentioned. Were it not so we should be driven to the position that of a pair of pineal eyes originally present one has disappeared entirely in Sphenodon and the other has disappeared equally completely in the lizards. The improbability from a physiological point of view of this having happened need not be accentuated.

Here again, then, there seems to be up to the present no sufficient reason for departing ‘from the view that pineal and parapineal organs were primitively median in position, one in front of the other. As to their original significance we have no obvious clue: the absence of convincing evidence that they were originally eyes does not of course preclude the possibility of their having been originally some kind of sense organ.

N EUROMERY.-——It has been noticed in various Vertebrates, particu- larly Elasmobranchs, Amphibians and Birds, that the neural rudi- ment while still in the form of an open plate is sometimes divided up by numerous and regular transverse markings. Whether this appear- ance of segmentation is caused simply by the active growth in length of the medullary plate or, whether on the other hand it has some deeper significance has not been conclusively determined. The name neuromere has been given to the apparent segments. That these are really primitive morphological segments as is believed by many and as is implied in the termination “ -mere ” seems improbable.

The existence of a clearly marked segmentation of the nervous system where it oecurs—in the phyla Annelida and Arthropoda——is brought about by the concentration of ganglion-cells in serially repeated masses, in correlation with the presence of serially repeated appendages (parapodia in Polychaeta), and there is no suflicient evidence to show that such were ever present in the ancestral Vertebrate. The fact that the longitudinal muscles are divided into myotomes would not be sufficient by itself to account for the external form of the central nervous system being segmented, for in that case the segmentation would be still clearly marked in the many fishes where the myotomes remain practically unm odified.

During later stages, after the neural tube has become closed in, “ neuromeres ” are particularly conspicuous in the brain region. 102 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

They are sometimes very distinct in the hind-brain of fowl em- bryos of about the fourth day (see Fig. 236 in Chap. X.). It is however an outstanding characteristic of the head region as compared with the trunk that the segmentation of its mesoderm has become blurred and in great part obliterated. It is under those circumstances improbable that a primitive segmentation of the central nervous system, which is in its nature linked up to a segmentation of mesodermal structures, should have remained particularly distinct in a region where the mesodermal segmentation itself has become particularly obscure.

The appearances mentioned seem to be adequately explained by the active growth of the developing brain within its confined space, combined with the presence round it of mesodermal tissue with vestigial segmentation. It will be noticed in the figure referred to above that the dividing lines between the “ neuromcres ” are spaced out at exactly the levels where We should expect to see boundaries of mesoderm segments were the existing series prolonged forwards. Segments are no longer visible in this region but there is, as will appear later, convincing evidence that the series of segments did formerly extend through this region now occupied by continuous mesenchyme. _

It may well be that the individuality of the segments, no longer visible as such, isstill expressed by a difference in consistency of the mesenchyme, suflicient to mould by its varying resistance the actively growing hind-brain as it presses against it.

DEVELOPMENT or ‘THE PERIPHERAL NERVES.——Tl1e development of the peripheral nerves of Vertebrates has been the subject of a large amount of , investigation, partly on account of its intrinsic interest and partly on account of its bearing upon physiology and pathology. In spite of the labours of numerous investigators the problen1—-for We may take it that the mode of development is fzmda.- mentally the same throughout——has not yet by any means been satisfactorily decided.

While bearing in mind the undesirability of making use of modern facts merely to support, or to undermine, old hypotheses, it will be convenient to approach the question by stating shortly the three prevalent views as to the main features of the development of peripheral nerve-trunks neglecting differences in detail. For shortness these three views may be termed after their most prominent supporters (1) the His view, (2) the Balfour view and (3) the Hensen view.

THE HIS VIEW (Outgrowth theory).-—This hypothesis may be said to have been founded by Kupffer in the embryological portion of Bidder and Kupffer’s work (1857) on the spinal cord. As however Kupffer later on gave up the view, in favour of that of Balfour, the hypothesis now under consideration is commonly associated with the name of His, who played the main part in’ building up the theory and who fully deserves to be regarded as its principal founder. II NERVE DEVELOPMENT 103

It is to be noted in passing that Kupfl’er’s original observations were made upon Mammals and those of His (1868) upon the Fowl. In other words, in both cases the embryos were such, in regard both to the minute size of their cell elements and to their high position in the Vertebrate scale, as to be unsuited to provide a reliable basis for the generalization that has been built upon them.

'l‘he His View as expounded by one of its most distinguished supporters S. Ramon y Cajal (1909) may be summarized as follows, the case of the motor nerves being taken for the sake of simplicity. Each motor nerve-fibre arises as an outgrowth from a neuroblast, or young ganglion-cell, lying within the spinal cord. The fibre sprouts out from the neuroblast, makes its way to the surface of the spinal cord, perforates that surface, and proceeds to grow freely through the mesenchyme. The free end of the fibre forms a “cone of growth,” commonly shaped somewhat like a grain of barley and with a pointed end. This “cone of growth” shows an active amoeboid movement, by which it insinuates itself through the interstices of the mesenchyme. Sometimes it may be seen to flatten or mould itself against obstacles in its path.

In the Fowl these processes take place during the third and fourth days. Eventually (about the fifth day, in the Fowl, in most cases) the growing nerve-fibres reach their destination and become joined up to the muscle cells which form their definitive end-organs.

The essential feature of the His view is that the nerve-fibre (which already shows the characteristic specific reactions of a nerve- fibre, e.g. on impregnation with silver salts, and is therefore not merely a strand of undfiferentiated protoplasm) sprouts out from the central nervous system and grows through the intervening mesenchyme with a free end until it becomes joined up secondarily with the end-organ.

The His view at the present day rests upon a large body of observed facts. In studying the embryology of almost any Verte- brate it is easy to find in sections what appear to be freely ending nerve-fibres sprouting out from the spinal cord. Some of the most beautiful preparations of this kind have been made by Ramon y Cajal and others by the use of silver impregnation methods.

Perhaps the most striking evidence, which has recently been adduced in favour of the His view, has been obtained by experi- mental methods, especially by Harrison (1908, 1910). In one set of experiments which have been regarded as particularly convincing Harrison removed small pieces of embryonic spinal cord from Frog embryos at a period just before that at which the motor nerves became visible, and was able, by using ordinary antiseptic pre- cautions, to keep them in a living condition for relatively prolonged periods mounted in a drop of sterile lymph under a coverslip upon a slide. The lymph soon -clotted and held the piece of spinal cord in position. Harrison now observed in many cases little projections 104 |*‘.MllllrI.Y(f_)il.O({Y 0|“ '|‘ll|4l il.()W”E|l \'|Cfli'l.'|‘l*}ilrlfl{x\'l"lC.f fill.

making their appearance from the pieces of spinal cord which he identified as rudimentary inotor nerves.

Any possible doubt as to the correctness of this identification was removed by Burrows (1911) repeating the work on the chick and obtaining the specific staining reaction of neuro-fibrils’ in the structures in question. These nerve-rudiments when l<(i-.pt alive under the conditions mentioned were observed to increase rapidly in length, the rate of growth being in one case as high as 56;; per hour. The end of the rudiment (Fig. 57) was somewhat enlarged and projected into fine protoplasmic tags which showed active amoehoid movement. It is this amoeboid protoplasm at the free end of the fibre which, in Harrison’s belief, is. the active agent in the extension of the nerve-fibre.

As to the method by which it is, in the actual body, guided along the proper path to its destination, Harrison does not commit himself, but he appears to have a leaning towards the view held by Ramon y

chemiotaxy. _

In the words of their author (1908) these “experiments place the outgrowth theory of His upon the firmest possible basis,—-that of direct observation. The attractive idea of Hensen must be abandoned as un- tenable.”

It should be added that the His theory fits in very well with current views in physiology and pathology—- in particular with the fashionable neurone doctrine, according to which the cellular units which compose the nervous system are not in organic continuity with one another. Ob- viously this hypothesis and the Flu.‘ 57.--~ Viewpof outgrowing nerve- Outgrovvth hypotllesis, according to

".‘:'.?l.‘;i:T...‘.‘..‘-Lil..1"I§“;’$.-..il'i’;.'i which the  is for. a me

five minutes later than A. (After Separated by 3: gap fI'0n1 1tS end‘

Harrison. 1908-) organ, lend one another mutual support}

’ It must be borne in mind, however, that the histological basis of the neurone theory is not universally admitted to be beyond suspicion. Its main foundation consists.ol' observations by the Golgi and similar methods ol' metallic impregnation. In reparations made in this way single cellular units are frequently picked out witiout the reaction taking place in neighbouring units arranged in series with them. A ganglion-cell A with its axon aml ts-rminal branches stands out deep black in the preparation, while the ganglion—eull B, I'1u.\;t to it in the series, shows no reaction. Snell an observation obviously suggests discontinuity.

The P()S'.sll)l(_E fallacy in these observations lies in the"fact that the stain used is not a true stain in the ordinary sense of the word but merely a precipitation of metal upon

Cajal that it is mainly a matter of" II NERVE DEVELOPMENT 105

The His view is concerned primarily with the actual functional nerve-fibres. As regards the primitive sheath (Gray Sheath of Schwann), in which these fibres are enclosed, the His view regards it as being formed by mesen- chyme cells which apply themselves to, and spread out over, the surface of the originally naked nerve-fibre.

(2) THE BALFOUR VIEW (Cell -chai n theory). ———While Schwann(1839) long ago described the multicellular structure of nerve—trunks in the foe- tuses of mammals, it was F. M. Balfour (1876) who really founded the view that the nerve -trunk arises in development from a chain of cells. F10. 58. —Section through the dorsal part of the trunk Balfour fuund in E1aSInO_ of a Torpedo embryo. (From Balfour's Embryology.)

branch elnbl-yos the (1.:-, dorsal root; g, spinal ganglion; why, myotome; N, noto- nerve_trunk was repre_ n, nerve--trunk; m:, cavity of spinal cord; v.r, ventral sented by a chain of

cells in early stages (Fig. 58, 22.7")’, and similar observations have been made by subsequent observers. According to this View the whole nerve- trunk is niulticellular in origin, the cells not only forming the sheath of the nerve-trunk but also giving rise to the nerve-fibrils which come into existence traversing the cellular strand from end to end.

011 the question of the origin of the cells which constitute the nerve-rudiment opinions vary. Most supporters of this view have regarded them as having emigrated from the spinal cord (e.g. Balfour, van Wijhe, Dohrn): while others (Kolliker) have looked on them as mesenchymatous in nature. Sedgwick took this latter view and as he regarded the mesenchyme as a continuous syncytium, the bridges connecting the cells being primitive——-persisting from the

4n—-_ 4-» ——-1-.:.— -a—-_ —u— ._-1— -——-.._~——j~ 44

the surface of the cell and its processes. We know from the recognized unreliability of the method that the occurrence, or not, of this precipitation is liable to be decided by extremely delicate chemical differences. We know further that the axis cylinder, however it arose in development, is morphologically and physiologically a prolonga- tion of the cell-body (ganglion-cell), and therefore that its metabolism is under the control of the nucleus of that cell-body. The individuality of the cell and its pro- longation, due to the metabolic control by its own special nucleus, is probably qulte enough, in itself, to account for a chemical character of its surface sufliciently different from that of its neighbours to influence the precipitation without there being, as the neurone theory assumes, any absolute discontinuity. mo EMBRYOLOGY or THE LOWER VERTEBRAVTES cu.

thalamencephalon. In each of these the lower wall shows histological characteristics of retinal tissue and each is in continuity with the brain-——-in the case of the parapineal organ directly and in the case of the pineal by an elongated stalk containing nerve-fibres.

The parapineal organ lies in some cases (GeomIa.——Dendy, 1907) slightly to the left of the pineal and its nerve-fibres have been traced _into the left habenular ganglion while those of the pineal organ have been traced to the right habenular ganglion. In neither case does the outer wall of the vesicle show any signs of thickening to form a lens———-so that neither organ can form an image»-but the overlying tissue is comparatively transparent so that diffuse light stimulus can reach it. '

According to Studniéka the two organs develop as evaginations of the brain roof one (parapineal) in front of the other. The para- pineal evagination soon loses its lumen and becomes solid and it is noteworthy that at first it is continuous on each side with the habenu- lar ganglion of that side. Later on it becomes by differential growth shifted far forwards, away from the region of the habenular ganglia, and it loses its connexion with the right ganglion while it remains connected by nerve-fibres with the left. ’

The two questions of special interest which present themselves in regard to the pineal and parapineal organs are (1) were they originally ocular in structure and function and (2) were they paired

or unpaired?

(1) It is obvious that the presence of an eye-like pineal or para- pineal organ in certain Reptiles and in Lampreys, and of a large parietal forainen in the skull of various extinct Vertebrates suggests the possibility of these organs having had the form of visual sense organs in the ancestral Vertebrate. Against this however must be set the fact that in all other Vertebrates than those mentioned, including such relatively archaic forms as Elasiuobranchs, Cross- opterygians, Dipnoans and Urodeles, there is no trace whatever of eye structure.

It seems highly improbable that a well-developed visual organ once present on the dorsal side of the head in the ancestors of Vertebrates should have disappeared without leaving a trace in all the varied groups, with their very different modes of life, outside the limits of the Lampreys and Reptiles. To the present writer it does not appear that the evidence, so far as it exists at present, is anything like convincing that the pineal eye is an ancestral feature of Vertebrates in general rather than a mere secondary development.

(2) Various recent investigators of the pineal organs are inclined to look upon them as being originally paired structures, the pineal organ in the strict sense being the right-hand member of the pair and the parapiueal organ the left. This is perhaps most .clearly suggested by the Lainpreys in which the parapineal organ is con- nected by nerve-fibres with the left habenular ganglion and the 11' NERVE DEVEI.OPMEN'J.‘ 107

be, on the one hand, comparatively archaic-——it should belong to one of the relatively more primitive groups of Vertebrates—-and, on the other ha11d, its histological texture should be as coarse as possible, its cell elements being of large size.

Amongst Vertebrates investigated up to the present time in regard to nerve—development Lepzlclusiren (Graham Kerr, 1904) is unrivalled in its combination of these qualifications and a. summary will now be given of the main features which‘; have been made out from the study of the development of the motor nerves in this animal. It will be convenient to commence with the fully formed nerve-trunk and then work. backwards towards the earlier and more obscure stages.

Fig. 61 represents a portion of nerve-trunk from a fully developed larva of stage 34. The nerve-trunk consists of a cylindrical bundle of nerve-fibrils, dotted over the surface of which are the numerous large nuclei of the protoplasmic sheath. The sheath itself is so thin as to be practically invisible even under a high-power immersion objective except in the angle close to a nucleus where it is distinctly visible.

Fig. 60, D is taken from a larva. ten days after hatching. At this stage the nerve-trunk, when examined superficially, has the appear- ance of a thick strand of protoplasm containing numerous nuclei or a chain of cells. Careful examination of well-fixed and well- stained specimens shows however that this conspicuous mass of protoplasm is really only the sheath, the true nerve-trunk (n) being visible traversing it from end to end. Scattered about in the thick sheath of this stage there are still to be seen granules of yolk (black in the figure) which have not yet been used up.

Fig. 60, C is taken from a larva at the time of hatching. At this stage the nerve-trunk is a well-developed bundle of nerve-fibrils, just as in the later stages, but throughout the greater part of its length it is devoid of a sheath of protoplasm. In the section figured the sheath is visible as a mass of nucleated and heavily yolked protoplasm enclosing a portion of the nerve-trunk towards its outer end. This mass of protoplasm is obviously just a condensed part of the general mesenchyme which is scattered about in the form of irregular heavily yolked masses throughout the spaces between the main organ systems.

The mass in question is identical with the rest of this mesenchyme in its various features and every here and there it is continued into it without a break. The section figured shows the whole length of the motor nerve-trunk from the ventral root to the myoblasts or muscle cells which form the myotome. Towards its outer end the trunk_ is seen to break up into numerous diverging strands which are directly continuous with the protoplasm of the myoblasts (see below, p. 204). _

Fig. 60, B is taken from an embryo about three days before hatching. At this stage the myotome l1as barely commenced to 108 EMBRYOLOGY OF THE LOWER VERTEBRATES

FIG. 60.-—~Port.ions of t.x-ansvex-.~u: Ht:(1:t.i(')l1s tln-«.>u_«,_;'h _\'«.mn;_: l.upi«[lo.~:ire11.'< t'o illustrate the development. 01' Lhv spinal m:r\'v.s: (\'v'nh':Ll I'()ut.s


A, stage 2-1 : J), .-at.:ng<- 2'.) 4.-. m, nn_mtu_m1t-: -u. m~r\'o'-trunk ', .<.«-, .<}»in:|l curd ; sh, slmzttlx.

CH. II

NERVE DEVELOPMENT

'- .~w(.'.1,inm': t}I1'r)11;4'}1 _vnun;_,-' I.«'-piul«\.~'in-n.< to illustrzltu

m, Inyulcimm; -21., IH‘l'\'t‘.-1ZI‘llll](; .-.4-,i,spi|m| uni-«I ; sir, slu-n1.|n

FIG. 60/\.———Porti0ns of tr:m.wc-r the develolnment. of flu» spinal m.-rvv.~: (ventr:ml root.


B. .~zI..-ago 25 ; 0, stage 27.

109 110 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

recede from the spinal cord, but yet each motor nerve is already present as a distinctly fibrillated trunk bridging across the narrow gap between spinal cord and myotome. A few mesenchyme cells have wandered into the gap but they have not yet begun to con- centrate round the nerve-trunk.

‘ Fig. 60, A is taken from an embryo of stage 24 at a time when myotome and spinal cord are still in close contact with one another. In specimens which were extended in one plane under normal salt solution while still alive and subjected to the action of the fixing agent in that position, it is found that the myotome is frequently pulled slightly away from the spinal cord (as in the specimen figured) and in such cases it is found that the nerve- trunk already exists in the form of a bridge of soft granulai‘ protoplasm (n) with- out any trace of fibrillation, connecting spinal cord and 111y0t0me. That these bridges are really the nerve-trunks is indicated by their occur- rence one to each myotome, apart from the fact that a con- tinuous series of stages have been observed between them and the fully developed nerve-trunks. '

In summing up we may FIG. 6l.~——l’art of transverse section of Le.1)i<l0.s"i')'(’n, take the various stages in

(stage 34), showing a portion of nerve-trunk. their proper chronological in”, myotome; N, notoehord; n, nerve-trunk ; n.S, nucleus Sequcnde.

of nerve sheath; S1, pi-irnary sheath of notochord; XI, . e . lateral branch of vagus. The nerVC'trunk 18

already present as a proto- plasmic bridge at a period so early in development that spinal cord and myotome are still in contact with one another.

(2) As the embryo grows and the myotome recedes from the spinal cord this protoplasmic bridge increases in length and becomes fibrillated.

(3) As the nerve-trunk lengthens amoeboid masses of mesen- chymatous protoplasm collect round it and gradually spread out over its length to form the protoplasmic sheath.

In stages later than those figured the sheath protoplasm insinu- ates itself in amongst the nerve-fibrils of the trunk, dividing them up into bundles or nerve-fibres. As the myotome resolves itself into the various muscles of the adult each piece retains its primitive e nerve-strand, drawing it with it, as i.t becomes pushed about by the processes of differential growth, as its own special nerve.

It should be mentioned that the most important point in the II NERVE DEVELOPMENT 111

above deseription——the existence of the motor trunk in the form of a bridge of protoplasm between myotome and spinal cord at a time when they are still in close proximity—-has been confirmed for an- other very primitive group of Gnathostomes, the Elasmobranchs (Paton, 1907), as is shown in Fig. 62.

It will now be convenient to review the facts just described for Lepidosiren in relation to the general theory of nerve-development.

(1) It is clear in the first place that the His view is put out of court, seeing that before there is any development of nerve-fibrils the motor nerve-trunk already exists in the form of a bridge of proto- plasm connecting spinal cord and myotome.

(2) It is equally cl.ear that the Balfour View is inapplicable: the nerve-rudiment cannot in early stages by any possibility be re- garded as a chain of cells, seeing that its total length is greatly less than the diameter of a single cell-nucleus.

(3) While the nerve-rudiment forms a primary connexion be- tween spinal cord and myotome, in the sense that it is in existence before these organs begin to recede from one another, there is no evidence by which the connexion can be traced back to intereellular bridges or plasmodesms (StraS_ FIG. 64'2'.r——~l‘:n'l .(')l'llI.‘.'l.l-l.’\'\."(£.l‘.1\‘:"lit-?(.ffll(ill tllrplugh burger, 1901) of early. e-.<7- SesInen- is "u'.‘$"';..Z;'t'.'»"’l‘;L£le.t£{l‘K12"'§II.’nTn§§iI tation, stages in the development (After Stewart Paton, 1907.)

Of egg, as would be case 'In'_1/,my0t'..nl'm'; n,mirvv-11-unk;s.r, .~:pin:ll ('n1'(l. according to Hensen’s theory.

(4) The primitive protoplasmic bridge gradually becomes fibril- lated but there is no means of determining with any degree of certainty how these fibrils are developed.

It is s11ggeste(l 1 that the development of the actual nerve-fibril is simply the gradual coming into View of a pathway produced by the repeated passage of nerve i.mpulses over a given route.

It is clear from the study of the simpler organisms that one of

‘the most ancient properties of living protoplasm is that of the trans-

mission of impulses through its substance. Although nothing is really known as to the precise nature of living impulses it is reasonable to suppose that they involve changes in the distribution of energy analogous to those involved in the passage of an electric disturbance. If this be the case their passage between two points will be determined by the relative potential, and the route along which the impulse passes will be that of least resistance. If the conductivity of the

1 Graham Kerr, 190-=1. ‘It has been pointed out th.-xi similar .-'1lg'gcsti0I1S in regard to the nervous system. in general were made long ago by .l'.l.a_-rburt; Spencer. 112 EMBRYOLOGY OF, THE LOWER VERTEBRATES CH.

living substance were uniform the path would be a straight line joining the two points: if the conductivity were not uniform on the other hand the path would be diverted along routes of high conductivity where the total resistance would be at its minimum. Looking at matters from such a point of view we should regard a motor ganglion-cell at the moment of functioning as a centre of high potential and its muscle ending as of low potential, while a sensory cell at the moment of its functioning would be a centre of high potential and the central termination of its nerve-fibre as at relatively low potential.

111 early stages of evolution, whether phylogenetic or ontogenetic, we may take it that vital impulses fiitted hither. and thither in an indefinite manner within the living substance and that one of the features of progressive evolution has been the gradual more and more precise definition of the pathways of particular types of impulse, as well as of the transmitting and receiving centres between which they pass.

We may then regard the appearance of neuro-fibrils within the protoplasmic rudiment of the nerve-trunk as the coming into view of tracks, along which, owing to their high conductivity, nerve impulses are repeatedly passing} It may be that as each successive passer-by causes a jungle pathway to become more clearly defined so each passing impulse makes the way easier for its successors, and makes it less likely for them to stray into the surrounding substance.

The special physiological meaning of the differentiation of the fibril would simply be the increase of its conductivity —~—-possibly towards one specific type of impulse——but correlated with this are optical and staining peculiarities which, though unessential in them- selves, make the fibril recognizable to the eye as a definite structure.”

The nerve-trunk in Lepidosiren is seen to be at first naked and later on to acquire a sheath formed by concentration of mesenchyme round it. This sheath is at first richly laden with nutriment in the form of yolk granules but these are gradually used up as the nerve- trunk goes on with its development, the products of digestion of the yolk being doubtless passed on to the developing nerve-trunk. This as well as the marked increase in the number of nuclei in the sheath seem to indicate that the main r6le of the sheath is to look after the nutritional needs of the nerve-trunk.3

We have dealt, so far, only with the motor nerve-trunks. In regard to the general method of development of sensory nerves, there

1 Paton (1907) shows that impulses are actually transmitted across the protoplasmic bridge at a very early stage in the case of Elasmo ranchs.

9 The hypothesis here outlined in connexion with the embryonic development of nerve fits in well also with certain of the phenomena observed in the regeneration of nerves which have been severed and joined together a ain [see Trans. Roy. Soc. Edliné, ]xli, p. 126, also Mott, Halliburton and Edmunds in Proc. Roy. Soc., B, vo . 8 .

3 The medullary sheath of nerve-fibres is non-cellular and appears to be produced by the secretory activity of the protoplasm of the axon. II NERVE DEVELOPMENT 113

exists the same divergence of opinion as in the case of the motor nerves, and in endeavouring to decide which View has upon its side the balance of probability, it is well to bear in mind similar conditions to those alluded to on p. 106. Bearing these in mind, it is of interest to notice that in Lepidosriren (Elliot Smith, 1908) the process of development of the olfactory nerve takes place along exactly similar lines to that of the motor trunks. And it is significant that, in the opinion of those well qualified to judge (Retzius, Golgi, Rainon y Uajal, van Gehuchten, Kolliker, Elliot Smith), this nerve has advanced less from the primitive condition than has any other nerve, and in its general arrangements has undergone extraordin- arily little complication during ontogeny.

Already at a time when the olfactory organ has not yet com- menced to recede from the wall of the hemisphere the olfactory nerve exists as a stout pri5toplas- mic bridge (Fig. 63, I) which gradually increases in length as the olfactory organ recedes from the hemisphere. This observation seems to indicate clearly that the mode of development of the sen- sory nerve-trunk is fundamentally the same as that of the motor: that it develops out of a pre- existing protoplasmic bridge be-

tween Celltre and end-0rgan. FIG. 63.-—--All t3d.I'l_V Sl.fl,g(§ Of the Oll'fl.(3t.Ol‘y nerve of Lcpz.zlnsi7~en. (From Elliot Smith,

1908.) REMARKS UPON THE GENERAL ¢-.11, lateral wall of hemispliere; ulf, olfactory

PROBLEM OF NERVE DEVELOPMENT organ; 1, olfactory neI'\'e. The nuclei semi in the

region where the olfactory nerve enters the lu-.mi- It; will be adlnitted by most sphere belong to the olfactory bulb.

Zoologists that we are justified in believing that the process of nerve—deve1opment is probably fundamentally the same throughout the Animal Kingdom. It will also be clear, even from the short and imperfect statement which has been given here, that the detailed study of the phenomena of nerve—development has led, in the minds of different‘ observers, to widely divergent conclusions as to the exact nature of the process. The subject is one to the discussion of which we may devote with advantage some further space. It is in itself of great embryological and physiological interest. It presents many problems still unsolved. And it may be taken as a type of biological controversy with which it will be to the student’s advantage to become acquainted.

In approaching the question from the present-day standpoint it appears impossible to get round the fact that in two of the most

VOL.‘ II I 114 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

archaic groups of Vertebrates (Elasmobranchii and Dipnoi) the motor nerve-trunk is already present as a protoplasmic bridge at a time when myotome and spinal cord -have 11ot yet commenced to recede from one another. It does not seem possible to explain the appear- ances recorded in these cases by any conceivable errors of observa- tion. But if such bridges exist in these relatively archaic groups, the balance of probability is entirely on the side of their represent- ing the primitive mode of development of nerve—trunks in general, and of a fundamentally similar mode of development occurring in other Vertebrates though possibly in a modified and less distinct form.

On the other hand appearances of the kind which led to the original formulation of the His view, and which are still adduced in its support, and which are easily observed in series of sections through almost any type of Vertebrate embryo—-nerve-trunks passing out from the spinal cord and ending freely amongst the mesenchyme—-are peculiarly apt to be misleading.

Such a misleading appearance is produiisd sometimes by compara- tively simple causes-—-by breakage of the nerve-trunk or by the nerve-trunk passing away out of the plane of a section and‘ being unrecognizable when out transversely in a neighbouring section. In other cases the appearance of a freely ending nerve-trunk is due to the portion of nerve-trunk which has received its protoplasmic sheath being distinctly visible in a stained section, while the delicate peri- pheral portion which is still naked is practically unrecognizable. On account of such liability to misinterpretation a very large proportion of the observational evidence which supports the His view is open to suspicion.

A physiological difficulty which has been raised against accepting the His view is that involved in the idea that the free end of the growing fibre tracks down and finds its appropriate end-organ. It is pointed out that it never makes a mistake—-never becomes joined up to a wrong end cell. And yet; if it be the case that nerve-fibres do grow outwards with free ends in the way involved in the His theory, certain experimental results show that such fibres do possess a very decided power of making mistakes. This is brought out clearly by the beautiful experiments of Braus (1905).

In the experiments in question Braus made use of the method, invented by Born and developed by Harrison, Spemann and others, in which portions of one amphibian embryo are grafted upon the body of another, when the grafted portion (“parasite ”-—Braus) pro- ceeds to develop as part of the individual (“ autosite ”--Braus) upon which it has been grafted. ,

In the experiments which are most important in their hearing on the point now under discussion the early rudiment of the pectoral limb was grafted upon a host in the region of the head. In this position the rudiment went on developing into a perfectly normal limb containing a normal arrangement of the limb nerves. N ow the II NERVE DEVELOPMENT 11 5

iniplanted limb _in such. a case (Fi g. 64) is Situated in a region inner- vated by the facial nerve and the study of sections showed that the nerves in the i1upl:uited limb were continuous centrally with branches of the facial nerve.

"If we attempt to interpret this vxperilnent on the oiitgimvtli view we find ourselves compelled to admit that the facial fibres concerned made the serious mistake of growing into a limb rudiment; and then continued on their mistaken course until finally they established the muscular connexions normal for the nerves of such a limb. Braus repeated this type of experiment in a number of cases and there appears to be no question as to the accuracy of his observations. If accurate, however, they provide a formidable, if not unsurmountable, difficulty for the outgrowth view—a difli- culty which is by n0‘means got rid of by the suggestion (Harrison, 1908) that after arriving in the limb the nerves are “ merely guided in their growth by the structures present in the transplanted part.”

A similar difficulty is seen in post- embryonic nerve-development in the fact well known to surgeons that functional continuity can be established between the cut cent'ral stump of one nerve (ag. spinal accessory) and the severed peripheral portion Fm, 64,——Young Toad (Bom- of another bimttor) on which an addi-

- - - tional limb has been graltecl And so again in the development of in the head region (After

anastornoses between peripheral nerves such B,-ms, 1905,)

‘as the well-known “ dialyneury ” of Gaster0-

pod molluscs, or the short circuiting of the left pulmonary nerve over the dorsal side of the oesophagus which has come about in the evolu- tion of the Crossopterygians and Lung-fishes.

All such cases present great if not insuperable difliculties to the His view.

Again much of the evidence which is brought to the support of the His view is seen when looked at critically to be less convincing than it appears to be at first sight. Thus for example with the experiments of Harrison already described, which are rega.rded by their author as settling the whole question. Their true value will become more apparent if we bring Harrison's results into correlation With tlw results (h-.seribcd above for L6P’l:(l08"l:7‘81&.

As has already l)(*.("]l shown, in this animal the motor nerve-trunk is represented at an early stage by a bridge of soft fragile protoplasm. These bridges require a very favourable object and very careful technique for their detection, and it is clear that one could not expect to see them in comparatively coarse preparations Inade by excising a piece of living unfixed spinal cord. Tlioro is tlu-rol'o1'e no guarantee that sueli pi-otoplasmic nerve-rudiments were not already present in the pieces of spinal cord investigated by Harrison. 116 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Let it be assumed that such an experiment is repeated upon Lepddosiren with a small piece of spinal cord rudiment with the protoplasmic bridge attached to it (Fig. 65, A). The piece of spinal cord is well supplied with food material in the form of yolk and, if kept under suitable conditions, it would go on developing. So also might the protoplasmic bridge, for every one agrees that the metabolic control of the motor nerve is exercised by the central ganglion-cell nuclei within the spinal cord. If this happened and the process Went on quite normally we should get in succession stages such as those shown in Band C of Fig. 65.

N ow these would be inter- preted by Harrison presumably as demonstrating the outgrowth view, whereas all that they really show is that, given suitable con- ditions, the motor nerve increases in 1ength———a fact which of course is obvious. What is needed as a demonstration of the His view is not merely to show that a nerve-trunk increases in length but to show (1) that it ‘normally has a free end and (2) that it grows within the body at a greater rate than the tissues in which it is embedded, so that there is brought about a difl'er— cntial movement in which the FIG. 65. —-—-Drawings taken from the szune free (ind Pushes its a'y.thrOug.h

preparations as those illustrated in Fig. 60, the tlssues S“1T0undmg 113- ,Th]sS showing a piece of spinal cord with the has 1101'; been Sl10W1l by Harri~

developing motor nerve but ignoring the SOn’S experinwnts mm. could it niyotonie which is iii the actual embryo ‘

continuous with the outer end of the nerve. possibly be Shown by this type of experiment. In Lepi(‘]0s’i'/"6-72. the study of sections shows as has already been pointed out that, although the motor nerve-trunk grows actively in length with the increase in bulk of the body, at no period from the earliest stage figured has it a free end;‘it is throughout connected with its end- organ.‘ r In a word, it appears to the present writer that what are commonly regarded as the most convincing pieces of evidence in favour of the His View are by no means convincing. Views resembling that of His in that they also involve an out-

1 The actively moving pseudopodium-like tags which Harrison observed at the end of his outgrowing nerve-trunk are believed y the present writer to he meson- chymatous in their nature--- ssibly shreds of sheath protoplasm. It is a general feature of embryonic mesenc iyme that its protoplasm shows active amoeboid move-

ment. ' II NERVE DEVELOPMENT 1 17

growth of the motor trunk from the spinal cord, but differing from it in the essential feature that the outgrowth is simply protoplasmic and not fibrillar, have been enunciated by some modern workers such as Dohrn and Held. Dohrn (1888) describes the motor nerve-rudi- ment as arising by a “plasmatic outflow from the neural tube” but Paton later on finds that at the stage referred to by Dohrn the nerve-rudiment is already continuous at its outer end with the protoplasm of the myotome.

Held (1909) also regards the motor trunk as arising by outgrowth from the spinal cord at a time when the myotome is still com- paratively close to it. It has to be borne in mind in interpreting such sections as Held figures that there is more liability to error in demonstrating the absence of continuity than in demonstrating its presence, owing to the extremely fragile character of the nerve- trunks during early stages in development and their consequent liability to rupture during the ordinary processes of preparation which precede. section—cutting.

It is sometimes said that the diiliculty attaching to the His view involved in the idea of the nerve-fibre tracking down its own particular end-organ disappears if the view is taken that the out- growth takes place at a stage so early as that indicated by Dohrn and Held. But as a matter of fact this involves, as indicated, a distinct departure from the View enunciated by His according to which not merely undifferentiated protoplasm but definite fibrillated trunks grow out from the spinal cord. Further ii‘, as Held believes, the individual fibrils grow out in the substance of the protoplasmic outgrowth each one has still to seek out the particular portion of the myotome which will eventually be converted into its own proper muscle-cell—-a view which, looking to the comparatively undiffer- entiated condition of the myotome cells at these early stages, is even more difficult to comprehend physiologically than the» outgrowth towards a specialized muscle.

The embryological evidence upon which the His view rests is seen, when submitted to critical examination, to be unconvincing. The same is the case with the observational evidence upon which the Balfour View rests. The nuclei and cell-bodies which commonly give a multicellular appearance to the nerve—rudiment are quite reasonably interpretable as sheath-cells, 1J.e. mesenchyme elements which have collected round and it may be migrated into the, at first nonccllular, nerve-trunk.

In Lepidosireaz, with its coarse and heavily yolk-laden mesen- chyme, it is comparatively easy to distinguish such elements from the actual nerve-trunk embedded in them, but in most Vertebrates this criterion is not available and there is no certain means of distinguishing in ordinary microscopic preparations the protoplasm of the nerve-trunk from that of the sheath-cells.

The primitive protoplasmic bridge described in 1902 for L6p'id08’i'r6’Ib as representing the motor trunk at a time when 118 EMBRYOLOG-Y OF THE LOWER VERTEBRATES CH.

myotome and spinal cord have not yet commenced to move apart, confirmed later in the case of the motor trunks of Elasmobranchs by Paton, and in the case of the olfactory nerve of various Vertebrates by Elliot Smith and others, seems to rest upon a secure basis of Observation. It is difficult, therefore, to avoid the expectation that the progress of future research will show such a primitive protoplasmic bridge between centre and end-organ to be the normal forerunner of nerves in general.

But, if this he so, we are faced by the question as to the actual mode of origin of such bridges and here we pass into a region where direct observation is either impossible or unreliable. Those who accept Hensen’:-s views in their entirety would look upon them as representing inter-cellular connexions persisting from the earliest segmentation stages. Reasons have already been given (p. 37) for disbelieving in the persistence of such bridges between the cells of the segmenting egg. The connexion appears certainly to arise at some later period-—hut exactly when seems to be a question incapable of answer by direct observation.

When considering these general problems regarding the nervous system it should be borne in mind that the nervous system has for its main purpose the keeping of the various parts of the body linked together into an organic whole, in spite of their increasing differentia- tion and specialization. It has for its function the providing of exquisitely specialized pathways by which the living impulses can traverse the whole length of a relatively immense body at least as readily as they originally did the minute blob of ancestral protoplasm.

Bearing in mind this primary consideration will cause one to reflect that the evidence must be overwhelming before one is justified in believing that this organ system, whose most striking functional feature is continuity, has come in the course of evolution to be characterized by the structural discontinuity involved in the neurone theory of adult structure, or in the outgrowth theory of ontogenetie development.

Again it is important to bear in mind the high degree of probability attached to the view, originated long ago by O. and R. Hertwig (1878), that the nervous system of the higher metazoa, including Vertebrates, has been evolved out of a sub- epithelial nervous network of the kind still seen in some of the more lowly organized groups such as Coelenterata and Echinodermata. We may suppose that such a plexus was present in the far back ancestors of Vertebrates over the basal surface of both ectoderm and endoderm cells (as in modern Actinians, Havet, 1901) and that nerve- trunks became evolved as local condensations of such a network, just as we still see in the nerve-strands of a Medusa or a Starfish.

In this connexion, it is of interest to note that according to the protostoma theory of the fundamental structure of the vertebrate body, which will be found stated later, in Chapter IX., the points II NERVE DEVELOPMENT 119

represented by the two ends of the motor trunk were originally in close proximity, and a condensed strand of the network joining the two points would naturally be left as a bridge when they became separated by the deepening of the cleft between mesoderm and endoderin (cf. Fig. -66).

To sum up, in regard to the mode of development of the nerve- trunks, it seems reasonable 2l11_ the present state of our knowledge

(1) to reject definitely that portion of the Henseii view which looks on the protoplasmic bridges as having persisted from the com- mp menceineiit of segmentation, ‘

(2) to regard the His view of free—en(ling fibrillated outgrowth as non-proven and for various reasons improbable,

(3) to believe that the nerve- trunk already exists as a proto- plasmic bridge between centre and end-organ at a period when these are still in immediate contact, even although this has up to the present been definitely shown by actual ob- servation only in a few peculiarly favourable instances,

(4) to leave the exact period at which the protoplasmic bridges come into existence an absolutely open

pad?-




Fie. 66.-—-lllustratiiig the structure of a

question as being beyond the limit of reliable observation,

(5) as regards the sheath of Schwaiin, to accept the view that it is derived from nieseiicliynie. -

It will be noticed that little has been said so far regarding the mode of development of the actual neuro- fibrillae. Their origin is indeed unverifiable by direct observation, with any certainty, owing to their

liypotlietieal primitive Vertebrate at a time when the protostoma was still open. In the lower figure an entere- coelie pocket, the rllllllllellt of a mesodérui segment, is beeoining de- marcated froin the rest of the endo- derni by the downward spreadiiig of a split between the points a and I). In the earlier stage shown in the upper figure this split has not yet begun to develop, and the points a and 6 are seen in close proximity to one another on the outer surface of the ciidoderin.

minute size. They appear to spread outwards frcin the centre, and Held interprets this appearance by a kind of His theory on a minute scale, holding that each fibril grows out with a free end through the protoplasmic bridge. On the other hand if it be the case as suggested on p. 112 that the fibrils simply represent the specialized paths of nerve impulses there would be nothing surprising in their becoming visible first in the neighbourhood of the ganglion- cell from which the impulses start and from which also is exercised control over the nietabolisin of the nerve-trunk. Were this the

-m.p, niedullnry plate; p.8, pi-otostoma. 120 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

case we should get appearances which would closely simulate growth of freely ending fibrils——eentrifugal in the case of motor nerves and centripetal in the case of sensory nerves.

This view as to the meaning of the Iibrils bridges over a good many of the difficulties in the way of accepting the outgrowth view, either as regards the individual fibrils or the nerve-trunk as a whole. Thus the secondary establishment of anastomoses between peripheral nerves becomes less surprising if it be the case that undifferentiated protoplasm is liable to develop nerve-fibrils as a reaction to the passage of nerve impulses through it, for wherever there are nerves there’ must be a certain amount of leakage of the particular form of energy which constitutes the nerve impulse.

So also with the joining up of the central and peripheral ends of a severed nerve or of the central stump of one nerve with the peri- pheral portion of another. In such cases we should assume that indifferent protoplasm accumulating between the cut ends gradually becomes iibrillated in response to the passage of more or less imperfect impulses through it, the newly developed portions of fibril being necessarily, from their mode of formation, continu- ous with both central and peripheral fibrils, leading respectively to the “high-potential” and the “ low-potential” end of the nerve- fibre.

_ Again it is known that a mass of embryonic ganglionic tissue implanted into some abnormal portion of another individual may establish nervous connexions with the surrounding tissue. On the outgrowth hypothesis this demonstrates “error” on the part of the outgrowing fibres: on the functional view it simply involves the gradual differentiation of paths along which impulses spread out- wards from the high potential ganglion-cells into the low potential surrounding tissue.

On the whole, the present writer believes that this view, that the formation of nerve-fibrils is a response to functional activity, is at the present time the most plausible working hypothesis and also the one which is most likely to lead to fruitful research. Before leaving the subject it may be well to emphasize the fact that the solution of this general problem of nerve-development is to be sought in the study of Vertebrates of large-celled coarse histological texture, com- bined with a low degree of specialization of general structure. N 0 amount of observations upon small-celled highly specialized Verte- brates will ever lead to a really convincing solution with the methods new at our disposal.

Finally we would once more emphasize the fact that the kernel of the problem seems to centre round the origin of the fibrillae. Do they or do they not develop in a pre-existing bridge of protoplasm? Assuming that they do, the possibility of such bridges dating back to the period of segmentation seems to be definitely excluded. The question at what precise moment they do -become established seems to be of minor importance. II NERVOUS SYSTEM 121

While the present writer is inclined to believe that the junction is already in existence while end-organ is still in close apposition to the central nervous system there is no difiiculty in principle in the way of admitting that the bridge may in certain cases be formed somewhat later, as Dohrn describes, provided always that the gap to be bridged over is small and the bridge itself protoplasmic and not flbrillar. It is probably along such lines that we may look for a reconciliation between the supporters of His (the outgrowth view) and those who believe in the protoplasmic bridge view but it will involve dropping what are essential features in the outgrowth View as enunci- ated by His himself——(1) that the outgrowth arises at a time when the end-organ has already retreated to a considerable distance from the nerve-centre and (2) that the outgrowth is already fibrillated dur- ing the outgrowing process and before it is united to its end—organs.

SPINAL GANGLIA AND DORSAL RooTs.——As has already been in- dicated, the central nervous system of the Vertebrate consists in its most primitive condition of a specialized area of the ectoderm of the dorsal surface. It is further very characteristic of the Vertebrate that those ganglion-cells which belong especially to the sensory fibres have become concentrated into segmentally arranged clumps towards the margin of the nervous plate and have eventually come to lie out- side the limits of the actual plate, or tube into which the plate be- comes converted. These little detached pieces of the central nervous system are the ganglia of the dorsal roots or the spinal ganglia.‘

During actual ontogeny the ganglion rudiments in some cases (ag. Birds, Fig. 67, A) become distinctly apparent While the spinal cord is still in the form of an open medullary plate. They appear in the form of a continuous proliferation from the inner surface of the ectoderm in the angle between the medullary plate and the external ectoderm. In such cases the two rudiments become carried in towards one another, as the edges of the medullary plate curve inwards to form the neural tube, and undergo fusion across the mesial plane. There is thus formed a median unpaired plate or tract of cells lying just over the roof of the neural tube. and between it and the external ectoderm. This is known as the neural crest (Marshall).

More usually the ganglionic rudiment makes its first appearance after the closure of the neural tube and in such cases the paired stage of the rudiment is slurred over, the neural crest being formed by proliferation of the roof of the neural tube. This is well seen in the case of Elasmobranchs (Fig. 67, B, 0).

However it originated, the plate-like neural crest splits into two

‘ There is reason to believe that this is an instance of a widespread tendency in evolution for groups‘of ganglion-cells to undergo gradual shifting towards the direction from which their most frequent afferent impulses come. In other words there is a. tendency to shorten the afferent path by shifting the cell-body. This principle of neurobiotaxis has been developed by Ariens-Kap ers in his various papers (e.g. 1913). It is particularly conspicuous in the changes whic 1 have come about in the position of the ganglionic centres of the various cranial nerves within the brain in the difl'erent groups of Vertebrates. 122 EMBRYOLOG-Y OF THE LOWER V ERTEBRATES C11‘.

lateral halves and then grows outwards on each side opposite each

myotome, each outgrowth representing a single spinal ganglion. o

Eventually these l')I'(‘-itli apart "but in some of the more primitive Vertebrates the intervening portions of neural crest persist for a time in the form of a distinct longitudinal coiumissure (Fig. 68) linking up the series of‘ spinal gangliaito one another (Elasniohranchii --—Ball"our: Dipnoi).

The mode of development of the fibres forming the dorsal root, Whether by outgrowth from the ganglion-cells of the spinal ganglion

“or by difl'erentiatio.n of an already existing protoplasmic bridge, comes

C.

Flu. 67.———Illn.~'traling the mode of origrin of the spinal ganglia.

Miiwle.r11ln'-_yo with four lll(':~2(_Nlt'l'lll segrr1¢mt..s' (:cl't¢-r .\'c-inn.-i_\'r. 19043); It and ('3, 'l'ur/mrlu .1 mm. embryo (aftc.-.r Dohrn, 1902); cut, ectoderm; g, rudiment o1'g.‘mi«,;-1i«_»n; s.v', spinal cord. under the general controversy as to nerve-development and need not

be specially discussed. URANIAL NERVEQ. Tilt: (leveluplilellt of tile (.'.1':1.Il1al nerves has

heen investigated by many workers and an i1nnn"-.nse alnonnt of detailed observation has been accumulated. There is however great discrepancy in detail between the results obtained by different W01‘ko1's, and much of the observation seems to be perilously near the limit of p1‘nin1.l)le error. Consequently the material seems hardly ripe for tn-.at1m-ant in a text-book of a general kind and nothing of the sort will he attempted here beyond noting one or two points of particular iniportaneel

1 A modern account of the development of cranial nerves will be found in Neumayr (1906). . II ORAN IAL NERVES - 123

In the first place We find in the head region as in the trunk a tendency for the nerve-fibres to come off from the central nervous system in segmentally arranged clumps, and for the motor fibres to be situated more ventrally and the sensory more dorsally. In the head region however the dorsal root becomes reinforced by a large mass of motor iihi-es which have become shifted dorsalwards and incorporated with it.

A neural crest develops resembling that of the trunk and in the Birds it can be seen similarly to have a paired origin, arising before the complete closure of the medullary tube. This neural crest of the brain region forms an anterior prolongation of that in the trunk: it is quite continuous with the latter, it develops outgrowths similarly, and the intervening portions here also persist for a time as a longi-

tudinal commissure. A number of the most important cranial nerves


FIG. 68.—l-lrr'z.u(/u'u..v, slag:-. ‘.33, 9 mm. long, .~<lmwing ganglia of cranial and spinal nerves. (Alter Scaiulnon, 1911.)

int, intestine; 1, lens; Z13, liver‘; pan, pancreas; sp.g, ganglia of spinal nerves; Th, thyroid; V, ventricle; v.c, visceral cleft; y.st, yolk-stalk; IV, V, etc., ganglia of cranial nerves.

are simply prolongations of the outgrowths in questi0n——V, VII, VIII, IX and X. I

A conspicuous feature in the development of the cranial nerves is that in portions of their length they receive components directly from localized thickenings (placodes) of the ectoderni (Kuplfer, Beard) a possible remi.niscence of the time when nerve-trunks became evolved out of a plexus in direct relation to the external ectoderm.

I. The Olfactory nerve is unrivalled amongst all the sensory nerves of the Vertebrates as a subject for investigation on account of its large size, its short uncomplicated course and its retention‘ of

comparatively primitive conditions even in the adult. Research

should therefore be specially concentrated upon its mode of development.

In the case of Lepidosdren, as already indicated, Elliot Smith has shown that the olfactofy nerve is simply .a. drawn — out primary connexion between brain and olfactory o1‘gan, already present at a period before these organs have begun to move a] part.

In other vertebrates (Elasmobranchs ———1lol1n, 'l‘eleosts, Amphibians l24 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

—-Cameron and Milligan) there is evidence that the same mode of development holds. One of the important points to be settled is whether the nuclei which are seen scattered about in the young nerve-trunk and which give it a syncytial appearance are not really immigrant sheath nuclei. The conditions in Jmyniclosiren where it is easy to distinguish the heavily yolked sheath protoplasm make it appear probable that this will be found to be the case.

II. The optic nerve is not a true peripheral nerve comparable with the other cranial nerves but simply a narrow isthmus or stalk connecting the main brain with its outlying portion which forms the retina. Its development is mentioned in the description of the eye.

III, IV and VI. The oculomotor, pathetic and abducent nerves appear to agree exactly in their n1ain developmentalfeatures with ordinary motor nerves of the trunk (Neal, 1914).

It is not proposed to say anything here regarding the topography of the cranial nerves but some points regarding it will be touched upon later on in connexion with the segmentation of the head.

SYMPATHETIC.-—The sympathetic ganglia, as was first shown by Balfour (1878) for Elasmobranchs, are derived directly from the spinal (or cranial) nerves. In its earliest recognizable stage the ganglion forms a swelling on the course of the nerve just ventral to and continuous with the spinal ganglion. With further development the ganglion bulges more and more pronouncedly towards the mesial plane at about the level of the dorsal aorta. The nerve-trunk in this region now splits longitudinally and the ganglion becomes shifted farther towards the mesial plane, lying immediately over the posterior cardinal vein and remaining connected by a slender bridge ——-the ramus communicans-—-with the spinal nerve from which it has become split oil’.

In Sauropsida the sympathetic ganglia arise in similar fashion. In Amphibia and Sauropsida, where the sympathetic ganglia are in the adult connected by a longitudinal trunk, this latter is said to arise secondarily, the ganglia being at first quite separate. In view, however, of the difficulty of detecting such nerve-trunks in early stages of development it will be well not to dismiss altogether the possibility that the ganglia are after all in continuity from the beginning.

From the basis of the sympathetic system so laid down extensions apparently sprout out into the various tissues which are eventually innervated by this system-—but again there has to be admitted great possibility of error. The problem of the mode of development of these obscure portions of the nervous system will probably only be satisfactorily settled after we know with certainty the processes at work in the development of the main nerve-trunks and ganglia.

THE ORGANS or SPECIAL SENSE.—-We may take it that in the-

early stages of the evolution of the nervous system, while this system was still a diffuse network, there, were already present

"scattered sensory ce1ls——-cells specialized for the reception of im- I1 OLFACTORY ORGAN 125

pressions from without. Local concentrations of such sensory cells and their further specialization for the better perception of some particular type of stimulus has led to the evolution of the various organs of special sense.

The special sense organs of the vertebrate fall into two categories —(I.) the organ of vision, perhaps the Oldest organ of special sense, which is developed within the limits of the central nervous system and (II.) the other organs of special sense which have probably arisen more recently from the sense cells of the skin outside the limits of the central nervous system.

As the organs belonging to the second category have evolved less far from the primitive condition they will be considered first. They appear to have become specialized functionally in two different directions, those in the neighbourhood of the mouth for the apprecia- tion of differences in chemical composition—the organs of taste and smell-—and those on other parts of the body surface for the apprecia- tion of vibrations of the surrounding medium———the lateral line organs and the organ of hearing.

OLFACTORY 0RGAN.—-—Tl1e olfactory organ arises in the form of a localized thickening of the ectoderm on each side towards the anterior end of the head. Later this thickened ectoderm becomes depressed below the general surface so that it assumes a saucer- and later a cup-shape; its external opening eventually becomes comparatively narrow.

In many of the Elasmobranch fishes the olfactory organ retains throughout life the condition of a simple inpushing of the skin opening to the exterior on the ventral side of the snout. In many Vertebrates on the other hand characteristic changes come about in the relations of the external opening. These will best be understood by considering first what happens in the lung-fish Protopterus as shown by Fig. 69. In A and B the olfactory organ is visible as a little rounded dimple on each side. In C the dimple has become a deep groove running obliquely from before backwards and outwards. It is further seen that this groove is becoming involved in the sinking in of the skin to form the buccal cavity. In D and E the groove has become a deep slit narrow in its middle part and dilated at its two ends. Finally, in the stage represented in F, the margins of the narrow part of the slit have undergone complete fusion so that the continuous slit of the preceding stage is now represented merely by its terminal portions which form two widely separated rounded openings——the anterior and posterior nares (oéf 1 and 04/’ 2).

Turning to the other Vertebrates we find various divergences from this simplest mode of origin of the external and internal nares as seen in Protopterus and Oemtodus. In the Actinopterygian fishes the phenomena are quite similar to those described, only here differential growth leads to the gradual shifting of the olfactory organ and its openin s from the ventral side of the snout up to its dorsal side. The resu t is a. topographical reversal of the positions of 126 EMBRYOLOGY or THE LOWER VERTEBRAIITES an.

anterior and posterior naris: the morphologically posterior naris coming to lie in front of that which was originally anterior. '

In the Amphibian and Amniote the upper lip, which completes the huccal cavity in front, is situated between the anterior and posterior narial openings, so that the latter opens into the huccal cavity

Flo. 69.-——Vcntral views of llutltl region of larva of Protoptc-rus at stages 31 (A), 32 (B), 34 ((3), 34 (D), 35 (E), and 36 — (F), to illustrate the development of the olfactory organ.

c.o, celnent organ: e.g, external gill; olf, olfactory organ; olf 1, anterior (“external ") naris; olf2, posterior (“internal “) naris. In C the curved line running across the ventral side of the head is the posterior margin of the mouth: the darkly sluuled grooves passing inwards and forwards from its

outer ends are the olfactory rudiments.

(internal naris), while the former remains outside (external naris). In the developing Amniote embryo (cf. Fowl, Chap. X.) the general arrangements, while essentially the same as those of Protopterus, are somewhat obscured by the modelling of the face region. The ridge which forms the upper lip, or anterior boundary of the buccal cavity, is out across by the olfactory slit, here a wide and deep cleft, into a u L OLFAOTORY ORGAN 127

median portion (median “nasal process) and a lateral portion (maxillary process).

The ridge hounding the olfactory involution on its outer side remains for a time separated by a distinct groove from the maxillary process but as the latter grows forwards it oblitcrates this groove as Well;as the superficial portion of the -cleft which separates it from the median nasal process, the deep portion of the cleft remaining as a definitive canal leading from olfactory organ to huccal cavity.

In Amphibians, as in Lepialosiren among the lung-fishes, the posterior naris is frequently formed as a secondary perforation which

Flo. 70.—~I-lorizontal h'L‘(‘.l-l()llh' through the olfactory organs of I’nlg/p(¢:ru.s* of stages 25 (A), 26 (B), and 27 ((3).

c.o, cement organ; olf, olfactory rueliment: opt, optic stalk; '1‘Im.l_. cavity of tlialaniencephalon.

breaks through from the posterior portion of the olfactory organ into the part of the huecal cavity lined by “ endoderm.” This is a secondary modification of a type which will be discussed in the next chapter in the description of the development of the huccal cavity in these forms. _ The first rudiment of the olfactory organ has been described as a thickening of the ectoderm. A.s in the case of other nervous or glandular developments of the cctoderm the superficial layer (Fig. 70) takes no part in its formation. Commonly it degenerates and disappears over the actual olfactory epithelium. Again, as frequently happens in the development of primitively hollow organs, the rudi-

ment may be for a time solid, forming a simple solid downgrowth in 128 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

which a cavity makes its appearance secondarily, the actual involu- tion of the surface being delayed or reduced or absent (Fig. 71).

Sometimes, as is well shown in the case of Poly/pteras (Fig. 70, A), the olfactory thickenings are at first continuous across the mesial pla11e and this fact, taken in consideration with the fact that in the Lampreys the olfactory organ of the adult is unpaired, obviously suggests the possibility that the olfactory organ of Vertebrates in general was originally unpaired (Kupffer). Though this must be admitted as a possibility the evidence does not appear to be suflicient to give the idea probability .in view of the fundamentally paired character of the portions of the brain associated with the olfactory organ, even in the case of the cyclo- stomes where the organ as a whole has an unpaired appearance.

After the olfactory involution has become definitely established it under- goes various complications of form, differing in detail in the various groups but consisting for the most part of

lining epithelium so as to bring about an increase in its area. In the Elasmo- branch these cutgrowths take the form of parallel grooves which gradually become converted into deep slits separated by thin partitions —-—- the Flo.’7l.—~Longitudinal vertical se<-,- .SChnelderi.anfO1dS' In Crossolmgrygians “on through ,..,,,w,te,.,,3 (smges instead of numerous folds with free 28-29), showing the olfactory rudi- edges complete septa are formed which "“’“‘ “S “ ‘“i"k.°"i"g °Lf “W .‘l°"1’ radiate out from an axis formed by layer of the eetoderm in which a . _ _ . . i cavity has developed secondarily. l-’he_ 01fa’Cl-‘Olly negrve and dlvlde lfhe .cavity as seen in transverse section into distinct chambers, the lining of which in turn forms deep folds. In the higher forms the outgrowths of the olfactory lining are fewer in number and the projections left between them form the turbinals which have characteristic arrangeinents i.n the different groups. Amongst the Reptiles a conspicuous development of the olfactory apparatus is the Organ of Jacobson. This arises as a pocket-like outgrowth of the lining epithelium, on its mesial side and near its ventral edge, which becomes gradually constricted off from the olfactory organ and opens into the buccal cavity in the region of the posterior nares. In Chelonians, Crocodiles and Birds this organ has disappeared except for a possible vestige in the form of a transient bulging of the olfactory lining. . A diverticulum which may correspond to J acobson’s organ makes its appearance in Lung-fishes and Urodeles but in this case it becomes gradually displaced outwards until it lies external to the olfactory

cavity.

bulgings outwards on the part of the’ ll OTOCYST 129

A curious, possibly adaptive, arrangement has been noticed in late developmental stages of certain Sauropsida, where for a time the external nares are plugged by a proliferation of ectoderm (Aplm-3/ac——— ’l‘. J. Parker, Sp/wnmlon-—Dendy). Such temporary obliteration of a channel at a period of development where it is unnecessary or harmful is a phenomenon which occurs fairly frequently: examples of it will be met with later in connexion with the alimentary canal and the excretory organs.

(_)'1‘0cYsT.—'l‘he Vertebrate possesses a pair of otocysts situated one on each side of the hind—brain. Each arises in the typical fashion familiar in the invertebrates, by a sensory portion of ecto- derm becoming depressed below the surface of the skin and eventu- ally isolated as a closed vesicle. As in the invertebrates certain of the lining cells of the otocyst secrete otolithic masses of Calcium carbonate.

'l‘he otocyst of the Vertebrate however shows two developments which do not occur amongst the invertebrates. Firstly, in connexion with the primitive function of the organ, that of balancing, the wall of the growing otocyst becomes moulded into the three semi- circular canals which are arranged in planes at right angles to one another. These canals have for their function the analysis of any rotatory movement into its components in these three planes. And secondly a special region of the otocyst wall becomes specialized in connexion with a new sense—~that of hearing——and grows out into a curved horn-like pocket, the lagena, which may become greatly enlarged and spirally coiled, in Vertebrates in which the sense of hearing is very acute, forming the organ known as the cochlea.

The development of the otocyst may be described as it occurs in the Fowl (liothig and Brugsch, 1902). The otocyst begins to make its appearance during the second day of incubation as a thickened area of ectoderm on each side of the hind—brain. This thickened area becomes depressed below the general surface, forming a saucer- shaped depression which gradually deepens till it forms a deep pit. The lips of this pit, especially dorsally, grow inwards so as to constrict the opening which is finally completely obliterated, the original open depression being thus (:0I1Ve1'te(l into a closed, somewhat pear—shaped, sac the otocyst. The wall of the otocyst remains for a time con- tinuous through a solid bridge with the outer ectoderm (Fig. 7 2, A) but as a rule this bridge persists merely for a very short time and only a small cellular tag attached to the otocyst remains to mark its position.

As development goes on the otocyst increases in size by growth of its wall and this growth is especially marked ventrally and later- ally with the result that the point which was originally connected to the ectoderm becomes displaced so as to be situated on the mesial side of the otocyst. This portion of the otocyst wall now comes to project upwards as a distinct pocket-like outgrowth the recess (Fig. 72, B, 9'). External to this a wider bulging of the wall fore-

VOL. 11 K 130 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

shadows the anterior vertical semicircular canal (am) and a little later a similar bulging more ventral in position-——the horizontal canal (lac). The lagena also is foreshadowed by a slight downward bulging of the floor of the otocyst.

With further development the posterior vertical canal rudiment appears also as a bulg- ing of the otocyst wall continuous with that which will form the anterior canal (Fig. 72, C-F, The three canal rudiments come to project more and more prominently, the recess assumes the tubular. shape of the endolymphatic duct and the lower portion of the otocyst (saccule) With its projectillg lagenar pocket and en- dolymphatic duct be- comes more sharply marked off from the rest of the otocyst (utricle). The pouch-like rudi- ments of the semicir- cular canals, as they come to project more freely from the utricle, assume a flattened form and finally the central FIG. 72.-—ll1ustrating the development of the otoeyst in the portion of the wall on

Fowl. (A-G after Rothig and Brugsch, 1902; H after each Side bulges in__

R t ' 5‘! I I e Zn") wards and fuses with

A and B, early stages; C, towards end of seventh day; 1), to- 0 wards end of eighth day; E, five days; I*‘, towards end of ninth n Other"

day; G, towards end of twelfth day; H, adult. A is a view from In llhit‘-l Way the in front, B from behind, while C-H represent the left otocy-it as gmltral pm;-fion of the seen from the left side. a. v, anterior vertical canal; h.c, horizontal - ,

canal: lag, lagena; pm, posterior vertical canal; 9', recess; s.e, Oavlty of each, Pouch endolyniphatic duct. be 0 em 6 S O b 11 terated

while the persisting peripheral part takes the form of a curved tuhe——the definitive canal. At first the space subtended by the canal is traversed by a continu- ous septum formed out of the fused walls but this soon disappears leaving the canal as a freely projecting arch which opens into the utricle at each end. The ampulla. appears at an early stage as a

dilatation of the canal rudiment at one end. As will already have been gathered, the three canal rudiments do II OTOCYST 131

not appear synchronously——the anterior vertical appearing first, then tl1e horizontal and finally the posterior vertical. The same order is followed in subsequent development the anterior vertical canal keep- ing ahead of the other two—probably a foreshadowing of the greater importance of this canal, parallel to the sagittal plane, in the function

of flight. It should be noticed that the posterior vertical canal assumes its

Fm. 73. ——De.velopn-lent of the otocyst in Lung-fishes as H(7!t‘l| in transverse sections. (From drawings by M. C. Cairney.)

A, Lepidosirerl, stage 21; H, I’-rotupterus, stage 23: (.7, l.:'pz'¢.los'z're-n, stage ‘._’.\‘. N, nut.ochm-cl; at, otocyst: rh, rhomliencephalon. In Fig. 0 the rudime-.ntol'1.he emlolympliatie duct is visible as an out- growth from the otocyst wall dorsally and mesially.

position at right anglesto the planes of the other two secondarily. At first its rudiment is continuous with and almost in the same plane as the anterior canal but as it assumes the tubular form it swings outwards and forwards slipping, as it does so, over the horizontal canal in the way indicated in Fig. 72, F, Gr, H.

The otocyst of Vertebrates in ;__ge.ne1‘al develops along similar lines to those described for the Fowl while presenting modifications in detail. The sensory epithelium is a development of the deep layer of the ectoderm and in cases where the ectoderm is distinctly two 132 EMBRYOLOGY OF THE LOWER VERTEBRATES on.

layered during the early stages of otocyst development the super- ficial layer is seen to pass over the rudiment unaltered (Fig. 73, A). In Lung-fishes the cavity of the otocyst appears secondarily in the midst of an apparently solid downgrowth of the deep layer of the ectoderm (Fig. 73, B) but the examination of earlier stages (Fig. 73, A) shows that here also there is an actual involution of the deep layer although there is at first no patent cavity.

In the Elasinobranchs the otocyst retains throughout life its con- nexion with the exterior, the connexion hecoming drawn out into a slender tube. In Birds the recess, and therefore the endolymphatic duct, represents the remains of this original connexion, but curiously enough in certain other Vertebrates 6._(]. Lung-fishes (Fig. 73, C) the last connexion of otocyst with external ectoderm lies lateral of and somewhat anterior to the endolymphatie duct which latter here develops as an independent outgrowth of the otocyst wall. This is to he looked on as a secondary modification of the more primitive arrangement seen in Elasmohranchs. The structure named endo- lymphatic duet in Tel:-osts also arises as a secondary outgrowth of the otocyst wall.

The endolyinphatic duct or recess commonly persists in the adult as a conspicuous blindly ending diverticulum of the otocyst wall. In Lung-fishes and Amphibians its wall proliferates actively giving rise to projections which in the Lung-fishes and some Urodeles, e.g. the Axolotl, meet to form an irregular sac over the roof of the fourth ventricle. In the Anura the irregular thin-walled sac formed in this way spreads forwards and also laterally until it hecomes continuous ventrally so as completely to surround the 11ind—brain. An unpaired prolongation of this sac extends tailwards immediately dorsal to the spinal cord within the vertebral canal. Paired outgrowths of this extend outwards along with each spinal nerve and expand at their ends round the spinal ganglia to form the calcareous bodies so con- spicuous in the adult frog. The whole system of outgrowths is con- spicuous in the adult from the white otolithic particles in its interior. The vertebral portions become eventually broken up into a network of irregular tubes which is interpenetrated hy a network of capillaries (Coggi, 1889).

Somewhat similar outgrowths of the endolymphatic duct make their appearance in Sauropsida although in this case they do not undergo the wide extension that they do in the Anura. In the Geckos however they do hecome extended so as to form a large superficially placed irregularly lobed sac which covers over a great part of the neck region close under the skin (Wiedersheim).

LATERAL LINE ORcANs.——']‘hese sense-buds (neuromasts), which are found arranged in rows on the head and body of fishes and aquatic amphibians, take their origin in linear thickenings of the deep layer of the ectoderm which spread along the surface of the head and body and eventually become segmented up into separate pieces. In correlation with the function of these organs, which II N ERV 0 US SYSTEM 1233

apparently is to detect slow vibrations in tl1e water and is therefore closely allied to hearing, it is of interest to notice that the ectoderlnal rudiment from which tl1ey arise appears to be in some cases continuous at first with that which gives rise to the otocyst.

The. sense organs are in correlation with their origin at first placed superficially but as de.velopme,nt goes on they in most cases become depressed beneath the surface either in isolated pits or in continuous grooves. The latter may in turn remain ope.u or may become covered in to form tubes except where at intervals openings remain leading to the exterior. This is the condition which is reached in the adults of the majority of fishes.

'l‘he lateral line. sense organs being correlated with an aquatic habit commonly degenerate on the assumption of a terrestrial existence. Various Anura however which remain purely aquatic after metamorphosis retain their full equipment of lateral line organs. 3

ORGAN O14‘ l’INKUs.~—ln Le])'£(l0s’i7'('n. and 1"I'0to/)l6'r'us a peculiar organ of special sense. lies deeply embedded in the tissue on each side of the. head in close contact with the wall of the auditory capsule. This organ, discovered in I’7'olop/crus by Pinkus, has been shown by Agar (1906) to be developed from the eetodermal ingrowth which forms the outer end of the spiracular rudiment.

l_§_¥_E‘—-As the eye develops in the same general manner, differing only in detail, in the dilferent subdivisions of the Vertebrata it will be convenient to describe first its development in the Fowl——the Vertebrate of which it is easiest to obtain material for practical study.

The first obvious rudiment of the eye consists of a projection of the side wall of the thalamencephalon which juts out at right angles to the axis of the body and gives a characteristic ha1nmcr- shape to the fore-brain region (see Fig. 231, Chap. X.). A transverse section across the head near its front end in a chick about the middle of the second day of incubation shows (Fig. 234, D) the thalamencephalon extending out on each side as the optic outgrowth.

As development goes on and mesenchyme accumulates between the brain-wall and the ectoderm the proximal part of each optic outgrowth becomes constricted, from above downwards, to form a relatively narrow optic stalk (Fig. '74, A, B, D, 0.3). The optic outgrowth is closely apposed against the inner surface of the external ectoderm and a slight thickening of the latter soon becomes apparent just where it is in contact with the surface of the optic outgrowth in (Fig. 74, B, Z). This thickening is the first rudiment of the lens. The lens-rudiment gradually becomes sunk below the general surface to form a saucer- and later a cup-shaped depression. As the rudiment becomes involuted in this way, the outer wall of the optic outgrowth also becomes 134 EMBRYOLOGY OF THE LOWER VERTEBRATES C11.

invaginated to fcmn u. cup—1ike struct;ure—the optic cup (Fig. '74,

Flu. 7v1.———]')«_:v.-I..1.nn-nt. of the eye as seen in h'aI1s\'el'.~u_- .s'«-ctions of Fowl e1nln'_vn:~'.

A, latter half ()f.\'P,('nnal .1.-ny nt‘in(:nb:1ti«-n; H, mm] of so-ennui «lay; C, 21_;(1a,ys; I), 3 «l:1_\-s: E, Intim- half offiftth day. wt, 0_\;h'.'I'I1:11 c.-ct=0«lem'1 : I, la-nn; ~.r_, rmlixm-Ht of rye; n.s, optiv stalk; ;:.1, pi;_;'nn-n1 layer of retina. ; 7', l‘(‘.till:l ; Hm], wall of Llmlamumuo-plmlol1.

B_ and C). The cup-like lens-rudin1ent- b'ec.0mc-s g1‘a(‘l11all_y con- 11 EYE 135

stricted off and finally completely separated from the outer ectoderm (Fig. 74, U and 1)). '

In the meantime a marked difference becomes apparent between the two layers forming the wall of the optic cup. The layer next the cavity of the cup becomes greatly thickened its cells becoming tall and columnar: it forms the rudiment of the visual layer or retina in the strict sense. The outer layer of the cup-wall on the other hand dcgenerates, it becomes thinner and later it de- posits melanin pigment in its cells. lt forms the pigment-layer of the retina.

The invagination of one wall of the optic outgrowth within the other is not confined to that portion of the outgrowth in proximity to the external ectoderm as might be supposed from the description so far. The invagination involves also the ventral wall of the rudiment towards its outer end and for some distance along the optic stalk. The result is that the wall of the optic cup is interrupted by a gap ventrally the ghoroid fissure 1—-and that the optic stalk for seine distance from the optic cup has_a deep groove along its ventral side.

The cavity of the optic cup, as is the case with cavities generally in the embryonic body, becomes filled with clear fluid secreted into it by the surrounding cells. This fluid becomes jelly- like later on and forms the basis of the vitreous body.

As development goes on the eye increases greatly in size and assumes a spherical shape, the lens blocking up its opening towards the skin and the choroid fissure becoming obliterated by its lips coming together. The site of the fissure remains apparent for some time owing to the formation of pigment in the pigment-layer being delayed in its immediate neighbourhood.

As the eye increases in size the Retina for a time grows more actively than the rest so as to be thrown into wrinkles (Fig. '74, E). The lens which was a hollow vesicle becomes solid its cavity being filled up by a great thickening of its deep wall, the cells of which grow out into a tall columnar form (Fig. 74, D and E).

The essential parts of the eye as an optical instrument have now been laid down-—the lens for the production of an image, the retinal wall of the optic cup for the reception of that image and the conversion of its light waves into nerve impulses, and the optic stalk which will become the optic nerve for the transmission of these impulses to the brain. To these essential parts there are added various accessory structures developed from mesenchymatous cells which accumulate round the parts of the eye already formed. In particular there is formed a protective capsule of tough con- nective tissue -——-the sclerotic with its transparent portion the cornea, covered externally by the ectoderm forming the corneal

1 The term “ choroid” fissure is in reality misleading, having been adopted when the fissure was interpreted as a cleft in the choroid, in the days betore the existence of the pigment-layer of the retina was recognized. 140 EMBRYOLOGY OF THE LOWER VERTEBRATES (311.

along the deep surface of l)esce1net’s niembraiie and there settle down to form a single layer of flattened cells. ()n the deep side of this corneal endothelium a split gradually develops in the jelly-like inatrixr this contains a watery secretion (aqueous humor) and becomes the anterior chamber of the eye.. The portion of matrix lying superficial to l)eseeniet’s inenibrane becomes colonized by cells from all round its margin. lt l'oifi'ns the main portion of the cornea, while a thin layer lying next the ectoderm remains uncolonized and gives rise to Bowma.n’s membrane. -

VITREO U S .l‘i()L)Y.~'~,lllH‘. cavity of the optic cup is l'i'o1n the beginning filled with clear fluid which l<eeps it distended and there is no an- parent reason to assume that tliis arises otherwise than by the same method as holds with the eyes of many iiivertelirates as a secre- tion of the surroiiiidiiig retinal cells. The fluid gradually acquires the jelly—like consistency charactei'isti(-, oi" the .i'ully—fo.r1ne(l vitreous body.

Anioebocytes wander at a coni- paratively early perioifl into the Vitreous rudiment —— i.n the Fowl embryo about tlie third day-Mark Hill (talk)and at a later period a continuous mass of inesencliyine tissue projects into it through the choroid fissure. This mass of meseiichynie develops :1. Flu. 78.~—Semi<lizigraniniatie figure of the IlPl3WO1‘l{ Of l")l(f)()d-V8SS(‘.lS COI1l3l1l1l('_)11S

l’iS‘.’°ted eye Of.“ V°””l""“"‘ ""‘b"Y". with those of the surrouiidiiig tissue. (ltima. 8 min.) to show the course of . A . . . , 5 g the optic iierve-fibres (after Asslieton, In thii llmrii l)r1m1l’1Ve \ ”rl’el)ra‘teS 1892). this niesenchyinatous mass reaches c.f,Wall of cl10I‘0i(lllssuI'e; g/,gaiiglion-cell; T10 g1‘Ud:l3 (lE‘:V(‘/lUp1l’l811l} but lll t»l1C

  • 3 i'”"__“"i""““'~ S“I’I’.°““"3~ °““3 73 l*’"_**? “-15 Teleostei and the Sauropsida, the

nerve-libres: 12.1, pigment layer of retina; n _ -_ - - _ A p(]‘r($iI-’i‘_“'-lflcéfillg ’ most highly specialized groups amongst the non—1namnialian Verte- brates, it does so and persists throughout life, as the falciform process with its 111118018-lll)1",SfOl‘ the purpose of accommodation in the one case (Teleostei), and the highly vascular, and probably mainly lllltritive, pecten in the other (_Sauropsida).

OPTIC N ERVE.—-—AS already indicated the optic nerve is not strictly speaking a peripheral nerve at all. It is a slender drawn-out portion of the brain analogous with the olfactory tract of a teleostean fish, con- necting the main portion of the brain with the small highly special- ized portioii which has become converted into the optic cup. Its function being a conducting one the main mass of this stalk-like portion of the brain is composed of white substance or nerve-fibres.

These fibres instead of passing outwards over the rim of the cup 11 RETI N A ' 137

RE'1‘INA.——The fully-developed retina—which it will be re- inenibered is Inorphologically a specialized portion of the brain-wall -—-is an organ of extreme complexity. Its structure even in the adult is by no means completely worked out, and our knowledge of the details of its histogenesis is most imperfect. The most conspicuous feature is the great increase in thickness, the retinal cells becoming slender and columnar in form. Later on the nuclei are seen to become arranged i_n layers, this being an expression of the fact that the cells are also becoming specialized into layers of different structure and function. The details of development of these are almost completely unknown and there is here an interesting field for investigation.

The layer of visual or percipient cells lies on the proximal side of the retina, and their rods and cones—the special parts of the visual cells which are believed to have the function of converting light

, Fm. 76.— Illustrating the development of the rods in Lep2'o'om,'rc7-1-. The upper side of the. figures represent the side turned away from the lens.

A, B, C, D from stage 35 ; E, fully «Ieveiupecl visual cell at stage 38, tlxed during exposure to light: E*, similar element killed in the «l:u'l\'. 11.2‘, annul:u' vacuole; f._q, fatty glolmle.~< stuinetl black by osmic acid; ‘m.-.l, external limiting membrane; n, nucleus of visual cell; 7-, rod.

waves into nerve i1npulses——are at the ends of the cells which point away from the lens. To reach these rods and cones the light rays have therefore to traverse the whole thickness of the retina. This remarkable arrangement of the retina, precisely the opposite of what we should expect, is one of the characteristic features of Vertebrates. Its morphological significance is however at once made clear by a consideration of the main facts of development of the eye as already outlined. These, in fact, show that what becomes the proximal surface of the retina, 73.9. the surface which faces away from the lens, was originally part of the inner surface of the brain rudiment and therefore of the outer surface of the ectoderm before it became involuted to form the brain.

The visual cells develop therefore on what was originally a part of the outer surface of the. body and their rods point in a direction which was originally o1.i_twards. The mode in which the rods develop is illustrated by Fig. 7 6 which is taken from Lepidoszwnrz, (Grahain 138 EMBRYOLOGY OF THE LOWER VEIWEBRATES CH.

Kerr, 1902). Similar observations have been subsequently made in the case of Amphibians.

'J‘be first obvious step in the specialization of the. visual cell is the. appearance of a fatty globule in its protoplasm. The end of the cell turned away from the lens now grows out into a projection and pushes back the fine cuticular limiting membrane (external limiting membrane) which has developed over this surface of the retina, into a little pocket. 'l‘he oil globule which gradually increases in size passes into this pocket (Fig. 76, A, f.g) and lies in it cnsheathed in protoplasm. 'l‘hc protoplasm now becomes heaped up into a little conical protuberance (Fig. 76, B, 7') which is the rudiment of the rod. At iirst the limiting membrane is distinct over the surface of the red but gradually, as the latter assumes a cylindrical shape, its protoplasm takes on a clear structureless appearance throug_»,hout: it apparently becomes in fact converted into cuticular material. This cylinder of euticular Inaterial increases in size, assumes a character- istic appearance with alternating discs of dimmer and more trans- parent material as seen in the fixed specimen and the rod is complete (Fig. 76, E and Ei‘).

The rods complete their development sooner or later according as they are nearer or farther away from the optical axis ol the eye and their time of development shows great variation in dill'erent individuals. The cones in those Vertebrates in which cones are present are merely specialized rods.

LENs.—-—The lens shows in its early stages, in various groups of Vertebrates, departures lrom the normal condition as described for the Bird, of exactly similar kinds to those seen in the development of the Otocyst. In particular, the lens tends to develop out of a solid downgrowth of the deep layer of the ectoderm. This is well seen in Elasmobranchs (Fig. 77, A-E) where a rounded solid lens-rudi- ment is formed by proliferation of the ectoderm, this rudiment becoming isolated and developing a cavity secondarily. It is of interest to notice that even here a slight dipping down of the external surface into the lens rudiment is apparent for a time (Fig. 77, B).

In Amphibians, Lung-fishes and Teleostoinatous fishes the lens arises in a manner intermediate between what occurs in Elas1no- branchs and what occurs in Sauropsida. In the forms mentioned the lens arises as a downgrowth of the deep layer of the ectoderm (Fig. 77, F-I) and in some cases this downgrowth is simply an invagination of this layer, the only difference from the Sauropsidan condition being that here the opening of the invagination is closed by the superficial layer being continued across it (Fig. 77, J, K).

As regards the later stages in the development of the lens all that need be said is that it undergoes an enormous increase in size--by absorbing nourishment from its surroundings, for it has no blood- vessels-——the cells of the deep wall becoming greatly elongated and taking on a clear glassy appearance, while the superficial wall remains as a layer of cubical epithelial cells over the outer surface of the lens. II EYE 139

SCLEROTIC, CORNEA, CIIOROID.-—These portions of the eyeball are gradually differentiated out of mesenchyme which becomes concentrated round the primary parts of the eye. hi the case of the

cornea the first stage in the developmental process consists in the accumulation between lens and ectoderrn of a clear jelly-like secretion

Fm. 77.—-Variations in the early stages of the development of the lu_-ma‘. A-I4}, I'ri.~<ti.ur1L.-: I"-I, ._\'im_1nn (after Rab], 1898).: J, K I’hyl£o-metlwsa (after Jimlgvtt-, 189:’).

continuous, and identical in character, with that which fills the optic cup. , _ As development goes on (Knape, .1909), a thin layer of this jelly-like material, about midway between the lens and the ectoderm, becomes condensed to form the rudiment of Descemet’s membrane. Amoebcid cells from the mesenchyme round the optic cup creep en. III THE ALIMENTARY CANAL 145

f

becomes extended into the form of a thick-walled tube-——the rudiment of the intestine (eat). From the stage of Fig. 80, E, onwards active growth of the true tail or postanal _region is taking place, and it is noteworthy that, during this process, the endoderm retains for a considerable time its continuity with the mass of actively growing unclifferentiated tissue at the tip of the tail and becomes drawn out into a cylindrical postanal ‘gut (pay). This remains conspicuous for a time but eventually clisintegrates and disappears completely. The main mass of yolk-cells, forming the ventral wall of the middle part of the enteron, gradually shrinks in volume, as the yolk is absorbed and carried off by the circulating blood for distribu- tion to the growing and developing ialSS11(3S of the body, and eventually the gut Wall is no thicker in this region than it is elsewhere.

BUCCAL CAV1'rY.-——The alimentary canal of the adult Vertebrate commences with the buccal cavity which is in part at least-as shown by the presence within it of placoid and glandular‘ elements corresponding with those of the skin-——ston1odae.al in its nature. The stomodaeuui however is not as a rule developed, as is so usually the case in the Invertebrates, from a simple involution of the ectoderm forming a depression of the surface below the general level. It arises rather by the walling in of an area on the ventral side of the head through »the development of ridge-shaped outgrowths. These ridges may he termed respectively the maxillary ridge and the mandibular ridge accordingly as they give rise later on to the upper or to the lower jaw.

The roof of the buccal cavity, or at least its anterior portion, is simply to be looked on as part of the primitive ventral surface of the head, delimited by the maxillary ridge on either side. The floor, on the other hand, represents the mandibular ridge (Fig. 80, H, mm) which has grown forwards in a direction parallel to the roof. The inner wall of the buccal cavity is in close contact with the anterior extremity of the endodermal alimentary tube but for a time the two cavities remain separated by a thin membranous diaphragm made up of the apposed layers of ectoderm and endo- derm. This may conveniently be termed the velar membrane as the organ known as the velum in Amp/m'0a3us or I’et7°omy/zen consists simply of the remains of this membrane.

The formation of the anterior, stomodaeal, portion of the buccal cavity is seen in its simplest form in some of the lower holoblastic

,Vertebrates such as Urossopterygians, Lung—fishes or Urodele

amphibians.

In Fig. 69, D (p. 126) in the case of I’7*()t0/ate7°us, or in Fig. 100 (p. 178) in the case of Pol]/pie-7"ws (see also Fig. 80, H), what will become later the anterior part of the buccal roof is seen to be simply a portion of the ventral surface of the head, bounded behind by the transverse mandibular ridge———the rudiment of the lower jaw——and on each side by the longitudinal maxillary ridge.

As is well shown in the figure of Polg/pterus, and as is also the

VOL. II L 11 EYE 141

all round, as they possibly did originally, have become crowded together during the. course of evolution into a single large bundle. on the ventral side of the cup. In accordance with the general principle of economy of tissue this bundle of nerve-fibres has become sunk into a deep notch in the wall of the cup-—the choroid fissure-—-so that it passes directly to the optic stalk.

While this is probably a correct statement as regards phylogenetic evolution, matters are somewhat simplified in the development of the individual, inasmuch as the choroid fissure is brought about not by the notching of the a.lready formed cup rim but by the rim ceasing to develop at the site of the choroid fissure while it grows actively everywhere else.

As regards the development of the actual nerve-fibres, all that need be said is that they first make their appearance in the wall 01' the optic stalk ventrally and that they increase rapidly in number, passing between the epithelial cells of the stalk, loosening them out,and causing them in great part, if not entirely, to degener- ate. The individual fibres certainly for “ the most part become diffel-entialtcd in a FIG. 79. Transverse sv('ii()l1 through the still open neural cmltripetfll (h-rection plate of [man palmstrzs near its anterior end, showing the

_ _ . position of the optic rudiments (E) already marked out by ‘I/.6. l'[‘01I1 l':llO 1‘£'.l;11ld: l:0- the formation of pigment (after Eycleshymer, 1895).

wards the brain, but

whether this means that they are actually sprouting out from ganglion- cells of the retina as is generally believed, or on the other hand that their fibrils are simply becoming differentiated centripetally within a continuous prc—existing protoplasmic connexion, remains to be demonstrated.

EMBRYOLOGY AND THE EvoI.U'r1oNAnY ORIGIN or THE EYE.-The peculiar reversed position of the Vertebrate retina may perhaps be taken as an indication that that organ had already come into exist- ence, though no doubt in a very simple form, at a stage in Vertebrate phylogeny when the central nervous system had not yet sunk down below the surface. It is therefore of interest that in certain Verte- brates the rudiment of the retina does actually become apparent during embryonic development at a period when the medullary plate is not yet closed in. Thus Eycleshymer (1.895) has described i11 Rana pnlustmls and in Amblg/sloma how a patch of pigment appears for a time on the surface of the medullary plate (see Fig. 79) in the position which will later on form the optic outgrowth.

Although we are perhaps justified in believing that the eye of existing Vertebrates was already present as a patch of epithelium sensitive to light in the far back evolutionary period when the fore- 142 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

runner of the central nervous system was still a portion of the outer surface of the body, we do not, in the present writer-’s opinion, appear to be justified in connecting up the eye of typical Verte- brates with the “eyes” of Amp/mloams or of Tunicates. It seems more probable that the eyes of these highly specialized creatures are organs which have developed independently Within their own groups.

l’ITUITARY Bonr.——'J‘o be included amongst the derivatives of the ectoderm is that enigmatical organ the pituitary body (“ Hypophysis

cerebri”). This——the “anterior lobe” of the pituitary body in '

mammalian anatomy-«arises normally as an iugrowing pocket of ectoderm on the ventral side of the head, situated as a rule close to the hinder limit of the stomodaeum but in the case of the Cyclo- stomes just outside its anterior boundary. This pituitary involution extends inwards beneath the infundibulum. In the Cyclostomata it retains its original form of a tube communicating with the exterior, but in the gnathostomatous Vertebrates its outer end becomes gradu- ally eoustricted into a narrow duet which in the great majority of cases becomes finally obliterated, so that the organ new forms a closed sac lying immediately underneath the infundibulum. The wall of the sac sprouts out into numerous tubular projections between which develops highly vascular niesenchyme, providing the rich blood supply necessary to the definitive function of the organ as a ductless gland.

As regards variations in the development of the pituitary involu- tion: it may arise as a solvld ingrowth of ectoderm (Lung-fishes, Am- phibians); it may be two-lobed (Teleosts) or three-lobed (Laeertilia) in its early stages ; its external opening may become secondarily dis- placed up on to the dorsal side of the head (Lampreys); its inner end may come to open secondarily into the pharynx (Myxinoids). As already indicated the wall of the infundibulum in the Gnatho- stomata comes into intimate relation with the pituitary body in the restricted sense, forming the so-called posterior lobe, or cerebral

portion, or nervous portion, of the pituitary body.

LITERATURE

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Agar. Anat. Anzeiger, xxxv, 1910.

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Balfour. Phil. Trans. Roy. Soc., clxvi, 1876.

Balfour. Monograph on the development of Elasmobranch Fishes. London, 1878. Bidder und Kupfier. Untersuchungen iibvl‘ die Textur des‘Riickenmarks und die

Entwickelung seiner Formelemente. Leipzig, 1867.

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Kerr, Graham. The Work of John Samuel Budgett. Cambridge, 1907. Klinckowstrom. Zool. Jalirl). (Anat;.), vii, 1894.

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Scammon. Keibels Normcntaf'cln zur Entwicklungsgcsclriclitc, xii, 1911. Schwann. Mikroskopischo Untersuchungen iiber die Ubercinstimmung in der

Struktur und dein Wachsthum der Thiere und Pflauzen. Berlin, 1839.

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Winkler. Arch. Entw. Mecln, xxix, 1910. CHAP'l_‘l+3R III THE ALIMENTARY CANAL

THE alimentary canal or enteron‘ of the V crtebratc consists of a tube passing from the mouth to the anus. The wall of this tube is known technically as the splanchnopleure in contradistinction to the somatopleure or body-wall (Ballour). It consists of an inner lining epithelium, the endodcrm, cnsheathcd in a complex coating of mesoderm——the splanchnic mesodern1———consisting of connective tissue, blood-vessels, lymphatics, nerves, and coelomic or peritoneal epithelium.

As is commonly the case in other metazoa the cndodermal lining is in the Vertebrate more or less encroached upon at’ the oral and anal ends of the tube by the spreading inwards of ectoderm. The parts of the tube which come thus to be lined with ectoderm are known as stomodaeum and proctodaeum (L-ankester, 1876) while the intervening region lined by cndoderm is known as the mesenteron. In the Vertebrata there is very slight develop- ment of proctodaeum but an important section of the buccal cavity is, as will be seen later, stomodaeal in its nature.

It is also customary in embryological writings to use the some- what loose expression foregut for the anterior portion of the alimentary canal (reaching back to the pylorus or to the opening of the bile-duct), which in the meroblastic vertebrates becomes differentiated oil‘ from the yolk-sac comparatively early in de-

velopment. . _ . A good idea of the blocking out 01 the main regions of the

alimentary canal in one of the lower vertebrates is got by inspecting sagittal sections of embryos and larvae at different stages of develop- ment such as those shown in Fig. 80. From the gastrula stage (A) on to the stage illustrated in Fig. 80, C, the endoderm forms a simple sac with its opening posterior (anus) and with its ventral wall greatly thickened owing to the fact that its cells contain the main store of yolk. From the stage of Fig. 80, D onwards the foregut (f.g) becomes gradually constricted oil‘ in a tailward direction from the mass of yolk, while at the opposite end of the

body, correlated with the outgrowth of the posterior trunk region of

the embryo and the backward shifting of the anus, the yolky mass 144 152 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Wards of the communication between olfactory and buccal cavities, a process which reachesits extreme in the Crocodilia where the palate extends back to about the level of the glottis.

STOMODAEAI. GLANDS.———Whereas in the majority of Fislies the stomodaeal lining possesses only isolated gland-cells, in the air- breathers on the other hand there are developed definite multicellular glands. These originate as a rule from solid down—growths of the

F10. 85.-View of the roof of the mouth in three species of Lizard (A, 19'_(/«ernw Ifingjivf ; B, Mabuia. qmfnqmflaemfata; U, Lygosonm. ‘l“ll.f(.’.S'('(’I'l.S‘), illustrating the shifting back of the communication between nose and month. (_ After Voeltzkow. 1899.)

rh, recess into which primitive posterior nan-s open ; pal, palatim.-. ; pt, pterygoid ; tr, transverse hone; no, vomer.

lining epithelium which develop a cavity secondarily. In Urodeles there is, as already mentioned, a special aggregation of these glands forming the gland-field in front of the tongue, while a single gland of considerable size develops from the roof of the mouth in the region between the olfactory sacs (lnterniaxillary or internasal gland).

In terrestrial Reptiles glands are present in numbers on ‘the roof of the mouth (Palatine), beneath the tongue on each side of the middle line (Sublingual) and along the edge of the month just external to the row of teeth -(Labial). The poison glands are specialized and enlarged labial glands of the upper jaw except in 146 EMBRYOLOGY 014‘ THE LO\V'ER VI|+}l't'l.‘EBRA’l.‘el'<}S (“.11.

case with I’roto1;le'2--u..s:, the huccal roof‘ in froiit, that is to say in the neighbourhood of the mesial plane, 1)&Lb'SGH without interi-upticm into the external skin: in other wo_rds the maxillary ridge is not con- tinued to the mesial plane so as to meet its fellow. In later stages

I~‘1o. 80.——Sagitt9.1 sections through P()l;/p/.(:.-e:l.s‘.

A. 3t9-L'.0 14.: 13. St--".‘-W W: C. Stage 20: 0, stage 2: ; E, stage-24 +. (1., mos; m-, :ll.1'llI--llt.I'l‘()Il ; e-ml, "-l1‘l"J"1<‘J‘||l1 "71’: ‘‘“l'‘_‘(‘- W‘ ll"-.\' 3 f-’’- ‘‘”\'“.V H!‘ forvln-uin: f.!7. fun-gut: N. uo1..nclmr(l 2 2).,/I. l""i"“"‘.V "*1" (If hruin ll-mr; pin, pnneul rmlivm-n1"; .-_:_-, ¢-;u-i1_\' of spinal cum"! ; 3/, yolk,

the roof of the mouth would be hidden in a view from the ventra side owing to the forward growth of the lower jaw. ' The anterior portion of the buccal cavity in Urodele A11l1)l'1il)ifl.l1S arises in a manner essentially similar to that descrihed above. In the G-ymnophiona and the Amniota a characteristic 1'nodifi.ca- tion of the mouth margin is‘ brought ahout hy the fact that, as III BUCCAL CAVITY 147

already mentioned, the maxillary ridge is cut across by the olfactory groove and so divided into the outer maxillary process and the inner median nasal process, the latter of which is continuous with its fellow across the mesial plane, forming with it the so-called

fronto-nasal process (see Chap. X.).

Fm. 8OA.~-—eS:t;;'lll:I.l Ht'('-ll()IlH txhroligh .I‘u/_:/}af:‘:'u.s".

1.‘, _\.1_ug., 3.3; G‘ smge -39; H, St:l‘L‘D‘[| :;-_>_ n, anus; u_«-, :ll|l.f‘l'l0l‘ (-onnnissllrn; 4-lo, optic cliiusrnu:

l clones.‘ ml enlmic c:l\'it.- : 1-]. heart : hm. halu-nnlar u-onnnissui'e: l1'., li\'<'l': m.r. m:m«lil.ul:u* r-i«lgo- , . . , Y _ _ _ 2,, pjt;ujt;n~y -;m-ulutimi ; pm, [_'M)h‘l.('l'lH1‘('l)]]1l|)iSfil|1‘!'; ,.n._:7, pH.~4l..'{ll:ll _-.;ut.: yum, pzuu-I-o-:it'Ir rudiment‘; /"rm. pineal rudiim-.nt.; 5’, .<t«mi:u:ln : y, yolk.

It is of interest to notice that in various Vertebrates the buccal opening is at first elongated in an antero-posterior direction instead of from side to side. Such is the case with Scylliuqn (Sedgwick, see Fig. 81) and ’1'o'rpedo amongst Elasinobranchs. In these cases the slit-like mouth is bounded on each side by a longitudinal ridge. 148 EMBRYOLOGY or rm: LOWER VERTEBRATES (:11.

Later on each ridge becomes sharply bent, about the middle of its length, in such a way as to give the buccal opening a rhomboidal shape and at the same time to mark oil’ the ridge into a maxillary portion in front and a mandibular portion behind. In Anura a somewhat similar arrangement is found.

“ENnonEuMAL” SECTION or BUCCAL CAvrrv.-—'l"‘he fully developed buecal ea.vity has incorporated in it a posterior portion- varying in relative extent in dil'l'erent Vertebrates—which is de- rived not from the ectoderm but from the anterior portion of the “endodermal” enteric rudiment. The simplest way in- which this portion becomes added to the anterior portion is seen in those Vertebrates in which the anterior part of the cnteric cavity is patent throughout develop- ment. In this case the velar membrane simply r u ptures——-its reinnants soon be- coming absorbed-- and the stomodaeal cavity is thrown into open communication with ‘ the cnteric cavity. This is the case in certain Anura (Rana) and in Amniota.


Fm. 8l.—--Ventral \'lt‘\\‘ of head region of embryos of Sag‘:/lliu-nz ¢-mu}-u/u. (After Sedgwick, 1892.)

A, 7—8 mm. ; 13, Sll_L','l)ll_\-' more :ulv:uu-ed than .\ ; C, H -12 mm. ; I), '16 mm.

brat-es no velar mem- brane is present, owing to the fact that the foregut either becomes solid for a time (Pol;/pteru.s, Fig. 80, D—Gr) or is so at the beginning (Teleostei, Urodela, Lepirlosvlren and Protopterus). In such cases the peripheral layer of the yolky foregut rudiment gradually assumes an epithelial character and the yolk along its middle breaks down, so that a cavity arises—continuous with the stomodaeal cavity and forming the hinder section of the definitive buccal cavity. The pro- portion which this posterior portion bears to tlfe anterior section derived directly from the outer surface is very different in different groups. It apparently attains its maximum" in Teleosts where

-it forms practically the whole of the. buecal cavity.

' Points of critical importance to the germ—1ayer theory are raised in this connexion by the fact that teeth, organs belonging originally to the outer surface, are developed in this posterior region of the buccal cavity from yolky “ endederm.” This is well seen in a Urodele, ‘or a lung-fish such as Lepidomlren or Protopterus (Fig. 82). The attempt is made to get round this difficulty by assuming that the layer of epithelium which makes its appearance over the surface of the buecal rudiment, and in relation with which the teeth develop, is really an ingrowth from the ectoderm.

In many V erte- T III BUCCAL CAVITY 149

It is, as a. matter of fact, quite continuous with the ectoderm,

FIG. 8‘2.—Sagittal section tl1rou;;l1 lu-.:ul I't’gi0Il of a Protopturus larva (Stage 33).

Inc, buccul ts-uvit-y: h.I, :ml<-riur l_u_nn1<l:u‘_\' of tongue; N, nulm-lnm‘41: pin, pine":-al lmtly: /W-I‘. pm-uphysi.-.: l’iI., pit: 5t.:u-_y bc«l_\': Th, tllymitl ruuliment; Lu, H.-rtum «-ptimluv. Th.-. p()sil.inn nr dvlllul rudiments is in<li<-.atml by Hue hm upwzu-d pr(r_ic:(:l.i()nS of the dOI".~m| wall of the buccal cavity.

but exaniinution of carefully prepared celloidin sections (Fig. 83) shows that at its inner end the epithelium passes by imperceptible

A 8

FIG. 83.—Sagittal N'(_‘llHllH t.h1'ough the 1'egi<_n1 of the lulucul cavity of (A) Lcpi¢.io.s£'rc1t, stage 30, and (B) .lmI:[.;/.s-Iowa, 7'5 mm. in i(‘ll§'.'_,'iiI.

b.c, bu(‘c::l opitllvlilllll ; 4'-cl, l‘L'tmlBl.'lll : _I/, .s'(_)litl Illziss of yolk-(.'I-ll.~: in pu.\'itiun 01' i.|ll(.‘('2tl vzivity.

gradations into the ordinary yolky endoderm, with no trace of the sharply defined edge which it would possess were it a 1uyer__of 150 EMBRYOIJOGY OF THE LOWER VERTEBRATES CH.

ectoclerm pushing its way inwards. It extends inwards simply by a process of delamination from the yolky “ endoderm.”

The real lesson to be learnt from these cases is that the charactcrs of one gcrm—layer are liable to spread over its boundary into l3C.5]‘l‘ll'.()Ty belonging to another layer or, in other words, that the territories of the various layers are liable to be separated by an indefinite debatable zone rather than by a mathematically sharp li.ne. It follows that the apparent position of an organ-rudiment in relation to such a boundary is not necessarily to be taken as

Fm. 84. —-Sagittal sections illustrating the development of the tongue in l_.l1'm_lclc..~'.

.\ and I5, Triton; C, Sula.-armndra (after Kallius, luol); _«_/.1‘, gland tie1d;__M, mandibular arch ; 7».I, priuuu-y t.ongLu.-.

giving any definitive proof as to which of the two cell-layers that organ belongs to.

Tim 'I"<mcUE.——The tongue is a portion of the buccal floor which becomes demarcated off from the rest by a split formed by a down- growth of the lining epithelium of the mouth. is mode of develop- ment is well illustrated by what happens in Urodele Amphibians_as described by Kallius. Here tlwru develops first a primary tongue, ensheathing the anterior and ventral portion of the hyoid arch (Fig. 84, pt), which becomes marked off, except at its hinder end, by a deep groove in the floor of the mouth.

A liorseslioasliaped thickening of the buccal epithelium now III BUCCAL CAVITY 151

develops external to, and parallel with, the groove bounding the primary tongue, and consequently lying on the floor of the month between the primary tongue and the lower jaw. The thyroid involution is situated between this thickening and the tip of the tongue.

The ectodermal thickening develops numerous glands, each originating as a solid ectodermal down-growth, and is known as the gland-field. Externally it is bounded by a shallow groove. Later on the cleft or groove separating the gland-field from the primary tongue becomes obliterated by fusion ol‘_its walls, and the gland-field becomes raised up in a dorsal direction (Fig. 84, B) the tongue-tip shrinking backwards so that eventually the demarcation between primary tongue and gland-field disappears (Fig. 84, 0). Meanwhile the groove bounding the gland-field externally becomes deepened. It forms the outer limit of the definitive tongue which is thus a compound structure, its tip and edges developed from the original gland—field, its postero-median part from the primary tongue.

In the fishes the tongue remains non-muscular and non-glandular: it is simply the primary tongue. In the Axolotl the tongue appears also to be a primary tongue, the gland-field making a transient appearance as a rudiment but eventually undergoing atrophy (Kallius).

In the Amniota the tongue is, as in the terrestrial Urodeles, a compound structure, the primary tongue rudiment becoming fused with an elevation of the floor of the mouth lying in front of the Thyroid rudiment. This elevation, called by His the tuberculum impar, represents morphologically the gland-field of the Urodeles.

The tongue of Cyclostomes is remarkable for its complexity: it has complex muscular and skeletal arrangements and on its surface it develops the horny spines which function as teeth and simulate teeth in their appearance. In Bdellostoma. the tongue develops as a cushion-like swelling of the floor of the mouth at an early period while the velar membrane is still intact. In J’et9°0m_2/zen, on the other hand, it does not develop until the time of metamorphosis.

It has already been shown how the olfactory organs come to communicate with the buccal cavity by the posterior nares. In the Amniota these become sunk into a recess in the roof of the mouth and in the higher Reptiles, as in the Mammals, this recess becomes shut off from the buceal cavity by a horizontal shelf which grows in from the side and meets its fellow to form the palate. How this has come about in evolution is illustrated by the three Lizards shown in Fig. 85.

In ontogeny the mode of origin may be similar,’ the palatine out- growths meeting and fusing with one another in the middle line (Crocodiles) or, as happens more usually, a median ridge or--septum extends backwards from between the primitive posterior nares: and the palatine processes meet and fuse with its ventral edge. In the two cases the physiological result is the same—-the shunting back- 170 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

twists upon itself, in such a way that points upon its ventral surface would move towards the embryo’s right side. (In other words the lung-rudiment rotates about its long axis in a counter-clockwise direction as seen from behind, its front end remaining fixed.) The

two lobes are the right and the left lung-rudiment but on account of

the rotation just mentioned which extends through more than 180° the left lobe at this stage represents what was originally the right side of the rudiment.

The two lungs of Lepiclcisiren or Prrotoptems are thus reversed in

A 5 C D

Fm. 96.-—-Dissections of mid-gut of L8})?:d()8t’I‘8‘?2- at stages 32 (A), 35 (B), 36 (C), and 37 (D), showing the modelling of the intestine and also the later stages in the development of the lungs. Seen from the dorsal side. ~

c.c, eloaeal caecum ; int, intestine; (.1, left lung; (Ii, liver; mun.d, Wolfllan duct; pan, pancreas ; ph, pharynx; r.(, right lung; sp, spleen.

position-—-the right lung of these forms being homologous with the left of other Vertebrates. An important detail is that in early stages the original right lung, '£.e. the definitive left, is decidedly larger than its fellow (Fig. 95, B). In later stages this inequality disappears, the smaller lung overtaking, the other in its growth (Fig. 96). .

In the ease of most individuals the lungs assume their dorsal position simply by growing directly tailwards, the oesophagus being pushed out of the way towards the left side (Graham Kerr, 1910). In certain specimens however, which doubtless in this respect retain I11 PH ARYN X 153

Ifeloderma. where they are the enormously enlarged sublingual glands.

Similar localized developments of the buccal glands occur in Birds and some of them may reach a great size as, for example, the enormous sublingual glands of the Woodpeckers.

PIIAI’.YN.\'.-—-The part of the alimentary canal which follows immediately behind the lmccal cavity is highly characteristic from the fact that in Vertebrates it is concerned with the function of breathing. The special organs which are developed to carry out this respiratory function fall into two groups one represented by the Lung‘——adapted for respiratory exchange with the atmosphere, the other by the Gills~—-adapted for respiratory exchange with gases in solution in the water. As the balance of probability is in favour of the latter being the more archaic they will here be considered first. .

The gills are seen in their most typical and familiar form in the various groups of Fishes where there is present upon each side of the pharyngeal region a series of visceral clefts—-slit-like openings lead- ing from the pharyngeal cavity to the extei-ior—-—separated from one another by masses of solid tissue known as the visceral arches or gill septa. The walls of the clefts are highly vascular and their surface is commonly raised into conspicuous plate-like projections- the respiratory lamellae —which serve to increase the area of respiratory tissue.

In the most archaic arrangement, seen in Elasmobranch fishes, the front lip of each cleft, except the first, is prolonged backwards to form a small valvular flap overlapping the external opening. In the Holocephali, Teleostonii, and Dipnoi the anterior one of these flaps, that projecting back from the hyoid arch, becomes greatly enlarged to form the operculum which overlaps not merely one but the whole series of clefts lying behind it. Correlated with this the outer portion of each succeeding septum, which in the Elasmobranch gave origin to its valvular flap, has disappeared, leaving only the portion lying next the pharyngeal cavity.

The cleft lying in front of the hyoid arch—-the spiracle is usually modified, its respiratory tissue having been reduced and even its opening being diminished in size or completely absent, but its general relations in the adult are such as to permit of no doubt as to its serial homology with the clefts behind it.

Most usually there are on each side six clel'ts——a spiracle and five branchial clefts~ —but there is reason to believe that there was a greater number present in primitive Vertebrates——seeing that the number of persistent clefts becomes on the whole less as one ascends the vertebrate scale and that here and there among the more archaic forms a greater number than the usual is found (Bdellostoma, up to 14, Notidanus cinereus 7, N. griseus 6).

In a few of the more archaic Vertebrates there develop during larval life, in addition to the visceral clefts with their respiratory 154 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

lamellae or internal gills, respiratory organs of another type——-—the external gills. As there is some reason to believe that these are more ancient organs than the gill-clefts they will here be considered first although they are much less familiar than the clefts with their internal gills. The branchial organs will therefore be considered in the follow- ing order: (1.) External gills, (II.) Visceral clefts, (III.) Internal gills. (L) Ex'1‘1«3RNAL GI[.I.S.—~Tl1e true external gills are organs which are commonly confounded with the ordinary or internal gills developed in the walls of the gill-clefts. They appear however to be quite in- dependent of these in their origin and they would probably have attracted more attention and interest than they have done had it not been for the fact that they occur in their typical form in only three subdivisions of the Vertebrates (Crossopterygii, Dipnoi, Amphibia) and that two out of these three groups comprise animals of extreme rarity, the developmental

mz_]V stages of which have not

been generally accessible

to emhryologists. 9-9-17 The typical External “C a W gill is a projection from ' ' the surface of the body - on the outer side of a

visc.eral arch. It con- sists of a core of mesen-

chyme with a covering Flu. 86.—-Diagrammatic longitudinal section through the (,f ectodenn; it is LI-a_

early rudiments of the external gills of Lepidosircn (Stage 25). versed by a vascular loop

e.g, external 1.',lll; end, endoderm; ma, \lHCel-‘ll arch; consistlng of ,the nlaln

m-, visceral cleft rudnm-nt. aortic avrcll Whlch P333353

out to its tip and then

doubles back; and it commonly has a pinnate form, paired projections

growing out so as to increase its respiratory surface. It is provided

with muscles by means of which the possessor is able to flick it sharply backwards so as to renew the water in contact with it.

The external gill as a rule is without any special skeletal support but in the larval Polypterus a short rod of cartilage projects into its base, and in the extinct Dolichosoma of the Gas coal of Bohemia there was apparently present a well-developed segmented skeleton within the substance of the external gills.

The external gill develops as an outgrowth from the tissue of the visceral arch at a period at which the clefts are not yet perforated. It arises as a bulging of the surface (Fig. 86) and in the author’s opinion the endoderm of the cleft rudiments takes no part in.its formation. At the same time it is only right to state that the pre- valent opinion in the past has been different. The outer surface of the visceral arch in the region where the external gill will develop is covered by a layer of cells thicker than the neighbouring ectoderm, and in some cases this thickened portion of the ectoderm shows in its

e.g.I There is however no definite evi-

III EXTERNAL GILLS 155

deeper portions a rich deposit of yolk, so as to look exactly like the yolk-laden encloderm. Grcil explains this appearance by supposing that true endoderm cells actually spread outwards and replace the deep layer of the ectoderm, so that the external gill-rudiment would be partly endodermal in its nature.

dence of any such process taking place and the present writer would interpret the appearances as mean~ ing simply that the ectoderm cover- ing the external gill-rudiment becomes thickened, and stores up a supply 01' yolk‘ in its deeper layers, as a physiological preparation for the active processes of growth which are about to take place as the external gill rapidly increases in length. In this he agrees with Marcus (1908).

The general appearance of the developing external gills is well seen in Hypogcopms (Fig. 87) or in Lepidoswlren (Fig. 200). In Lepvlolosiren there are present four B‘.1<_:, 37.---Hypogeophis eIul)i'y0s showing upon each side of the body. At ‘l3’,°1?1§“°“t_ ‘if89}}‘“ “"“""““l gm“- first the four are quite independent ( W m“’_ ") _ of one another but as development .,.;-;;ii;...;.;‘l:;1i:,l.‘.‘,‘.. ""‘«i~§.. ’.’.;...§i?{Z.‘{‘ ..“.f.§1‘,‘..‘ ...if.’£; g0(-3S 011 they b£‘.COII18 raised UPOII EL projecting in Bfi-mu the hyoid 8.I‘(‘ll, and also conlu-1011 base SO as to give the from the mandibular arch in front of it, are

__ . _ . p0ssil)l_\' u-xiurnal gill rudiments which do not 3:PP93—T3'l1(39 of 3' 3111819 Organ wlth go on with their development.

four branches (Fig. 200, B-E). The distribution of true external gills amongst the main groups of Vertebrates is shown in the following table:

1

,—-.-—-q-——.._.—_.__—j__..._ . __ - .. .- ._ . ...--..- -.

\

l 3 V I. II. III. I\'. ‘V. ‘ ' VI.” ' iscvral .\l'L'lI. ' _ . . . l-‘irsl ' .\'.4-Hnltl 'l‘ llI'( u Four‘: M‘l'"1lb“l‘w‘l Hymd‘ Jlr:1m-hi:ll. Iii‘.-lnuliinl. 3 Bram-hinl. 3 ll:'.'ui(.-lii:Il.g Elasmobranchii. . ; Crossopterygii . . 5 x . lhpnoi. . . . g x x x x Amphibia . . . - 'v.*" 2:. x y x x i Amniota . . ' i 4 I 1

  • r:..\'o'sLi_~,'iul.

In those animals in which they are well clovoloped the external gills are for a time the main functional breathing organs. They are 156 EMBRYOL()GY OF THE LOWER VERTEBRATES CH.

richly vascular and the renewal of the water in contact with their surface is provided for by a well-developed muscular mechanism by which they are sharply flicked from time to time, or, in early stages, by rich ciliation ol"tl1eir surface as in the Frog (Assheton, 1896) or 6'73:/ptolzramz/ms (Smith, 1912). They are as a rule merely temporary organs. As the respiratory function comes to be sufficiently per- formed by other organs their circulation becomes sluggish, their tissues moribund. They become invaded by leucocytcs and eventually unclergo complete atrophy. in .l’o~otopm~us distinct vestiges persist for a prolonged period while in various Urodeles they remain func- tional throughout life.

The external gills, highly vascular and projecting freely into the surrounding medimn, present tempting objects for attack by other organisms. They are therefore extremely liable to injury, and cor- related with this they present a high power of regeneration. in correlation also with the same fact we find that they tend to be eliminated from development in certain members of groups which are as a whole characterized by their presence. Such is the ease in the Amphibia where they are characteristic of the group in general but where in particular cases they are reduced (II;z/la n;'rb0'rea) or com- pletely absent (}>’ovmb'mator) although we must believe they were present in the ancestors of these forms.

This tendency for the external gills to become eliminated from development in the process of evolution raises the interesting morpho- logical question: were External Gills at any period more widely dis- tributed amongst Vertebrates than they are at present? And, if so, are their vestigial representatives still to be found in any cases where they no longer develop as functional respiratory organs?

This interesting problem, which offers an inviting field for research, has not yet had sufficient attention devoted to it. Even if it were the case that external gills once existed in the ancestors of forms in which they are no longer present as functional organs there is always the possibility if not probability that their disappearancehas been so complete as to leave no observable trace. Nevertheless such vestiges might persist and are worth looking for.

Under these circumstances it is of interest to note that already certain structures are known which are interpretable as vestiges of once-present external gills. Thus in Gymnophiona what appear to be transient rudiments of mandibular and hyoidean external gills make their appearance during development (Fig. 87, B). Again in the case of the Mandibular and Hyoid arches of Urodeles, on which no functional external gills develop, Driiner (1901) has found what appear to be vestiges of the muscles of external gills. Again in the larvae of various Urodeles there occurs in connexion with each mandibular arch a curious styliform projection known as the balancer, from the fact that the larva balances itself upon them as upon a pair of limbs (Fig. 88, b). Each of these has a vascular loop within it and it in fact appears to be the modified external gill of III . EXTERNAL G-ILLS 157

the mandilmlar arch which has lost its respiratory and taken on a supporting function.

While external gills occur within three main subdivisions of the Vertebrates, namely Teleostomatous fishes (Crossopterygians——the most a.rchaic of existing Teleostomes), Lung-fishes, and Amphibians, there are two main groups-—Elas1nohranchs and Amniotes---in which they are conspicuous by their absence. Having regard to the tendency of the organs in question to disappear (as in the cases already alluded to amongst the Amphibia) their absence in a special group would not in any case constitute strong evidence that they were never present in the ancestors of that group. As it happens however there is in the two groups mentioned a definite cause which seems quite competent to account for the dis- appearance of external gills, namely the de- velopment of'_ a new organ —— the yolk - sac with its highly developed vitelline network of blood-vessels»--which in addition to its primitive

function Inust neces- 88.%l‘hii'eie larval (_lt:l't'l()})lllt‘lll. of a ‘l'l(f\\'loW

sarily also function as it famiu/us) as seen from above. (After Egert, very efficient organ of '

respiratory exchange and what looks like :1 posit-rior 4-..\'tc-rnal gill is the ]w(:L-()1‘fll limh. SO render 311)’ Pre'eX-1st‘ In l"ii-_:.s'. ll and (l flu-1-Xternal _«_;'ill:~' lmve l)l',‘(‘ll cut away leaving ing respiratbry organ no only tll(‘ll,'l)-‘mill .~'i-urnps.

longer necessary.

Taking into consideration the presence of external gills in three archaic groups of Vertebrates it seems to the present writer to be clearly indicated that these organs are a very ancient characteristic of the Vertebrate phylum. The only alternative indeed is to regard them as having become evolved independently in the three groups in which they occur. It is diflicult to accept this as in any way probable having regard to the similar morphological relations of the organs in question.

It might be suggested that somewhere on the course of a large blood-vessel, such as an aortic arch, would be a most natural place for the development of a new respiratory organ. Such a suggestion however is entirely fallacious for simple physical reasons: for new breathing organs will tend to become evolved -not on the course of a

b, lnilaiurei-; !'.§I,i'_\t91‘l1:ll gillol'lir.-1. ln'anchial:1rch. In l«‘i_«,v-. A ' 158 EMBRYOLOGY OF THE LOWER V ERTEBI-LATES (311.

large vessel where the quantitative relation of surface to volume in the blood-vessel is at its minimum but rather where there is present a rich superficial network of capillaries, in which the ratio in question is at its maximum. .

(IL) VISCERAL CLEFTS. --The visceral clefts develop in what appears to be the most archaic method in Lampreys and Elasmo- branehs Where each arises as a lateral pocket (visceral pouch) of the pharyngeal wall which meets and fuses with a., much shallower, ingrowth of the ectoderm, the apposed portion of cndoderm and ectoderm breaking down so as to bring about a free communication between pharynx and exterior. Each cleft thus consists of a, usually much larger, inner portion lined with endoderm and an outer portion lined with ectoderm.

The most frequent type of modification of this probably primitive mode of cleft development is that so usually met with in the develop- ment of hollow organs, namely that the cleft-rudiment, instead of being a hollow pouch from the beginning, is for a time in the form of a solid lamina of endoderm, which only at a later period develops a cavity in its interior and becomes an open cleft. This modification is found in Teleostomatous fishes, Lung-fishes and Amphibians.

111 the young Elasmobraneh the gill-clefts are at first long slits "traversing the whole dorsi-ventral extent of the lateral wall of the pharynx. Each septum or arch grows back at its outer edge to form a valvular flap overlapping the cleft next behind it. In most cases this backgrowth fuses with the next septum at its dorsal and ventral ends so as to reduce the external opening of the cleft to a compara- tively small dorsi-ventral extent.

In all Gnathostonies, excepting the typical Elasmobranehs but including the Holocephali, the hyoidean backgrowth becomes greatly enlarged to form the operculum which overlaps the whole series of clefts behind it. Correlated with this the outer portions of the sub- sequent septa with their backgrowths become reduced. I11 these cases we frequently find a marked tendency for the edge of the opercular backgrowth to become fused with the body so as to restrict the size of the opening behind it. Thus in the Eel the opercular opening becomes reduced to a small persistent ventral portion, while in S3/mbmncltas the same holds but in this case the two openings have fused together to form a small ventrally placed median

ore. P A similar condition to this occurs in the tadpole of Discoglossus while in other Anura the persisting opening is displaced to the left side. Finally in Amniotes the fusion of opercular margin with body- wall takes place along its whole extent so that the branchial region becomes completely enclosed (see Chap. X.).

SPIRACLE.-——The spiraele or hyomandibular cleft always shows a considerable amount of modification. In Elasmobranehs its dorsal portion alone becomes perforate, although fusion of the pouch with the ectoderm takes place throughout its whole dorsi-ventral extent. .the Amniota the distal portion of the

I I r GILLS ' ' _ 159

hespiratory lamellae develop only on its anterior wall and these, as development proceeds, become vestigial forming the pseudobranch. In Teleostean fishes the spiracular pouch (Fig. 89 A, cc. I) flattens out and disappears (Goette) so that the pseudobranch (pa) on its anterior wall comesto lie on the inner face of the base of the operculum and appears to belong to the second cleft (Fig. 89, B). In Lung-fishes the soli.d endodermal rudiment never becomes perforate. It becomes gradually reduced during development while "its outer ectodermal portion becomes, as already indicated, converted into a special sense-organ. In Anurous Amphibians and in

cleft rudiment becomes greatly dilated to form the tympanic cavity, while the proximal part forms the relatively narrow Eustachian tube.

Just as the varying condition of the spiracle indicates a tendency for this cleft to undergo reduction so a similar but still more marked tendency exists for the gill clefts to become reduced at the other (posterior) end of the series. This is illustrated in the first place by the reduction in the number of functional clefts seen in passing from the lower Vertebrates to the higher. It is also frequently manifested in developmental stages. Thus 81110118317 the Amphibia we find Flu. 89.—-Horizontal sections through that in the Gry1llI'l()pl1l0Ifl& (Hyp0g(3- Salmon embryos explaining position Ophis, Marcus) 3 rudilnentary 7th cleft of pseudobranch. on inner surface of

_ . . operculum. (Atter (Joctte, 1901.) makes its appearance though it never _ ' L. ‘ reaches the eehederm, while the 6th ...;i;Z'.'..i‘.i1'.?."i...:f:I?f:...:f:f:...?:II%‘:'§1..?鑧?'.;....:iZ.; lS 0p9Il £01‘ 3. time. III UFOdBl8S EL l and II; II;/,l1_yoi«l ar«.'.l1; -Ti-, ope-rculum; rudiment appears and is for a time :"{"\"i"“l'jl‘-I3"Cf:-¥.g1*ll1tl'3'IIx2 1»-'vwlobw'=tnc1»; connected with the ectoderm but does °’ W” (T ' not become perforate, while in Anura this cleft appears only as a small and transient rudiment which never reaches the ectoderm.

(III.) INTERNAL G1LLs.——’_l‘he internal gills or respiratory lamellae arise as ridge-like or, at first, finger-like projections of the cleft lining. The chief matter of dispute regarding their development has been the question whether they belong to the endodermal or the ectodermal portion of the cleft lining. In cases where, as frequently happens, the lamellae begin to develop after the cleft is completely formed, the appearances are sometimes in favour of the one sometimes in favour of the other interpretation. Goette (1901) in fact goes the length of regarding the lamellae as being of endo- dermal origin in the case of the spiracle and ectodermal in the case 160 EMBRYOLOGY 0 F THE LOWER Vl1‘.lt"El3l\’.A'l‘l+]S (‘.11.

of the succeeding clefts, so that the spiracular pseudoln-ancli would on a strict interpretation of the germ-layer theory not be serially homologous with the other gills.

In the present writur’s opinion, as already indicated, such ul)S61‘- vations upon the first origin of organs which develop in the region of the blurred boundary between two layers are not to be taken as afihrding evidence ol' any serious importance in regard to the morphological nature of such organs. Greater weight however seems due to evidence obtained from cases where the hrst traces of gill laniellae are visiblq; at a period before the bounding inmnbrane of the cleft is ruptured, when the cleft consists still of two distinct pnnches—ene eetoderinal, the other endodermal——separatcd by a still eeuiplete partition. Such is the case in Aci_pc'nscr and (ioette shows that in this case the laurella-rudiments arise outside the

partition from what is undoubtedly an ectodermal surface (see Fig. 90, g.l).


Pl’ :3" '|‘he same discussion extends to the ' general lining cl’ the cleft~—as to how

go. much of the lining of the adult cleft is

/ ectodermal and how much endodermal.

-7" }octte and Morofl'(1902) hold that only

|"1u.90.—~ll0rizontal:-.m~1ionthrough the portion of Lhe cleft -in the innue- bmwhial reg.“ M. ymmg AOL d1ate_ IlBlg'lll)Olll‘l100(‘l of its pharyngeal peust-r showing the ('('f()de‘[‘[[]a1 opening 18 to be regarded as endodermal, <(>‘I3:.:':It1e01'1f)1(I)*51s)i1l1am011ne- (Atter all therest. being ectodermal. But here ‘ ’ ‘ ° again 11] view of the blurred character l.ll'iiI"‘ll‘.'ll”,Iiill,:i|l\(ill«l:ii(ill?:i:;‘,]ii;:.:iI('lii.llfllll: (.)f the boundary betvlreen the two layers p;,, ..,...._,. .,,~,,1...,_,..x_ it seems hardly profitable to speculate on the matter.

In certain fishes the gill-lamellae are for a time prolonged out- wards into long threads which project through the cleft opening into the surrounding fluid. Such is the case in the embryos of F.lasn1e- branchs, in which it is only the lamellae upon the posterior face of each arch that become prolonged, those on the anterior face not projecting beyond the edge of the septum. Eventually the pro- jecting part of the filament disappears while its attached basal portion becomes the definitive lamella. In a few Teleosts a similar temporary modification of the lamellae takes place ——~perhups the best example being Gymnarclms (Budgett, 1901; Assheton, 1907. See Fig. 199).

EVOLUTIONARY HISTORY or rm-1 BRANLIHIAL RESPIRATORY ()RGANs.——-As regards the early evolutionaryhistory of these branclnal respiratory organs one very generally accepted View looks upon the visceral clefts as being the most primitive, the internal gills as having developed next, and the external gills as being due to secondary extension of respiratory tissue outwards from the clefts. It seems however, bearing in mind what we now know regarding the development and distribution of external gills, at least equally ii’ llnl. III LUNG 161

more probable that the evolution of these organs has been in the opposite direction.

On this latter hypothesis the external gills would be regarded as the primitive respiratory organs, inherited probably from pre- vertebrate ancestral forms. The evolution of clefts between their bases would be explicable as an arrangement for pumping water over the surface of the external gills, while it could be readily understood that the respiratory tissue would then tend to spread inwards along the lining of the clefts, where it would be both advantageously situated for carrying out its breathing function and, at the same time, protected from the dangers to which external gills are exposed. The development of respiratory lamellae to increase the area of this respiratory tissue on the wall of the cleft would be a further and natural development.

The chief difficulty in the way of accepting this as a working hypothesis lies in the existence of animals admittedly near the base of the Vertebrate scale—such as Amp/2/ioxus and the Cyclosto_ mata-—in which there are no external gills and no vascular yolk-sac to account for their disappearance. This difficulty is undoubtedly a serious one but on the whole the present writer is inclined to think the difficnlty is not so great as to justify the immediate rejection of the hypothesis: it becomes less formidable when it is borne in mind that the forms mentioned although evidently archaic in some of their characteristics bear in others equally convincing evidence of high specialization. _

LUNG.—-In all the groups of Gnathostomata excepting the Elasmo- branch fishes the pharyngeal wall develops a great outgrowth which, as will become apparent later, is to be looked upon as home- logous throughout the series and as primarily respiratory in its function-——the lung. The lung appears in its most familiar and typical form in the tetrapod Vertebrates and its development in these will accordingly be considered first.

Here in an early stage of its development the lung is in the form of a pocket of the pharyngeal floor projecting downwards in the mid—ventral line. This pocket commonly makes its first appearance as a longitudinal groove or gutter in the floor of the pharynx at about the level of the last ‘visceral cleft. The groove becomes constricted off from behind forwards, so as to form a blindly ending pocket communicating in front with the pharyngeal cavity by a narrow opening—the g‘lottis—and extending back immediately ventral to the pharynx. The blind end of the pocket grows actively tailwards and becomes deeply bi1obed—-—the two lobes becoming respectively the right and left lung, while the unpaired portion connecting them with the glottis becomes the trachea or pneumatic duct.

While the lung passes in its early history through stages corre- sponding on the whole with those described there are differences in detail in different groups-—-the most conspicuous of these variations

VOL. II ' M 162 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

being, as is so often the case in the development of hollow organs, that the rudiment is at first solid and the cavity appears secondarily in its interior. This is the case in various anurous amphibians and in Jlepiolosofirevz and .1’-r0t01ate'/"as.

It has been indicated that the lung is primarily a ventra.lly placed pocket of the pharyngeal wall, that is to say its wall is a portion of splanchnopleure. It follows that the cavity of the lung is lined by endoderm while its outer layers (connective tissue, blood- vessels, muscles, etc.) are composed of splanchnic mesoderm.

As regards the further development of the lung, the main steps are concerned with its respiratory function and have to do with the increase of the respiratory surface. In such an animal as the Newt, where the lung retains a relatively primitive condition, the endo- dcrmal lining grows equally as the organ increases in size, so that even in the adult the lung has the form of a simple sac with smooth endodermal lining. In a Frog or a Lizard, however, growth activity is specially marked at particular spots so that at these spots the cndoderm forms outward bulgings into the covering of splanchnic mesoderm.

In these Sauropsida in which the pulmonary apparatus reaches its highest degree of evolution (Tortoises, Turtles, Crocodiles and Birds, in an ascending series) these pockets of the endodermal lining become more and more extensive, and more and more com- plicated, so as to give rise to a thick spongy mass, which forms the bulk of the lung, surrounding the now relatively small clear central space. The latter, forming as it does an apparent continuation of the bronchus or paired portion of the trachea, is spoken of as the intrapulmonary bronchus. Further the respiratory function becomes concentrated towards the terminal portions of the pockets, their proximal portions forming simply conducting channels-— branches of the intrapulmonary bronchus.

In the Chameleons, towards the end of development, a number of the endederm outgrowths bulge out beyond the general level of the surface of the lung upon its ventral side. These persist in the adult as large diverticula which when the animal blows itself out are inflated with air. In the embryos of Birds similar outgrowths make their appearance, four from each lung, but in this case as development goes on the outgrowths continue to increase in size and form the characteristic air-sacs of the adult bird.

THE LUNG or BIRDs.—-—As the Birds, in correlation with the intensely active metabolism as indicated e.g. by their high body temperature, stand pre-eminent amongst Vertebrates in the high stage of evolution which has been reached by their lung, the onto- genetic development of this organ will be followed out in a little more detail (Moser, 1902; J uillet, 1912).

In the Fowl the pulmonary diverticulum of the pharyngeal floor makes its appearance about the beginning of the third day. By the end of this day the rudiment is bifurcated at'its hind end, each lobe III LUNG 163

being the rudiment of a lung in the restricted sense and containing a prolongation of the enteric cavity lined by tall columnar endoderm cells. Outside the eiidoderm is a thick layer of inesenchyme and this in turn is covered by columnar coelomic epithelium.

The endoderm-lined cavity is destined to become the main intrapulmonary bronchus—-the mesobronchus. This remains un- branched until the fifth day when its cndoderm begins to bulge out, near the point where it enters the lung, to form the first ento- bronchus. During further development a. series of three other entobronchial outgrowths sprout out from the external surface of the inesohronchus close behind the first outgrowth. The four ento- bronchi so arising are closely contiguous and form a longitudinal row (Fig. 92, El-E4).

A set of similar outgrowths make their appearance spaced out along the mesial side of the mesobronchus posterior to the ento- bronchi: these are the rudiments of the ectobronchi. A third set of outgrowths on the lateral 0 side of the mesobronchus ejcf. are the rudiments of the Z‘ small secondary lateral bronchi (Campana). Of these sets of outgrowths the first and second are the most important and they are arranged in a slightly spiral row along the wall of the meso-

51: bronchus.

The mesobronchus, as FIG. 9l.—_Diagraiii illiistratiiig the arrangement of the it grows in length main air-passages in the lung of the Fowl as seen 3

h b from the mesial plane. (After J uillet, 1912.) assumes a Somew a ' l(’ is‘ c .. uni‘ an eii'.0i« '- m-shaped curvature. by M’ l'its?‘.1...;o§i3o:.:iii§?',..i,-,',....r:£.,.;~.)i.¢l.i.m" which the Group of ecto- ~

bronchi are: carried towards the (1'lO1‘Sa.fl facéa fof the lgng while the entobronchi are nearer the ventra sur ace c . Fig. 91 . Both ecto- bronchi and entobronchi grow rapidly parallel with and close to the surface of the lung-rudiment. They soon produce secondary branches as projections of their walls and these secondary branches increase greatly in length traversing the substance of the lung at first close to its median surface and, later, deep down in its sub- stance as well—-—the eiitobronchial branches growing in a dorsal and the ectobrauchial in a ventral direction.‘

The two sets of branches fits their t1% 1E:p‘pI‘01f1it(3l1 1(l)I1e another Iarg seen to alternate in position ( ig. 91). en t ey ave approac e closely each branch bifurcates and its two tips become closely apposed to the two tips belonging to the other serieswhich lie closest to them. About the thirteenth day these apposed tips become completely fused and their cavities continuous so that there is now

I/I///‘I/l-:’I"'.. T '

//((!l{!)ll‘Z‘llE};'; 164 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

established a series of channels running in a dorsiventral direction through the substance of the lung aml communicating dorsally with the ectobrcnchi and ventrally with the entobronchi. The channels in question are termed parabronchi (Fig. 91, par). These are embedded in an abundant matrix of mesenchymc which from about the tenth day becomes divided up into more or less prismatic masses each having in its axis an individual parabronchus—the prisms being delimited from one another by the development of intervening blood-vessels. The mesenehyme which constitutes the inner portion of this sheath round each parabronchus becomes later replaced by a layer of smooth muscle fibres.

At about the same period as the fusion of the parabronchial tips takes place, the Wall of the parabronchus begins to grow out into numerous little pockets arranged in radiating fashion. These extend outwards, perforating; the muscular sheath, and at a short distance from the parabronchus divide into branches which in turn elongate and become the air-capillaries of the fully developed lung. Judging from adult structure it would appear that the tips of these fuse with others to form the continuous air-capillaries so that the latter would be formed much in the same way as the parabronchi but it has not been possible, so far, to demonstrate this by actual observation.

The essential features of the development of the Bird’s lung as above outlined may be summed up in the statement that in this type of lung the diverticula of the intrapulmonary bronchus, which in other Vertebrates end blindly, become here joined together tip to tip to form continuous tubular channels. To allow this arrange- ment to function efficiently an apparatus is needed to force the air throu h the system of respiratory tubes: such an apparatus is provi ed by the air—sacs.

AIR-SACS.-—-The ventral part of the lung-rudiment is for a time formed of a thick mass of mesenchymatous tissue which has been termed by Bertelli the primary diaphragm, from the fact that it becomes continuous along its lateral margin with the side wall of the splanchnocoele, so as to form a kind of floor separating off the lung from the splanchnocoele which lies ventral to it. The air-sacs arise as outgrowths of the bronchial cavities and are on each side four in number: the first or most anterior giving rise to the cervical sac, the second by bifurcation to interclavicular and anterior thoracic sacs, the third to the posterior thoracic and the fourth to the abdominal sac. The rudiments sprout out into the substance of the primary diaphragm and become greatly distended within it, bulging out ventrally amongst the viscera so that the ventral layer of the diaphragm becomes stretched ‘out into a thin membranous wall delimiting the cavity of the air-sac on its ventral side. The dorsal part of the primary diaphragm, lying above the air-sacs, persists as the floor of the lung or secondary diaphragm (ornithic diaphragm of Bertelli, pulmonary aponeurosis of Huxley). III T AIR-SACS 165

The air—sac rudiments sprout out (Fig. 92) from the main pulmonary cavities-—the cervical from the first entobronchus, the interclavicular and anterior thoracic jointly from the third ento- bronchus, the posterior thoracic and the abdominal from the meso- bronchus. Later on additional secondary communications between the air-sac cavity and the pulmonary cavities are established (except in the case of the cervical air-sac) by means of the recurrent bronchi of J uillet. These arise in the ordinary fowl about the tenth day of incubation in C3,. the form of outgrowths of the wall of the air-sac either near its tip (interclavicular and anterior thoracic) or just before it emerges through the general surface of the lung (posterior thoracic and abdo- minal) as shown in Fig. 92.

These outgrowths burrow into the superficial layer of the lung, branch and become joined up, in a manner the details of which have not yet been worked out, with the system of parabronchi. The communications are visible in suitable preparations of the adult lung as groups of openings, each group leading in into the lung from the appro- /0’ ./ priate air-sac——those of the interclavicular and anterior 05- thoracic lying towards the FIG. 92.——])iagranuuatic view of the right lung of lateral edge Of ’Dl]6 ventral a Fowl embryo of the tenth day as seen from

surface of the lung, about the “:9 V°‘:"“‘(j{%:éri11;1::fi%::i'1s1 ;>1'is_iI{Ihe0fflt)1l:<; . a r-sac . c '-, ~ - level of the attaclllllent of entobronchi are shaded.

the bronchus’ and those of rib, abdominal air-sac; at, anterior thoracic air-sac: the POSl73ri0r th0ra»0i0 and car, cervical air-sac; 14."! and 164, llrst and fourth ento- abdonlinal sacs near bronchi; tc, iiiterclapicular air-sac‘; -nies,tn:)¢sst)ll;1;otrl1sch1is;

     pl, pUHl3('3I'10I‘bl'|0I‘8a('alL fllI"8{lC, T, I‘l,LllII‘0-ll P01. 1 ...

close to the direct opening between it and the corresponding air-sac. It would appear that the function of these recurrent channels is to conduct the air forced out of the air-sacs in the expiratory effort through the system of air-capillaries, the muscular coat of the parabronchi doubtless playing an important part in directing the passage of the air through the system of air-capillaries rather than through the parabronchi themselves. a The formation of the air-sacs does not exhaust the remarkable

proliferative powers of the wall of the lung in Birds. Further out-

III A IR—‘I'._LA DDER 167

FIG. 93.-—-Development of the air-bladder of n Teleost. (After Moser, 19,,4.)

A, Jmodeus, 5 mm., longitudinal section; B, Rh-Od¢'ll.$', 6 mm., longitudinal S(‘(!U()lI2 (.‘., Rlmdemz, 7 mm., trans-vt-rse section, showing small pouch-like mxtgrowtli of pneumatic duct; uml, emlu«lu'r1|1; cut, enteric cavity; 2, uir-bladder; Ii, liver; N, notochord: mt, pronephric chmnbur; p.41, pm-.Iu1m.tic duct; '_l/, yolk. 168 EMBRYOLOGY OF THE LOWER VERTEBRATES (in.

to form a kind of diaphragm perforated in its centre and capable of being thrown into vibration by air being forced from one chamber into the other so as to function as a sound-producing organ (e-._g. Gurnards). Other outgrowths may develop: thus for example in many Siluroids numerous branched projections are formed along each side of the air-bladder.

The air-bladder rudiment is at its first appearance in some cases approximately dorsal in position (Selma). In Jihodeus Moser (1904) has shown that the diverticulum is at first on the right side of the alimentary canal. The same observer found that during the early stages of development of the air-bladder the portion of alimentary canal from which it springs undergoes a process of rotation about its long axis in such a direction that a point on its dorsal surface is carried towards the left side.

Although the actual development has been worked out only in a few cases, we may infer safely from the adult relations (Rowntrce, 1903) that the amount of this rotation differs greatly in different

.members of the group Teleostei. Thus in Siluroids and Cyprinodonts

the glottis or pharyngeal opening is in the adult still to the right of the mesial plane; in others such as the genera 0smer'us, Olupea, Olwrocentrus it is practically median; in still others such as Mormyrids, Characinids, Gymnotids and Cyprinids it has passed the mesial plane so as to lie upon its left side, while in the case of the Characinids Jllacrodon, Erythmlnus and ]}r’b'ias'i'na the glottis has come to be completely lateral on the left side. This rotation of the gut in the region of the glottis is of much morphological importance as will be shown later.

In the young Rhodeus, 7 111111. in length, Moscr finds that a well- marked diverticulum from the pneumatic duct is present (Fig. 93, 0). Later on it gradually disappears. A similar diverticulum occurs in Salmo and in the Carp, and in all probability in numerous other Teleosts: its morphological significance will be discussed later.

ACTINOPTERYGIAN GANOIDS.——In these fishes the development of the air-bladder takes place on similar lines to that described for Teleosts. In Amda the additional detail has been made out that the rudiment is at first in the form of a longitudinally placed groove which becomes constricted off from the alimentary canal from behind forwards just as frequently happens in the case of the typical lung- rudiment of air - breathing Vertebrates (Bashford Dean, 1896; Piper, 1902). A rotation of the section of alimentary canal in the region of the glottis takes place similar to that which occurs in the Teleost.

LUNG-FISHES.-—In the adult Oeratodus an organ occurs which is equally lung and air-bladder. It forms an unpaired sac lying dorsal to the splanchnocoele just like a typical air-bladder, but the pneu- matic duct, instead of opening directly into the alimentary canal dorsally, passes round the right side and opens by a ventrally placed glottis. In Lepidosiren and Protopterus the general arrangement is m i LUNG _ 169

the same except that here the organ is deeply bilohed: a right aml a left lung or air-bladder occupying the place of the single organ of 0'e'ratodus.

The meaning of the ventral position of the glottis in these Lung- fishes, and, in fact, the morphological nature of the whole organ, is


Flo. 94.--Transverse sections through the endoderm of the pharynx showing an early stage in the development of the lung.

A, Polypterus, B, (}'era,tmIu.,g, and O, Bmn..h1',nato1' (O aTl5er Goette, 1875). 1, lung-rudiment; ph, pharynx.

demonstrated by the examination of early stages in development. In these the organ is found to be a perfectly typical lung-rudiment (Fig. 94, B)-——a mid—ventral projection from the pharyngeal floor of precisely the same kind as that found in te_trapodous vertebrates (C).1

Fm. 95.——Views showing early stages of the lung-rudiment of Protopterus as seen from the ventral side (stages xxxii, xxxiv, xxxv).

e.g, external gill; I, lung; oes, oesophagus; pan, dorsal pancreas; ,2. f, pectoral limb; Th, thyroid ; v.c, visceral cleft rudiment. (Cut surfaces are inclieatml by uniform light tone.)

Subsequent stages are illustrated by Figs. 95 and 96. The lung rudiment at first a rounded knob (Fig. 95, A) grows backwards and soon becomes bilobed (B). The figure does not bring out one im- portant fact namely that the lung-rudiment as it grows backwards

1 The projection is at first solid in the case of Lcpvidoseren and Protopter-us. III CEMENT-ORGANS . 179

head on each side as shown in Figs. 100, A, and 197, C, 0.0. A longj- tudinal section through the centre of the organ at about this stage (stage 26, Fig. 101, E) shows that the organ is covered hy the ordinary 2-layered ectoderm. Round the lip of the opening at its

Flo. l0l.——Illustrating the development of the cement-organ of l’oly_pterus. B represents part of a transverse section, the other figures portions of horizontal sections.

A and B, stage 20; C, stage 23; D, stage 24; E, stage 26. mo, cement-organ. The darker tone indicates ectoderm.

free end, the superficial layer of ectoderm stops, while the deep layer seems to dip down as a deep involution to form the secretoryepithelium (c.o) which lines the cavity. All the appearances seem to point to the secretory epithelium being ectodermal in its nature. How deceptive these appearances are will be gathered from an inspection

of ‘ig. 101, A-E. III LUNG 171

the archaic mode of development, the lung-rudiment (Fin. 97, Z) describes a spiral curve round the oesophagus so that the bifurcated

FIG. 97.———Porti0ns oi‘ l.l‘{Ll1SV8I'S0 sections through :1 Lepiulosiren larva (stage 34) to illustrate the eli:u1gin;_:' relations of lung to gut from at short distance behind the glottis l2tliW:ll'1lH. In .\ l.h«- lung is ventral to the :Llirn(-.ntary canal _; in B it is directly to the riglit ; in (3 it has lm_-mm: displaced dorsally ; while in D (where it is commencing to bifurcate) it has come to he llll(l-(iflrfial in position.

.4, aorti; gt, glomerulus ot'pron«_-phros; I, lung; N, notochord; ues, oesophagus.

hinder end of the rudiment, which will give rise to the lungs in the restricted sense, comes to lie dorsal to the alimentary canal. ~ The lungs continue their tailward growth in the substance of the 172. EMBRYOL()GrY OF THE LOWER VERTEBRATES OH.

dorsal mcsentery (Fig. 97, 1)) but eventually the portion of this mesentery containing the lung and dorsal to it becomes greatly thickened from side to side and finally merges completely in the roof of the splanchnocoele, so that in the adult condition the lungs lie completely outside the body-cavity——between it and the vertebral column.

In Uemtodus (Gregg Wilson, 1901; N cumayr, 1904) the lung is at first, as in the other two lung—fishes, ventral in position (Fig. 94, B) but in this case the originally left lung, which in Lepidostrevz, and I’7"utopte'rus is for a time during development reduced in size, seems to have disappeared almost entirely, being represented only by'a small and transient rudiment. Further detailed studies of the early stages in the development of the lung of Uemtoclus are much needed to make clear the origin and fate of this vestigial left lung. But it seems clear from what is already known that the monopncumatic condition of Ceratndlzzis has come about in evolution through the suppression of the originally left lung.

As the lung completes its development, its cavity becomes en- croachcd upon by two median longitudinal ridge-like ingrowths, one dorsal and the other ventral. It used to be supposed that these marked an incipient division of the l11ng into a right and a left half so as to bring about the condition seen in Leptdostren or Protopter-as —the monopneumatic condition being supposed to be the more nearly primitive. It will have been gathered from what has been said that this point of view is no longer tenable and that the n1ono- pneumatic condition of Cemtoalus is to he looked on as secondary and not primary.

CROSSOPTERYGIANS. — Of the two surviving examples of the Crossopterygian ganoids—the most archaic existing members of the Ganoid-Teleostean stem—a few stages in the development of the lung have been investigated in Poly/pterras (Graham Kerr, 1907). In the earliest stage observed the lung—rudi1nent was in the form of a mid- ventral groove formed by an outgrowth of the pharyngeal lining (Fig. 94, A, Z). This groove becomes deeper and towards its posterior end widens out ventrally so as to have a .L-shape in transverse section.

Posteriorly the lung-rudiment grows back into a pair of horn-like projections——the rudiments of the right and left lung. These extend backwards in the connective tissue of the splanchnopleure and they very soon show a marked inequality in their rate of growth the left lagging behind the right. As growth goes on this inequality becomes more and more marked, so that in a larva of about 30 mm. in length the right‘ lung extended right back to the cloaca while the left pro- jected back only about 3 mm. behind the glottis.

In these later stages another important feature is to be noticed, one which is correlated with the fact that the air-filled lung neces- sarily acts as a float in an aquatic animal. This feature is that the lung tends to assume a position symmetrical about the median plane. Thus in the anterior region where both lungs are present they are m AIR-BLADDER 173 ,

situated laterally, balancing one another, while farther back where only the right lung is present this shifts towards the mesial plane until it is symmetrical about that plane, lying in the dorsal mesentery (Fig. 98, A and B).

EVOLUTION on THE Am-BLADDER.-—-'l‘he facts that have been enunciated above, with regard to the development of the lung in Dipnoan and Crossopterygian fishes, are of much morphological interest. When pieced together with what has been said regarding the development of the air-bladder of Teleostcan fishes they afford data from which the evolutionary history of the Teleostean air- bladder can be traced out with a high degree of probability. That history may be stated in a few words to have probably been as follows:

1. The primitive condition was that of a lung, communi- eating with the pharynx by a ven- ' trally placed glottis ——-for we have seen that the‘ embryonic rudiment of the organ in the most archaic forms pos- sessing it is a typical lun,r_f-rudiment.

2. The organ became bilobed, growing back into a

right‘, lung and a, left FIG. 98.-—S_ections through the lungs of a larva of hula. Polypterus 30 mm. in length. ,__,.

'3_ In the for-Ins 1 f1:,1lnO!'O1l;llbeI'€0I‘l; Ba more postplrior; 1.4, aorta; mg, enteron ;i Ll, . . e ung; no 00101‘ ' opn Opls xonvp nos: 1». 1-, pm monary ve us; Wh]-ch took to 3' rnl, right lurig; 1:, lntcrrenal vein.

purely swimming

existence, and became specialized in the direction of adaptation to this, l)l]_(_‘.1‘(‘. ('..'Hn(__‘. about an asymmetry of the lungs, the right lung increas- ing and the left lung diminishing. Why this should have happened is not yet absolutely certain: it may probably have been in adapta- tion to active movements of ‘lateral flexure, for we see the same thing taking place in Grymnophiona, Snakes and Snu.l<o—like Lizards. That it has been the right rather than the left lung which has increased in size, is probably correlated with the rotation of this region of the alimentary canal in a counter-clockwise direction as seen from behind (seep. 168) which would tend to interfere more with the circulation through the left lung than with that through the right, by lengthening the course of the left pulmonary artery. Steps 174 EMBRYOLOGY OF THE LOWER VERTEBRATES GIL

in the development of this asymmetry are seen in Poly/ptems and in the Lung-fislics.

4. In purely aquatic creatures the dictates of adaptation would naturally cause the air-filled lung to assume a dorsal position. An initial phase of this is repeated in Polyptems where the right lung has become dorsal and median in its hinder portion. In the Lung- fishes a further step is takcn—--the whole of the lung becoming dorsal except the pneumatic duct which still remains to mark out the path by which the lung moved dorsalwards round the right side of the alimentary canal.

That the movement dorsalwards was round the right side was no doubt due to the right lung being predominant and the left reduced in size. In the case of Oezatodus the predominance of the original right lung has been retained, the other being completely obsolete except for a short period during development. In Lepidosvlzen and Protopterzzs, on the other hand, the lopsidedness disappears, the original left lung regaining during ontogeny its primitive equality in size with its follow.

5. In the Actinopterygians——those fishes which show the highest degree of evolution in adaptation to a swimming mode of 1ife———the lung has in the course of its evolution passed through similar stages to those exemplified by Poly/pterus and Oeratodus. Here again only the original right lung persists as the air-bladder, the vestige of the left lung being possibly represented by the little diverticulum found by Moser upon the pneumatic duct in early stages of development} In the Actinopterygians a further step onwards has been made in that the glottis has assumed a dorsal position. This is fully ex- plicable by the rotation which this part of the gut has undergone, aided no doubt by the principle of economy of tissue which would tend to bring about a shortening of the unnecessarily long pneumatic duct. In some cases there still persist vestiges of the ancient cellular respiratory lining of the swim-bladder (ag. LeZn'asz'na, Ezytlmnas).

6. Finally in the Physoclistic forms-——the most highly specialized of all-—the swim-bladder has become completely isolated from the gut, its respiratory function has gone and it subserves a mainly hydro-

static function. The outline given above represents a scheme of evolution which

in the light of modern research has a high degree of probability. Of course as in all such evolutionary speculations there exist details which are still difficult to explain. While most of the facts of com- parative anatomy fit in well with it, some do not——such as, for example, the nerve-supply and the blood-supply of the air-bladder of Am1Ia——-but it may be anticipated with considerable confidence that these difficulties will be lessened or disappear with the progress of

research. ' See p. 168. This matter affords an interesting subject for further research. III . THY R0 ID 17_5

DERIVATIVES OF PHARYNGEAL "WALL OTHER THAN THE RESPIRATORY ORGANS

THYROID.——The Thyroid gland arises as a mid-ventral outgrowth of the pharyngeal or buccal floor about the level of the Hyoid

FIG. 99.—Sagitta1 set-.t..i0ns through :mturior portion of :llilllI.‘Hl:U'_\' Uttlllll of l.¢jlm'do.w'n-1 illustrating the (l(‘.\'L‘l()plll('.lli- of the 'l‘hyroi«1.

A, I}, C from specinuyns of sttlgn 30; I), :-'et.‘l;.,'- 3| ; 'I‘h, thyroid: I, t.on:.rn¢-_

arch. In those Vertebrates in which the pharyngeal rudiment is solid at this stage the thyroid outgrowth is also solid at its first appearance (Fig. 99, A, T/2,) and develops its cavity secondarily by eytolysis. 176 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The Thyroid becomes gradually constricted off from the pharynx (Fig. 99, B and C) remaining for a time connected by a narrow stalk or duct with the pharyngeal or rather buecal floor just in front of the primary tongue (see Fig. 82, p. 149). This stalk of attachment becomes nipped across and the thyroid forms a mass (Fig. 99, D) or vesicle rounded in form or somewhat elongated in an antero-posterior direction lying in the mid-ventral line beneath the pharynx and just in front of the ventral aorta.

The originally simple vesicle undergoes a process of sprouting and division by which it becomes converted into a mass of rounded vesicles, each possessing a wall composed of a single layer of cubical epithelial cells and separated from its neighbours by highly vascular mesenchyme which penetrates in between the vesicles to form the stroma of the organ.

During later development the Thyroid undergoes characteristic changes of form in different subdivisions of the Vertebrata. Thus in Teleosts it frequently assumes a more or less diffuse character, the follicles being distributed in the neighbourhood of the ventral aorta and roots of the aifcrent branchial vessels. In the Amphibia and Amniota the organ becomes deeply constricted into two laterally placed lobes which may remain connected or may become separated, so that it assumes a paired character as happens in Amphibians

and Birds. With the processes of differential growth involved in the develop-

ment of the neck, the thyroid may undergo considerable displace- ment from its point of origin. Thus in adult Lizards it lies across the trachea well forwards from its hind end while in other reptiles and in birds it lies farther back close to the roots of the great arteries.

It is now generally accepted that the clue to the _phylogenetic history of the Thyroid is afforded by its development in Petromyzon (W. Muller, 1871). Here there develops a mid-ventral outgrowth of the pharyngeal floor, forming a short gutter in the branchial region, the lining of Which is composed partly of glandular cells which secrete a sticky mucus and partly of cells which bear powerful flagella. Morphologically this gutter is the same as the endostyle of Ampkiowus and during larval life its function is also similar: it appears to be in fact simply a shortened up endostyle. The slit-like pharyngeal opening becomes gradually reduced in length till it forms

merely a small pore. _ At the time of metamorphosis the pore becomes obliterated so

that the organ becomes a closed vesicle underlying the pharynx. This vesicle divides up into a number of small vesicles and its mucous secretion accumulates in their interior as a colloid substance like that of the Thyroid vesicles of the Grnathostomata. In a word, the endostyle of the Ammocoetes stage becomes the Thyroid of the adult, and there seems no reason to dpubt that the same has happened in phylogeny and that the thyroid of the Vertebrate is 111 BRANCHIAL BU DS 177

simply the modern representative of the endostyle of the proto- chordate ancestor.

An interesting feature is that while the physiological importance of the thyroid in the modern Vertebrate is that of a ductless gland for the production of internal secretion to be absorbed by the blood, it still goes on producing the mucous material used by the far back protochordate ancestor for entangling food particles, though that substance is no longer, owing to the disappearance of the duct, discharged into the pharyngeal cavity.

BRANCIIIAL BUDs.——There make their appearance in the develop ing Vertebrate a series of bud-like proliferations of the endodermal epithelium of -the branchial clefts which may be known as branchial buds. They appear at the upper and lower angles of the clefts and the series shows its fullest development in the Lampreys, where buds develop at the dorsal and ventral angles of all the clefts. In the majority of fishes investigated they have been found to appear at the dorsal angles of all the clefts eficept the first; in Urodele Amphibians at the dorsal angle of . clefts and at the ventral angle of II., III. and IV.; in Anura at the dorsal ends of I. and II. and at the ventral ends of ll.-V.; in Lacerta at the dorsal ends of I.-III. and the ventral ends of III. and IV.; in Gallus at dorsal and ventral ends of III. and IV.

The morphological significance of these organs is still completely obscure. Physiologically some of them appear to be of importance during the later stages of development preceding sexual maturity inasmuch as they give rise to that often bulky organ the Thymus. This arises by the fusion together of more or fewer of the dorsal buds, the others undergoing no further development. Thus in Lepvldosvlren (Bryce, 1906) dorsal buds III. and IV. develop into thymus while II. and V. undergo no further development: in Oeratodus (Grreil, 1913) II., III. and IV. give rise to Thymus while V. and VI. do not develop further: in H3/pogeopltds II., III., IV. and V. give rise to Thymus while rudiments on I. and VI. atrophy.

In regard to the much discussed histogenesis of the thymus all that need be said here is that the originally solid epithelial rudiment becomes in -the course of development loosened out into a sparse reticulum interpenetrated by mesenchyme richly traversed by blood- vessels and crowded with leucocytes.

The ventral buds, where they occur, become constricted off from the branchial epithelium forming simple rounded masses of epithelial cells (Amphibians) or they may be subdivided up by intrusive con- nective tissue into solid portions (Reptiles) or hollow vesicles (Birds). The small organs so formed are termed by their discoverer Maurer epithelial bodies: their physiological significance is quite unknown.

There normally develops in the Vertebrate either on both sides or only on the left side a small pouch-like diverticulum of the pharyngeal wall close to the ventral edge of the last gill cleft, what- ever the number of this be in the morphological series. The

VOL. II N 178 EMBRYOLOGY 01+‘ THE LOWER iVE_R'l‘EBIx’.A'1‘ES en.

divert culum becomes" separated from the pharynx and commonly gives 'se to numerous rounded vesicles somewhat resembling those of the yroid in appearance. The organ thus formed was named by van Be melen who discovered it in Elasmobranchs--—suprapericardial body—— iile Maurer has termed it the postbranchial body. Nothing is delini ely known regarding either its function or its evolutionary history, though it is someti.mes regarded as representing a vestigial last gill—pouch. A curious point is the tendency of the organ to unilateral development it makes its appearance only upon the left side in a large number of cases (Aca.nthz'as, Lepidosirevz and 1’7‘0t0pz1e'ms——-see Fig. 109, B-—most Urodeles, some Lizards).

CEMENT ()1z.m\Ns or 'J;m.sosroMA'i'oUs FlSHES.———-It has long been _

FIG. l00.—— Ventral views of Polyptcrus larva to show the cement-organs. A, Stage 80; B, Stage 33; c.o, cement-org:-m ; «.71, olfactory ox-gun; m, mouth ; V, ventric-le of heart.

known that the larvae of Actinopterygian ganoids possess cement- orgaus on the head in front of the mouth. Balfour (1881) wrote of this as “ a very primitive Vertebrate organ, which has disappeared in the adult state of almost all the Vertebrata; but it is probable that further investigations will show that the Teleostei, and especially the Siluroids, are not without traces of a similar structure.”

The organs in question were generally regarded as being developed from a thickening of the ectoderm. Miss Phelps (1899) lirst stated that they originated from endoderm (Amie) and the present writer, at the time ignorant of her work, was greatly surprised to find himself forced to this same conclusion by the examination of I'5udgett’s material of Poly/pterrus.

The cement-organ of Poly/pterus (Graham Kerr, 1906 and 1907.), when at the height of its development,_ forms a stout cylindrical structure with a deep hollow at its free end, projecting from the 188 EMBRYOLOGY OF THE‘ LOWER VERTEBRATES


FIG. 108. Illustrating early development of the liver in Birds. A, 47-hour chick; B, 52-hour chick; C, 50-hour chick (after Brouha, 1898); D, fourth-day chick; E, 7 mm. embryo of the Roseate Tern--Sterne paradi.«n'aca.-(after

Hammer, 1807). M 1, rudiment of anterior (“1eft") bile-

CH.

portion of the rudiment. The gall bladder originates as a bulging of the floor of the bile-duct towards its anterior end. A The formation of the pos- terior and longer section of the bile-duct, Which will be extrahepatic in the adult, lagsin its development behind the anterior portions of the rudiment. Such differences in the time of appearance of different parts of the hepatic apparatus——1iver, gall-bladder, bile-duct—-—are to be looked on as mere secondary modifica- tions of development,——the

primitive condition being that i

of a simple pocket of the gut- wall such as persists in Am’- phioasus.

SAUROPSIDA.-—-The hepatic apparatus here again makes its appearance as a longi- tudinally situated pocket of the morphologically ventral Wall of the gut. In birds this is situated at first on the anterior wall of the yolk—stalk (Fig. 108, A). The diverti- culum grows actively into an anterior (dorsal) and a pos- terior (ventral) pocket (Fig. 108, C, Z73. 1 and Z7}. 2) while the intervening portion be- comes flattened out and incor- porated in the gut-wall.

There thus come to be two distinct liver-rudiments an anterior and a posterior. Of these each sprouts out at its end into irregular projections which eventually fuse and form a spongy mass, surround- ing the cavity of the ductus

duct; bd°. posterior (“risht") bile-duct; ent, cavity or venosus, and havingin its

fore-gut; gb, rudiment of gall-bladder; la‘. 1 and 2, anterior and posterior liver - rudiments; pa.‘n,\ dorsal rudiment of pancreas.

meshes blood-spaces which 180 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The first rudiment oi’ the organ is seen to bee simple pocket-like outgrowth of the gut-wall (A, c.o): this becomes more and more prominent (B, C): it becomes gradually constricted oh" at its base from the gut-Wall, its cavity becoming isolated first Finally it separates completely from the main endoclerm and its outer end undergoes fusion with the deep layer of the ectoderm. lts cavity then opens to the exterior and the fully functional condition is reached——the endodermal origin of the secretory lining being for a time betrayed by the conspicuous persistent yolk granules in its cells.

It will be noted that the exposed side of the secretory epithelium, that on which the secretion is extruded, is that which originally faced inwards towards the lumen of the alimentary canal. In other words the direction in which the extrusion takes place is morpho- logically the same as that of any other part of the glandular lining of the gut-wall.

As is the case in other forms the cement—org-an is a transient, purely larval, structure. About stage 31 (Fig. 197, D) degeneration commences: the gland shrivels up, the gland-cells becoming more slender and dark pigment making its appearance in their interior, the epithelium becomes penetrated by ingrowing blood-vessels, its cell-boundaries become indistinct. The process of atrophy goes on rapidly and by stage 36 (Fig. 197, F) the organ has completely disappeared.

An interesting variation from the normal course of development is found in specimens in which the cement-organ rudiments are more or less approximated to one another. This variation reaches its maximum in occasional individuals in which they are completely fused and form an unpaired structure, continuous across the mesial

lane.

P In the actinopterygian Ganoids the cement—organ develops along the same general lines as those just indicated. In the Sturgeons the development has been worked out recently by Sawadsky (1911) in Acipenser ruthenus. Here the organ forms a rounded projection, very much in the same position as that of Polypterus, but in this case each becomes divided by a groove so as to form two rounded knobs. These knobs eventually grow out to form the tactile barbels of the adult, the secretory epithelium being carried out on the surface of the barbel as it grows.

The secretory epithelium is here also endodermal, its rudiment being the gut-wall immediately dorsal to the position in which the mouth will develop later and being continuous across the mesial plane. The unpaired condition which occurs in Polypterus as a variation is thus normal in the case of the sturgeon. As the head increases in length the secretory epithelium becomes carried out on its ventral surface, looking just as if it were the thickened ectoderm of this surface. Finally the paired condition comes about, ‘the lateral

parts of the secretory epithelium coming. to be supported by the knob-like projections already mentioned. III CEMENT-ORGANS 181

Am/ia is of special interest in regard to its cement-organs as it was in this form that their endodermal origin was first announced.‘ The organs are for a time in the form of a pair of rounded knobs, one on each side, but these take on a crescentic shape so that together they form a circular wall, interrupted anteriorly and posteriorly. Each organ contains a pocket-like projection of the gut—wall which takes on a somewhat sausage-like form in correlation with the curved shape of the organ as a whole. This endodermal sac separates from the main endoderm and becomes constricted across, so as to form°a curved row of closed vesicles from six to ten in number. Each vesicle fuses with the ectoderm and develops an opening to the exterior so that it takes on the appearance of a cup at first deep and narrow, later shallow and wider, its lining continuous with the deep layer of the ectoderm.

When the larva reaches a length of 13-14 mm. it makes less use of its cement-organ and the latter commences to degenerate, sinking beneath the surface with which, however, it remains connected by a narrow tubular channel. By about the 20 mm. stage this has disappeared and soon there is no trace of the organ to be found even in sections.

In Lepidosteus the organ appears to be similar while in the other ganoids its development still remains to be worked out.

These cement-organs are of special interest and importance for more than one reason. In the first place they are of importance in revealing a quite unexpected pitfall in the way of the investigator trained to have implicit faith in the germ-layer theory, for they show how a particular organ may become transferred from one g'erm—layer to another even though not belonging to the transitional zone where the two layers are continuous. A very common modification of ontogenetic development consists in the slurring over or even omission of particular stages in early development. Were this to happen in the case of the early stages in the development of the cement—organ say of Polypterus, it is easy to see that the organ might have every appearance of being purely ectodermal in its nature, although it is, as a matter of fact, endodermal.

It appears to the present writer quite possible, if not probable, that this modification has actually come about in the Dipnoi and Amphibians, and that the cement-organs of these groups, although they develop from the ectoderm in those forms which have been investigated (p. 79), are really homologous with the cement-organs of the Teleostomi, their cndodermal stage having been eliminated from ontogenetic development. Further investigations are needed in the Amphibia——to see whether no trace exists, in any member of the group, of an original connexion with the endoderm.

As regards the original nature of these organs it is impossible to arrive at any certain conclusion. Arising as they do in the form

‘ Phelps (1899). The actual discovery seems to have been made by Rcighard. Cf. Reighard and Phelps (1908). 182 EMBRYOLOGY OF THE LOWER VERTEBRATES 011.

of endodermal pockets, they obviously recall gill pouches on the one hand and coelenteric pouches on the other. Their position suggests a pair of preniandibular gill pouches: their function, that of forming an excretion (cement), perhaps indicates rather coelomic afiinities and the present writer suggested (1906) their possible correspondenee with premandibular head cavities of other Vertebrates. Reighard and Phelps (1908) hornologize them wi th the anterior pair of head-cavities of Elasmobranchs while van Wijhc (1914) sup-


l+‘m. I02. -Larva of Sa'rr_'.('u_l¢I..r_t(?s amlon. (After Budgett, 1901.) P‘-’I't3 3' homology With"

the ciliated organ of Amphiowus.

Altogether these cement-organs are very interesting and puzzling structures and would well repay further investigation. A thorough comparative study should be made of their development in the archaic Crossopterygians and of their possible homologues in Elasmobranehs.

Little is known regarding cement-organs in Teleosts, though it is proliable they will be found to occur in various tropical fresh- water fishes. Budgett ' s (1901) found a large P, _ cement-organ on the he . I « , head of the larva of the '1 /if/“‘jX~” Characinid Sarcodaces odéie (Fig. 102, 0.0). Ina A 0 / ( larva believed to be that 9 ' of the Mormyrid IIype7°o- lg J

«an, or-ment-organ.

pisus babe he found six ’ ‘( Well-marked cement 6. glands on the head which 1 . in this case secrete fine " U threads by which the

larva hangs suspended ' ' F 1' 103 ——Teleostean l as in“ )()‘8l to be those of in the water until the "' p " s ““’*°' *1’! ~“ ’_

lk . _ _ _ ,. II;/_,;er0pos1ts bebe, suspended from the rootlets 111 the

18 H11_St‘3d t1_1P (15 1gd' nest. (From Budgett, 190].)

. 8 670 18 3.11

Gymnarclms also possess similar organs—-very small in the latter case 0

(Assheton, 1907).

The organs in these various fishes present, the appearance of being ectodermal thickenings: we have as yet no information as to whether, as may be suspected, they really originate from the endoderm.

DIGESTIVE TRACT. —- The respiratory region of the alimentary canal is succeeded by the true digestive _.tract and this shows more or less pronounced differentiation into successive portions-— III THE ALIMENTARY CANAL 183

oesophagus, stomach, intestine and its subdivisions, cloaca. In correlation with the digestive and assiniilative function of the intestinal endoderm this serves during early stages as the favourite storehouse of food-yolk, and the concentration of yolk in the abapical portion of the unsegmented egg is to be looked on as a foreshadowing of the fact that this portion of the egg will later become the endoderm. .

In the holoblastic Vertebrates the mass of heavily yolked endo- derm cells becomes, as it were, modelled into a tubular shape by the

Fnz. 104. ----Illustrating the modelling of the yolk in .I¢.-/4././¢_:/u/;/u'.s-. (After Sarasins, 1889.)

A and B illnslmlu the sauna st::;.:«-, 13 representing a view from the-. dorsal side. The small—celle«.l epithelial port.-inn or the g-nt.-\m,ll is seen passing down the centre of l"i_«_;. ll. (3, I). and }*3r'np1"t-sent later stages «h-awn from the ventral side; F (7 cm. embryo) vent:-o-lateral view frmn the right side.

reciprocal activity of endoderm and splanchnie mesodermg the rudiment so formed undergoing active growth in length and differentiation of structure while the yolk is being assimilated.

In the two most archaic groups of holoblastic gnathostomes, the Crossclptm-ygians and the Lung-fishes, a feature of special interest is the development of the spiral valve. In Lepiciosiren, as is indicated by Figs. 105 and 106, this takes its origin by the solid mass of yolk-laden endoderm l)(‘('_fi(‘)llllllg modelled into a right-handed spiral coil——the deep incision xvhich separates successive turns of the spiral being filled up by ingrowing mesenchyme belonging to the splanclmic mesmlerni. There can be little doubt that this is a secondarily Il'l()(l.lllt__*.(l. mode of development, but nevertheless it is probable that the spiral coiling of the endodermal rudiment-v__is to be 184 EMBRYOLOGY OF THE LOWER. VERTEBRATES CH.

explained as a repetition of_ an ancestral condition in which the intestine as a whole was long and spirally coiled.

An important feature of such a spiral coiling of the gut rudiment is that it would necessarily tend to bring about a twisting of the

Flu. 105.~--l')issm-t.ion.s- of young In-1)i<lo.<in-..ns of stages 32 (A), 35 (B), 36 (C), and 37 (D), lmm I.lu_: vi-ntral side to show the modelling of the intestine.

g.b, gall-l')la_(lder; Ii, liver; V, \'c-uh-i«_-|(-,

alimentary canal just in front of the spirally coiled portion in a counter-clockwise direction as seen from behind, vie. a movement in which points on the ventral side of the alimentary canal would become shifted towards the 1-iglit side. A_s.a]ready indicated such a twisting of this region of the alimentary canal actually does take 111 THE ,ALIMEN'l‘AR'Y CANAL

place in' development causing the lung rudiment to shift dorsally round the right side of the alimentary canal.

In the more richly yolked Vertebrates the Ventral ‘[N)1'lJl('lllS of the gut-Wall are more and more clogged up with yolk and this results in a greater and greater concentration of de- velopmental activity in the dorsal wall. This is clearly in- dicated by transverse sections through the developing gut of Vertebrates wl1i_ch though rich in yolk are still holoblastic. Such sections (Fig. 107) show the dorsal wall of the gut to consist of small active cells ar- ranged as a columnar epithelium, while the side walls and floor consist of large coinparatively inert yolk -laden elements. It

"is only as development goes on,

and as the yolk is consumed, that the epithelial small-celled character gradually spreads ventrally.

In the actually meroblastic Vertebrates, the heavily yolked portions of the primitive gut- wall never undergo segmenta- tion at all, unless possibly as regards a thin superficial layer. They remain as a continuous mass of yolk, round which the epithelium gradually spreads. In this case the formation of all the important organs of the ali- mentary canal is concentrated in the dorsal. portion which he- eomes gradually folded off from the main mass of the yolk. This folding-off process takes place most actively in the anterior region, so as to form the tubular l'ore- gut, and also posteriorly, the intermediate portion re-

185

FIG. 106.—Dissection of Lepidosiren larva of stage 35.

t.o, tectum o1m2cunz..
pm, pronephros

oc.r, occipital rib

u 1

o. c, auditory capsule
Ii, liver
l, lung
ht, heart

h, hemisphere 186 EMB‘RYOLOGY or THE LOWER VERTEBRATES on.

maining for a time as a longitudinal groove opening ventrally towards the yolk. As the lips of this groove‘ gradually coalesce at each end the communication between the gut cavity and the,yolk becomes gradually narrowed down to the tubular cavity of the yolk- stalk situated at first behind the liver but later becoming shifted forwards by differential growth. Eventually this becomes obliterated and the definitive alimentary canal becomes completely isolated from what remains of the yolk. In many Telcostean fishes this isolation takes place at a very early stage in development. . _ _ The alimentary canal is, in correlation with its dlgestive function, necessarily a highly glandular organ. Primitively the secretory functions are carried out by unicellular glands, scattered about amongst the other epithelial cells of the endoderm, but in the Verte- brates, as in all the more complex Metazoa, special concentrations of gland cells and of secretory activity take place in localized portions of the enteric wall. Each of these specially glandular patches under- goes a great increase in its area, which causes it to bulge outwards as a simple or much subdivided Fro. l07.—'l.‘ransverse section through bind and comphcated pocket’? formlng

portion of intestine of a larva of Ich- flu CllSl':lIlCl: glandular appendage Of

thytlfih-1:8. (After Sarasins, 'l‘he the alimentary canal_ stage of development was that shown in


Fig, 104, F. ' The sheath of splanchnic LIVER‘ _"_Of these 81a’I?d“1‘“' megodgrm is omit,ted_ appendages, 1n the (3359 _Of Verte-

brates, the most ancient appears to be the liver, which is already present in Amp}:/ioasus. In this animal the liver originates in ontogeny (Hammar, 1893) as a pocket-like outgrowth of the alimentary canal wall on its ventral side and slightly posterior to the hind end of the pharynx. Apart from increase in size and relative narrowing of its base of attachment the liver in Amphiomus undergoes no further complication but retains its extraordinarily primitive pouch-like condition throughout life.

In the holoblastic Craniates the liver arises similarly as a ventral projection of the alimentary canal wall. This shows the customary modifications in correlation with the presence of yolk, arising in some cases in the more primitive fashion as a hollow pocket (Lampreys, many Amphibians, Cemtodus), in others (many Amphibians, Lep1Sdo- siiren and I’-rotopterms) as a solid knob of yolk-laden cells (Fig. 105, Z73). This grows rapidly in size, as it uses up its food-yolk, and becomes constricted off from the main mass of yolk by ingrowing mesenchyme, until its attachment becomes narrowed down to a slender stalk-—-the rudiment of the bile-duct.

The pouch-like rudiment of the liver undergoes an active process of sprouting into numerous secondary pockets, each of which becomes III LIVER 187

greatly elongated and branched, and gives the gland a tubular character. This character may be retained throughout life (Lampreys) but normally the tubules undergo anastomosis so as to form a net- work of trabeculae. While this is to be regarded as the primitive mode of development of the tubules it is to be noted that they more usually in actual fact show the modification of development which we have learned to associate with the presence of yolk, being at first solid and taking their origin not by a process of outgrowth but rather by a process of modelling by ingrowing mesenchyme.

In the meroblastic Vertebrates also the liver may be described as originating from a mid-ventral outpushing of the enteric wall. Variations occur in detail, in correlation with the varying relations of the hepatic portion of enteric wall to the fore-gut and yolk-sac. If this part of the gut-wall has already been folded off from the yolk-sac and incorporated in the fore-gut, then the early stages of development of the liver divertieulum pursue their normal course. If, on the other hand, it still forms part of the yolk-sac wall, the hepatic rudiment makes its appearance as a projection from this, and it may be in its first beginnings paired, its two halves separated by the longitudinal slit by which the cavities of the definitive gut and the yolk-sac are still continuous.

ELASMOBRANCHII.——The hepatic divertieulum at an early stage bulges out to form a conspicuous outgrowth on each side anteriorly ——the rudiments of the right and left lobes of the liver. The median portion between these becomes in its anterior region converted into secretory tissue while its posterior part becomes the bile-duet, with its dilatation the gall-bladder.

In Acanthzas (Scammon, 1913) the first rudiment of the liver, which makes its appearance at a time when this region of the enteron is not yet floored in but opens freely into the subjacent yolk-sac, is distinctly paired. In view of the unpaired condition in Amplmloxus and the holoblastic Craniates there can be little doubt that this condition in Acanthvlas is a secondary modification as indicated above. Secondary pockets soon make their appearance on the wall of the secretory portion of the rudiment, and grow actively into elongated and much-branched tubules. These fuse together secondarily to form the network characteristic of the fully developed liver. This network is bathed by the blood of the vitelline veins (see Chap. VI.).

After the embryo (Acamthias) has reached a length of 25-28 mm. the walls of the tubules, or trabeculae of the network,increase greatly in thickness so that both their own cavities and the intervening blood-spaces become relatively reduced and the organ assumes the compact definitive condition.

Whereas the tubules become throughout the greater part of their extent secretory in function the proximal portions, each common to a group of tubular branches, functionmerelyas ducts. These communi- cate with the main bile-duct formed from the posterior and median 274 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

of the genital portion of the peritoneal epithelium with the peri- toneal funnels, or with the nephrostomes, were in the form of open ciliatecl grooves or gutters on the surface of the peritoneum, that later on these became closed in to form tubular channels, and that in actual ontogenetic development in the modern amphibia the development from the coelomic epithelium has become obscured except for traces now at one end now at the other.

At their distal ends the cell-strands in the male can be traced gradually farther and farther into the genital fold until they come into immediate relationship with the cell-nests of gonocytes. In the female of Urodela and Anura the strands do not spread so far into the genital fold, nor are they, even in early stages, so well developed as in the male.

The fatty body is developmentally simply a portion of the genital fold which becomes specialized as a store-house of fat. In

Anura it is the progonal portion which undergoes this differentiation

while in Urodeles and Gymnophiona the rudiment of the fatty body is continued backwards as a ridge along the mesial face of the genital fold throughout its extent.

The fat is stored in the connective tissue of the organ, the fat cells being usually interpreted as immigrant mesenchyme cells which have invaded the rudiment by its base of attachment. It has also been suggested that these fat cells are peritoneal in their origin (Abramowicz, 1913)——a suggestion of obvious interest in view of the general tendency in the animal kingdom for potential germ~cells to undergo degeneration in order to provide nourishment for the germ- cells which become functional.

'1‘Es'1‘1s.———The development of the functional testis out of the genital fold is seen in peculiarly simple and diagrammatic form in the Gymnophiona. Here the strands of the urinogenital network, as they sprout into the interior of the testis, anastomose together along its axis so as to form a central canal——around which, embedded in the stroma of the organ, lie the rounded nests of gonocytes. Fusion

takes place between each gonocyte-nest and the wall of the central

canal and then each nest develops a cavity in its own interior and becomes a hollow ampulla opening into the canal at its inner end.

Various modifications of this simple scheme are to be found. In Grymnophiona themsel.ves ampulla—formation becomes suppressed except in localized regions between successive vasa etferentia, so that intervening portions of the testis are sterile and form merely thin tubular eonnexions between the bead-like fertile portions. Again the ampullae vary in shape: they may be elongated and tubular (Discoglossas) or, as in the majority of cases, flattened against one another by pressure. The “axial” canal again may lie close to the surface: it may become greatly branched, as in most Urodeles, or may form a complicated network as in most Anura.

0VARY.—--In the differentiation of the ovary (Bouin, 1901) the most important points to be noted are the following. As regards 111 PANCREAS 189

communicate with the just -mentioned vessel. This spongy mass, the trabeculae of which are at first solid and only secondarily develop a lumen, forms the secretory portion of the liver, while the proximal portions of,the outgrowths persist as the two con- spicuous bile-ducts of the adult bird (Fig. 108, D, E, bcl. 1 and M. 2). In such birds as possess a gall-bladder this is formed by a dilatation close to the point of junction of the posterior hile-duct with the gut-wall (Fig. 108, I), E, gb).

PANCREAs.—The pancreas, though in the adult a single structure, arises typically from three distinct rudiments, each of which is at first a simple pocket-like outgrowth of the splanchnopleure. One of the rudiments (cf. Fig. 80, H) is situated dorsally a little posterior to the stomach, the other two, which appear somewhat later, are ventral and arise as outpushings of the hepatic diverticul11m in the region of the bile-duct. The ventral pancreatic rudiments are commonly paired, arising one on the right and one on the left of the hile~duct. .

The three rudiments increase in size, secretory tubules sprout out from them and the two ventral rudiments become carried in a

' dorsalward direction, up the right side, by the rotation which the gut

undergoes in this region (see p. 168). The right ventral rudiment comes in contact with the dorsal rudiment and fusion takes place—- all three rudiments forming a single organ the three—1'old origin of which is indicated by its three communications with the alimentary canal.

Such may be considered the typical mode of development of the pancreas, but important variations in detail occur in the different groups. In Cyclostomes and Elasmobranchs only the dorsal pancreas is known to occur. Its development in the former group requires further investigation. In Elasmobranchs it arises as a longitudinal groove of the enteric wall dorsally and a little posterior to the open- ing of the bile-duct. It becomes constricted off’ from before backwards and in accordance with the rotation of the alimentary canal it becomes shifted to the left side and ends up by being ventral.

In Crossopterygians the three typical rudiments appear (Fig. 80, H) but their development has not been followed in detail. Eventually the pancreatic complex extends forwards beneath the liver and com- pletely fuses with it forming a tlfick layer over its ventral surface in the region near the opening of the bile-duct.

In Actinopterygian Granoids also (Piper, 1902; Nicolas, 1904), the pancreatic complex derived from the original three rudiments becomes fused with the substance of the liver, only its posterior dorsal portion remaining extrahepatic. The main duct of the pancreas is the persistent stalk of the right ventral rudiment which opens into the gall-bladder formed by the dilated terminal part of the bile-duct. Of the two other pancreatic ducts the left ventral apparently atrophies entirely, while the dorsal is said in the case of Am/ia to disappear but in the Sterlet (Acipenser ruthenus) to persist. 190 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

In Teleosts the early development agrees closely with that of the ganoids, only a doubt exists whether the definitive pancreatic duct (Duct of Wirsung) may not be formed by a fusion of the two ventral ducts rather than by the persistent right duct alone. During later stages great difl'erenees arise between dilferent members of the group. l n some (Silwrus, Esow) the complex forms a single compact gland, in others (flcomber, U3/_7)7"in'w.<¢) it becomes divided i.nto a number of independent lobes, in others, including the majority of the more familiar Teleosts, it becomes greatly branched and is diffused in the substance of the dorsal mesentery while in still others (Labridae, Syngnr.rt7tus) the condition resembles that of the ganoids a large part

of the organ being intrahepatic (Laguesse, 1894).

FIG. 109.-—-Dorsal View showing l'udiIm:nts of 1 orsal p:nu:reas .-mil luring in l:u'v:1c of Protopterus (stages 32 and 34).

I, lung; op, opercnlum ; pied, dorsal pancreas; p.b, pnstbranchial body; p.f, pectoral limb;

3:-, \'i.~'.«-o-ml t_'l¢'.fL rudiment.

In Lung-fishes the three typical rudiments make their appearance. In Protopterus the dorsal rudiment (see Fig. 109, A, pad) is a solid outgrowth (hollow in Lepidosiren) from the gut-wall, usually rounded in form but occasionally elongated in an antero-posterior direction as in the specimen figured (Fig. 109, A). The attachment to the gut becomes rapidly constricted to a narrow stalk and a cavity develops in the interior of the rudiment. The ventral rudiments appear a little later, as solid projections one on each side of the attachment of the bile-duct to the gut. The two ventral rudiments, as they increase in size, meet and fuse dorsal to the bile-duct, and later on the dorsal surface of the right ventral rudiment comes in contact and fuses with the ventral surface of the dorsal rudiment. The stalks of the three rudiments remain as three ducts, the two ventral opening just posterior (original right rudiment) and anterior m PANOREAS ' 191

(original left) respectively to the opening of the bile,-duct, while the dorsal opening is situated at the extremity of the spout-like pyloric valve.

The general course of development in Lepidosiren is similar and in both it is characteristic that the pancreas never bulges beyond the mesodermal coating of the splanchnopleure. It remains embedded throughout life in the gut-wall and is consequently not noticeable in an ordinary dissection.

In Oeratoalus (N eumayr, 1904) the development of the pancreas is similar though here the left ventral rudiment, which in Protoptems is smaller in size than the right, remains rudimentary.

The Amphibia are of special interest from the fact that it was a member of this group (B0m.b7Inat0r) in which Goettc (1875) first observed the origin of the pancreas from three separate rudiments. Groeppert (1891) was able to extend the observation to various other Amphibians, both Urodele and Anuran, and to show that in Urodeles the dorsal rudiment retains its duct, opening just behind the pylorus, while in the Anura this duct disappears. In both cases the ducts of the two ventral rudiments undergo fusion to form a duct of Wirsung which opens into the bile-duct.

In Reptiles (Lace7~ta-—-Brachet, 1896) the right ventral and the dorsal rudiments fuse to form the definitive pancreas, the left ventral atrophying (cf. Lung-fishes). According to Brachet the duct of the dorsal rudiment does not disappear but fuses with that of the right ventral to form the definitive pancreatic duct.

Birds show three rudiments which undergo fusion into a complex in the normal fashion, all three ducts remaining functional and conspicuous in the adult. Suppression of the left ventral rudiment occurs as an occasional variation.

The observed facts of development of the Pancreas clearly justify the conclusion that this organ of the modern Vertebrate has arisen in the course of evolution from three originally separate diverticula of the glandular enteric wall-~—-a pair arising from the hepatic pouch and the third from the dorsal wall. The precise localization of the rudiments at comparatively distant points of the enteric wall point to the probability that the nature of the secretion was originally different in the case of the ventral pancreas from that of the dorsal.

PYLOBIC GAECA.—The caeca which are present in the pyloric region in many actinopterygian fishes arise as simple outgrowths of the gut-wall. The interesting suggestion has been made (Taylor, 1913) that the simple circle of these caeca, which is apparently their most primitive arrangement, corresponds morphologically with the curious valve found in various fishes (Amia, Lung-fishes, Symbmnchus, Anguilla, etc.) in which the pyloric end of the stomach is prolonged back into a kind of spout which is ensheathed by the anterior end of the intestine. The circular prolongation forward of the intestinal cavity round the gastric spout might clearly give rise to a circle of pyloric caeca simply by_ subdivision into a number [of separate 192 EMBRYOLOGY 01*‘ THE LOWER VERTEBRATES CH.

portions each of which continued to open into the gut cavity at its hinder end. ,

RECTAL GLAND.——ThlS organ, which occurs in Elasmobranchs, arises as a simple pocket-like outgrowth of the gut-wall. The super- ficially similar caecum of Lung-fishes will be dealt with in con- nexion with the renal organs.

CLOACA.~——-In the more archaic Vertebrates the ducts of the excretory organs open into the terminal part of the intestine which is thus a cloaca. It is believed by many that the excretory ducts originally opened at the hind end of the trunk independently of the alimentary canal and it is natural to suppose that the openings of the ducts have become gradually shifted first into close proximity to the aims and finally on to the lining wall of the ali- mentary canal. This again suggests that the cloaca may really be a proctodaeum—~that the skin has been involuted to form its lining and that with this involution the renal openings have also been carried inwards.

Unfortunately the facts of ontogenetic development do not so far as can be seen at present fit this simple and attractive hypothesis. The cloaca is, except for a small portion close to its opening, of purely endodermal origin——the renal ducts open on what is part of the primary entcric wall. A suggested explanation of this fact dilfering from that mentioned above will be found in the chapter dealing with the renal organs.

A cloaca seems always to be developed though in some cases (e.g. Teleostean fishes) it flattens out and disappears later so that the renal organs and the gut come to have independent external openings.

The bursa Fabricii, a conspicuous glandular appendage of the dorsal Wall of the cloaca in young birds, has usually been regarded as proctodaeal in its origin but it is now known to arise in ontogeny from vacuolar spaces in a solid projection from the cloacal rudiment, dorsal to the stalk of the allantois (Wenckebach, 1888) and would therefore appear to belong to the mesenteron rather than to the

proctodaeum.

The anal opening of the Vertebrate, as may have been gathered from Chap. II., is to be regarded as representing morphologically a portion of the gastrular mouth or protostoma. In a large number of Vertebrates however the opening arises in ontogeny not in this way but rather as a secondary perforation, although even in such cases the perforation arises in the line of the closed protostoma.

TEMPORARY OccLUs1oN or THE ALIMENTARY CANAL.——The ali- mentary canal is, in correlation with its function, a hollow tube. In a large number of Vertebrates, however, there are more or less extended periods of development during which the cavity is com- pletely absent, either throughout the length of the canal or in certain portions. .

In its simplest condition this occurs as a special case of the III THE ALIMENTARY CANAL 193

temporary absence of lumen so frequently found in the ndevelopment of eventually hollow organs from a richly yolk-laden rudiment. An idea of how it has come about will be got from an inspection of the various stages of the development of the alimentary canal of Poly/pterus as shown in Fig. 80 on p. 146. During early stages the

'archenteric cavity is seen to be widely patent throughout, except

that there is no mouth opening. During the later stages of develop- ment, immcdiately prior to the canal becoming functional, its walls

throughout the region between the fore-gut and the cloaea become

closely apposed, so as almost entirely to obliterate the cavity. Later on the walls recede from one another and the lumen becomes again patent.

It would obviously be merely a slight accentuation of this modification of development for the cavity to be completely obliter- ated for a time. A still further modification would be brought about by the omission altogether of the original hollow stage from the ontogenetic record. This actually- occurs in the case of the fore-gut in those Vertebrates in which this region of the enteric rudiment is yolk-laden: where, on the other hand, the yolk is practically completely concentrated in the mid-gut region as in meroblastic Vertebrates it does not occur as a rule.

The most striking temporary occlusions of the alimentary canal during development have to do with its terminal apertures. Thus there is not a single existing Vertebrate, so far as is known, in which the mouth opening persists from the gastrular stage, or in which even any connexion has so far been traced between the definitive mouth opening and the protostoma. In every case, even in A'2n,p/viowus, the mouth opening develops comparatively late as a secondary perforation. This modification of development is in the present writer’s opinion to be attributed to the entire dependence of members of the Vertebrate phylum upon food-yolk during early stages of their development, the need for a functional mouth having thus disappeared.

The anteroposterior extent of this occlusion of the alimentary canal in the region of the oral opening differs in different sub- divisions of the phylum. It may include a large part of the stomodaeal as well as the cndodermal portion of the buccal cavity as in the Lung-fishes (p. 148) but more usually it is confined to the boundary between the two, 7§.e. to the site of the original mouth opening the closely apposed ectoderm and endoderm being at this level continuous across the site of the future opening as the velar membrane’ (p. 145). The secondary perforation by which the alimentary canal comes to communicate with the exterior at its front end is in the case of some larval Vertebrates (e.g. Lapi- dosdren) closely correlated with the commencement of pharyngeal respiration but where the development is embryonic it commonly still takes place long before the.existence of any obvious functional need (ag. Chick, fourth day). At its hinder end the archenteron is,

VOL. 11 o , 194 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

as has been shown in Chap. I., widely open to the exterior in all the lower Vertebrates during early stages and in various cases this opening can be traced either into direct continuity, or into less direct but still clear relationship, with the anal opening. The explanation of this lesser degree of modification of the development of the anal opening as compared with the mouth may probably be associated with the less accentuated delay in the functional need for this opening. At stages long before ingestion or inspiration takes place by the mouth, the formation of waste products during the digestion of the yolk necessitates an outlet from the entcrio canal at its hinder end. Where obliteration does take place during still earlier stages this is probably correlated with the fact that the need of the opening is still non-existent.

It is 01' interest to notice that obliteration of the anal opening which is of a directly adaptive significance may take place at a later stage. Thus in Lr,p2Talos2Iq°e71. during about the lirst two weeks of larval life, when large numbers of practically motionless larvae are lying crowded together in the nest, the anal opening, which had been continuously patent in earlier stages, is closed, so as to prevent the poisonous excretory products from finding their way out. So also in the case of the Elasmobranch embryo enclosed within its

.egg-shell. In the Amniota the perforation of the anus is delayed to

a relatively late period doubtless for a similar reason.

It is characteristic of the phylum Vertebrata that the anal opening no longer occupies its primitive position at the extreme end of the body but has become shifted forwards along the ventral side. This shifting has probably come about with increased specialization for swimming by lateral flexure of the body, the withdrawal of the alimentary canal with its surrounding splanchnocoelic cavity frrim the hinder portion of the body, leaving the space they occupied free for increased development of the lateral muscles. This shifting forwards of the anus, leading to the differentiation of a distinct postanal or tail region, has occurred in all Vertebrates, least markedly in the more archaic groups. It reaches its maximum in some members of that group of Vertebrates which is above all others highly specialized for active swimming, the Teleostei, in some families of which the anus has actually assumed a jugular position.

During the actual ontogeny of the Vertebrate the process by which the anus comes to occupy a position 1nore or less distant from the tip of the tail region is somewhat modified from that which probably occurred during phyletie evolution. We do not find that the anus remains at the tip of the tail during the growth in length and that it then gradually shifts forwards along the ventral side. What happens is that the opening at an early stage assumes a ventral position and that the tail region proceeds to sprout out dorsal to it. The process will be understood from an inspection of Fig. 80 (p. 146). In B the anus is at the binder end, in Cit has 111 THE ALIMENTARY CANAL 195

assumed a ventral position being overhung by the bulging tail rudiment, in D, E, 14‘, G the tail rudiment is seen to be extending actively past the position of the anus, the specially actively growing tissues being indicated by the darker shading.

In Fig. 80, G, a feature is well shown which occurs in the embryos of most Vertebrates—-—the postanal gut (pay). It was shown in Chap. I. how a connexion-—-the neurenteric canal--—~existed

in some Vertebrates between the cavity of the enteron and that of.

the neural rudiment at their posterior ends. Here, in the postanal gut, we have such a connexion still persisting in a drawn-out form though, as in the present case, it may be a solid strand of yolky cells and not a hollow tube. The postanal gut is a purely transitory structure which at a relatively early period of development dis- integrates completely.

In endeavouring to determine the morphological significance of the postanal gut it is necessary to bear in mind that the Vertebrate in early stages develops from before backwards and that the growth in length by the addition of new segments takes place at its hinder end where there is a mass of actively growing embryonic tissue forming a kind of “growing point.” The tissue of this, although to the eye quite undifferentiated, contains the elements which form all the various tissues such as nerve cord, notochord, myotomes, alimentary canal, etc. As growth goes on these gradually become differentiated out, the differentiation always proceeding from before backwards. If we now look at such a young Vertebrate as that shown in Fig. 80, Or, we see the typical Vertebrate structure, includ- ing alimentary canal (pay) extending right back practically to the tip of the tail: it is only at the extreme tip that the various organs merge together into unditl‘erentiated embryonic tissue. The only striking peculiarity is that the communication of the alimentary canal with the exterior, the anus, is not in the midst of the growing tissue of the tip, as it would be, for example, in a young Chaetopod worm, but well forwards on the ventral side.

This peculiarity, in the writer’s opinion, finds its explanation in the development from before backwards already alluded to. The appearance of the anus at a point relatively far forwards means that it and the organs related to it such as the excretory d11cts complete their development at an earlier period of time. As it is of functional importance that the organs in question should do so, in contradistinction to the purely motor arrangements farther back, we see a physiological reason why evolution should have brought about a development of the anal opening in its anterior position from the beginning, and the elimination of those stages in which it was situated farther back.

As regards the phyletic evolution of this part of the enteron, we may sum up probabilities as follows: that the alimentary canal with its surrounding splanchnocoele originally extended to the hind end of the body: that the anal opening came to be shifted on to the 196 EMBRYOLOGY OF THE LOWER VERTEBRATES III

ventral wall of the canal: tliat; it then underwent; a. gradual shifting forwards a.long the ventral side: that as it did so the now

postanal portion with its sphmchiiocoelc gradually atrophied the position they occupied becoming filled mainly with muscle. '

LI'1‘.ERAT U RE

Aaaheton. (.3uau‘t. Journ. Micr. Sci., xxxviii, 1896.

Aasheton. The Work of J. S. Budgctt. Cambridge, 1907. Balfour. CUI1l[)a.1‘zlt‘iVe Embryology, ii, 1881.

Brachet. Jollrn. dc l’Aimt. et. de la. Physiologic, xxxii, 1896. Brauer. 7.unl. Jiihrh. (Auul..), xii, 1899.

Brouha. Journ. «iv l'Au;it. rt do in Phys., x.\xiv, 1898

Bryce. Jouru. Aunt. and Phys, xi, 1906.

Budgett. ’l‘r.~ins. 7.001. Soc. Lond, xvi, 1901.

Dean, Bashford. Zooi. Jahrb. (Syst. ), ix, 1896.

Druner. Zool. Ju.1u'h, (Anu.t.), xv, 1901.

Egert. Zool. Anzcigcr, xiii, 1913.

Goeppert. Morph. Jahrb., xvii, 1891.

Goeppert. Morph. Jchrh, xx, 1893.

Goette. Entwicklungsgcschichto dcr U ukc. Leipzig, 1875. Goette. Zeitschr. \vis.s. '/1001., lxix, 1901.

Grail. Scmons Forschungsrciscu in Austmlien, i. Jena, 1913. Hummer. Arch. f. Anat. I1. Eiitivickltliigsgcscli., 1893.

Hammar. Anat. Anzeiger, xiii, 1897.

Juillet. Arch. zoo]. expér. [5], ix, 1912.

Kallius. Anat. Hcfte (Arh.), xvi, 1901.

Kerr, Graham. Proc. Roy. Phys. Soc. Edin.. xvi, 1906.

Kerr, Graham. The Work of J. S. Budgctt. Cambridge, 1907. Kerr, Graham. Quart. Journ. Micr. Sci., iiv, 1910.

Laguesse. Journ. dc l’Anat. et do it}. l’hy:~., xxx, 1894. Lankeater, E. Bay. Quart. Journ. M icr. Sci., xvi, 1876.

Marcus. Arch. 1'. mikr. Ana.t., lxxi, 1908.

Morofl. Arch. f. mihr. Anat., ix, 1902.

Maser. Arch. 1'. nukr. Anut., lx, 1902.

Moser. Arch. f. mikr. Anat., ixili, 1904.

Miiller, W. Jenaische Zeitschrift, vi, 1871.

Neumayr. Scmons FO1'SCh1l11g&l'ciS(5Il in Australien, i, 1904. Nicolas. Arch. Bio]., xx, 1904.

Phelps. Science, N.S. ix, 1899.

Piper. Arch. f. Anat. und Entwicklungsgesch., Suppl. Bd., 1902. Piper. Verh. Anat. Ges., Halle, 1902.

Reighard and Phelps. Journ. Morph., xix, 1908.

Rowntree. Trans. Linn. Soc. Lond., (2) ix. 1903.

Sarasin, P. and P. Ergcbnissc nziturwiss. Forschungen auf Ceylon, ii, 3.

Wiesbaden, 1889. '

Sawadsky. Aunt. Anzeiger, xl, 1911.

Scammon. Amer. Journ. .Ana.t., xiv, 1913.

Bedgwick. Quart. Journ. M icr. Sci., xxxiii, 1892.

Smith. Journ. Morph, xxiii, 1912.

Taylor. Quart. Journ. Micr. Sci., lix, 1913.

Voeltzkow. Abh. Senck. Ges., xxvi, 1899.

Wenckebach. Ontwikkeling en de bouwder bursa. Fabricii. Proefschrift. Leiden,

1888. Wijhe, van. Verhand. Konink. Akad. Wetensch. Amsterdarh, Tweede Sectie, Dee] xviii, 1914. Wilson, Gregg. Proc. Roy. Phys. Soc. Edin., xiv, 1901.


CHAPTER iv THE COELOMIC ORGANS

lN'1'RODUCTION.——The 1ne.soderm of A7np/w'oa;us consists in an early stage, as already indicated (p. 57), of a row of closed sacs arranged serially one behind the other" upon each side of the body. At this time the coelome of Amp/2/ioarus is in the extremely archaic condition of a series of metamerically arranged paired compartments ——a condition resembling that of the less modified forms of Annelids. The coelomic sacs gradually spread in a ventral direction until they meet. For a time after this happens the sacs of opposite sides of the body remain separated by a longitudinal partition the ventral mesentery. Similarly the apposed posterior and anterior walls of neighbouring sacs belonging to the same side of the body, form thin membranous septa like those of Annelids.

A highly characteristic difference from the Annelid arrangement begins to show itself a little before hatching in the ventral portion of the body, in as much as the transverse septa break down and disappear thus converting what was hitherto a chambered coelome in this region into a continuous space. There is no obvious reason why this loss of segmentation of the ventral portion of the meso- derm has come about in evolution. A general characteristic, however, of the phylum Vertebrata is the loading up of the ventral part of the endoderm with yolk and it may well have been that the loss of the mesoderm septa ventrally arose in correlation with the presence of a greater amount of yolk in the ancestral condition than exists in the present-day Amphioxus.

A further striking difference between the Vertebrate and the Annelid is expressed in the extent to which the coelomic wall gives rise to muscular tissue. In the Annelid practically the whole extent both of the, somatic layer lining the body-wall and the splanchnic layer covering the gut gives rise to muscular tissue. In Amphvlowus however, and the same holds for Vertebrates in general, the ventral portion of the somatic mesoderm, the portion which loses its segmental character-——loses also its capacity for producing muscle.

On the other hand the dorsal portion of the mesoderm, which retains its segmentation, retains also, and to an accentuated degree, its muscle-forming capacity. It separates ofl‘ from the ventral or

197 198 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

splanchnocoelic portion of the mesoderm in the form of a series of segmentally arranged sacs—-—the myotomes--— and the wall of these gives rise to almost the whole of the muscular system. The myotomes are at iirst, from their mode of origin, restricted to the dorsal side of the body, but as development goes on active growth of their ventral portions takes place and they extend downwards, overlapping and covering in the splanchnocoelie mesoderm right down to the mid- ventral line. In this way a muscular body-wall is provided for the ventral region of the body in which the original muscle-producing capacity of the somatic mesoderm had disappeared.

The evolutionary origin of this curious secondary muscularization of the ventral body-wall of the Vertebrate is unexplained but the suggestion may be hazardcd that it was associated with the loss of segmentation of the ventral splanchnocoelie mesoderm, the primitive mode of movement of the Vertebrate———by waves of lateral flexure—-— being only able to utilize longitudinal muscles divided into segments. We may take it that the splanchnocoelic muscular layer, as it lost its segmentation, would become less efficient for purposes of move- ment, and that, correlated with this, its territory would then tend to be eneroached on by the still segmented, and therefore more eliicient, dorsal portion of the muscular layer until eventually it came to’ be replaced completely by it.

As a result of the developmental processes which have just been indicated the mesodcrm of Amplmioazms, which for a time consisted of a metameric series of paired sacs, is now represented by (1) the segmentally arranged myotomes and (2) the unsegmented splanchno- coelic lining. To these a third element becomes added in the form of a pocket-like outgrowth from the myotome wall close to its lower end (Fig. 144, A, sol, p. 285). This grows first towards the mesial plane and then dorsally, insinuating itself into the space between myotome on the one hand and notochord and spinal cord on the other, until it occupies practically the whole of that space right up to the mid-dorsal line. This pocket-like divertieulum is the sclerotome (p. 286 . ‘

In )the typical Vertebrate a fourth derivative of the mesoderm segment is of importance: it takes the form of a connexion which persists for some time between the myotome and the splanchno- coelic mesoderm as a narrow stalk or isthmus. This-—the proto- vertebral stalk or nephrotome (Riickert, 1888) with its cavity the nephrocoele-—is of great importance from its relation to the nephridial organs but its existence has not up to the present been demonstrated in Amphiomus.

We will now proceed to trace out the subsequent fate of these various derivatives of the primitive mesoderm segments.

COELOMIC CAVITIES.--The only portions of the coelomic cavities which remain patent are the nephrocoeles (which will be dealt with later on) and the splanchnocoele or peritoneal cavity.

It may be taken as probable that the body -cavity of the IV COELOME 199

ancestral Vertebrate was divided up into segmentally arranged compartments by transverse septa, and into a right and left half by a sagittally placed partition supporting the alimentary canal and forming the dorsal and ventral mesentery ; in other words that the general arrangement was like that of a primitive Annelid worm. This seems to be indicated by the mode of development of the mesoderm in A7n_ph'i09ms.

ln Vertebrates above Amplmloams the segmented condition of the splanchnocoele has disappeared even from development.‘ The sagittally placed mesentery on the other hand still appears in ontogeny in the form of the partition remaining between the edges of the lateral mesoderm as they approach one another on the ventral and on the dorsal sides of the alimentary canal respectively. In correlation with the great increase in length, and consequent coiling, of the alimentary canal of the Vertebrates—a condition which probably existed even in the ancestors of those gnathostomes in which the alimentary canal is new short (p. 184)-the ventral mesentery disappears at an early stage of development throughout that portion of its extent which lies on the tailward side of the liver.

The dorsal mesentery on the other hand persists throughout life, serving as a bridge to carry the complicated connexions of the gut wall with the vascular and nervous systems, although perforations may appear in it, more or less extensive in different groups of Vertebrates. The complicated foldings and frillings which the dorsal mesentery undergoes, owing to its enteric edge having to keep pace with the increase in length of the gut, are of interest mainly to specialists in the anatomy of particular groups and need not be dealt with here.

In the fishes, in which the lung performs an important hydro- static function, that organ grows back in the substance of the dorsal mesentery, and in accordance with its tendency to assume a more and more dorsal position, the portion of mesentery lying above it may become incorporated in the dorsal wall of the splanchnocoele, the result being that the lung in the adult now lies entirely dorsal to and beyond the limits of the body-cavity (Dipnoi,2 Actinopterygii).

Apart from its primary segmentation, the splanchnocoele shows a tendency for special portions to become secondarily separated off from the main cavity. The most important case of this occurs at

1 While it has to be granted that the splanchnococle of the Vertebrates represents the ventral portion of the coelome which has lost its segmentation, care must be taken not to assume that this loss of segmentation has necessarily extended dorsal- wards to precisely. the same level in all Vertebrates. Like other anatomical boundaries the dorsal limit of the splanchnoeoele is doubtless fluctuating and vague. It is therefore wise not to attach too great importance to the exact position of the first rudiment of an organ which develops in one case on the dorsal and in another on the ventral side of the boundary between segmented and unsegmented mesoderm such as for example the gonad (p. 270).

'3 Ct‘. Graham Kerr, 1910. 200 EMBRYOLOGY OF THE LOWER VE.l~t'l‘EBRA'J.‘l4]S en.

the hinder end of the heart where there exists on each side a broad bridge by which the duct of Cuvier passes from the somatopleure to the sinus venosus. This bridge becomes extended headwards and dorsally on each side of the oesophagus until it meets the dorsal wall of the splanchnocoele thus forming with the oesophagus a floor separating the anterior portion of the splanchnocoele into two cavities, one dorsal and one ventral, each opening posteriorly into the main splanchnocoele. Of these two cavities the dorsal becomes completely obliterated by fusion of its splanchnic (oesophageal) and somatic walls from before tailwards. The ventral one on the other hand roofed in by the oesophagus persists as the pericardiae cavity.

The communication of this posteriorly with the main splanchno- coele is obstructed in the middle by the llattened headward surface. of the liver which is embedded in the distended ventral mescntery, while laterally the communication is for a time open. As develop- ment goes on however the opening on each side becomes obliterated by an ingrowth from the somatopleure which spreads downwards from the bridge of tissue containing the duct of Cuvier and the free edge of which meets and fuses with the mesoderm covering the headward surface of the liver. The pericardiae cavity comes in this way to be bounded posteriorly by a complete wall of tissue a large

‘part of which consists simply of the mesodermal sheath of the liver.

As the body of the embryo increases in diameter this wall of tissue keeps pace with it as does also the liver. The latter organ however in subsequent growth of its anterior or headward surface does not keep growing in continuity with the substance of the septum but becomes separated from it by a deep cleft, the region of continuity between liver and septum becoming thus restricted to a small area dorsal and close to the mesial plane. Similarly the region of continuity between the headward face of the septum and the wall of the sinus venosus which is at first of relatively considerable dorsiventral extent becomes reduced to a narrow bridge of tissue.

In the Elasmobranchs the isolation of pericardiae cavity from the main splanchnocoele is only temporary. A median pocket-like extension of the pericardiae cavity spreads tailwards immediately dorsal to the sinus venosus in the substance of the mesodermal sheath covering the ventral surface of the oesophagus. This develops on each side a communication with the main cavity of the splanchno- coele which persists throughout life as a crescentie slit on the ventral surface of the oesophagus (Hochstetter, 1900). This secondary com- munication between pericardiac coelome and splanchnocoele is known as the pericardioperitoneal canal.

In Myxinoids, throughout life, and in Petromyzon, during the larval period, the rudiment of the wall separating pericardiae from splanehnocoelic cavity remains in the form of a simple bridge

enclosing the duct of Cuvier so that the two cavities are in wide communication with one another. " l.V

In the Am- phibia. and Amni- ota. the pericardiac cavity becomes telescoped back into the general peritoneal cavity, its hinder well be- coming extended so as to form :1 thin membranous bag enclosing the heart and separat- ing it from the other viscera.

Apart from the walling oil" of the perieerdiac from the main periton- eel cavity there is found in the case of the Amniota a well-marked tend- ency for the latter cavity to undergo further subdivi- sion, special por- tions becoming more or less com- pletely wulled in by secondary fu- sions taking place between apposed portions of ‘the peritoneal lining.

FIG. ll0.——-Difi'erentia- tion of the myotorne as seen in transverse sections of Lepidosiren larvae.

A, stage 30;B, stage 31 +: 0, stage 32; D, stage 35 —: E, dividing myoblasts of inner wall from stage 36. ml)’, myoblasts of inner wall; mt", myoblasts of outer wall; mf, contractile flbrils; ma, vacuole. The contractile fibrils cut aemz-is

are shown as distinct black dots.

COELOME

20.1 202 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

For example in Birds ‘ the mesodcrmal coating of the lung upon its ventral side becomes continuous (1) with that lining the body-wall laterally so as to enclose the portion of splanchnocoele dorsal to the lung as a pleural cavity, and (2) with that covering the surface of the liver, forming a ventral pulmonary ligament which serves to wall in a pulmo-hepatic recess lying between it and the mesen- tery. A third connexion, the origin of which is associated with the development of’ the abdominal air-sacs, forms the thin post-hepatic septum which stretches from the ventral surface of the lungs obliquely downwards and backwards to the ventral body-wall.

Amongst Reptiles somewhat similar arrangements exist, diFf'ering in detail in the dififerent groups.

THE MYo'I‘oMF.s.-——'[‘hc developmental changes by which, in a gnathostomatous Vertebrate, the myotomes become converted into masses of muscle-librc-s are excellently shown by Lzepidosiren in which the cellular elements are particularly large and distinct. In this animal the myotome is at first solid, but later on develops a small cavity or myocoele by the breaking down of its central cells. This niyoeoele soon becomes obliterated by its inner and outer walls coming together. The cells of the inner wall assume a more regular shape, taking the form of large parallelepipcdal cells (Fig. 110, A, mb’), flattened dorsiventrally and stretching in an anteroposterior direction throughout the whole length of the myotome. The nuclei of these large cel1s——myoblasts or myoepithelial cclls—— divide, mitotically, so that they assume a syncytial character. Their protoplasm develops a longitudinally fibrillated appearance and presently distinct cross-striped contractile fibrils (mf) make their appearance in the protop1asm~—-each fibril running through the whole length of the myoblast or in other words from end to end of the myotome. The contractile fibrils, which as seen in a trans- verse section are arranged in a :3 -shaped pattern (Fig. 110, A, onf), become more and more numerous and soon fill up the inner two- thirds of the myoblast almost entirely, there remaining only a relatively small amount of perifibrillar protoplasm between them (Figs. 110, B, and 11]).

The outer end of the myoblast does not for some time develop any contractile fibrils but there appear in its protoplasm large vacuoles (vac) which form a broad clear band in horizontal sections—- of much use as a landmark to indicate the outer limit of the inner wall of the myotome. The cells of the outer wall of the myotome take the form of elongated cylinders stretching throughout the length of the myotome and in their protoplasm longitudinal fibrils make their appearance as in the case of the inner Wall myoblasts (Figs. 110, C ; 111, B). The longitudinal fibrils become fused at their ends with connective-tissue septa formed by mesen- chyme cells which wander in between consecutive myotomes.

1 For a well-illustrated account of the complicated arrangements in detail see Poole (1909). 1v MYOTOMES r 203

Some such mcsenchyme cells also penetrate into the substance of the inyotome and settle down there to form. connective tissue. The cylindrical myoblasts of the outer wall undergo active multiplication (Fig. 110, C) so that it comes to be greatly thickened, composed of many layers of muscle-cylinders-—those towards the outer surface going on dividing actively while those further in towards the mesial

FIG. 1'.l1.—Ditl‘erent-iation of the myotomc as seen in horizontal sections of llepidosireu larvae.

A, stage 3'! ; B, stage 31 -}-. -mb’, myoblast of inner wall ; mh", myoblasts of outer wall ; m f, contractile fibrils; mo, vacuoles; -y, yolk.

plane increase much in size as they develop more and more fibrils in their interior.

As the outer wall of the myotome continues to increase in thickness the myoblasts of the inner wall become relatively more and more insignificant. Eventually they divide up into muscle- cylinders like those of the outer wall so that it is no longer possible to distinguish the inner wall portion of the Inyotome from the outer wall part. The muscle-cylinders become the muscle-fibres of the adult, the undifferentiated protoplasm between‘-tthe fibrils persisting as the sarcoplasm the superficial layer of which may be somewhat

condensed to form the sarcolemma. 204 EMBR.Y()LOGY OF THE LOWER VER'l‘E]'lRA'l‘ES on.

A point to be noticed, of much morphological interest, is that the inner wall myoblasts of Lepidosvlren are for’ a time (Fig. 110, A) in the form of typical myoepithelial cells such as are familiar in some of the lowest invertebrates. They are, as indicated in Chap. II., in continuity with the central nervous system by a protoplasmic tail-like extension of the cell-body closely resembling that which occurs in Nematode worms (Fig. 112). The peripheral portion of this remains as a mass of granular protoplasm on the surface of the muscle-fibre—-—-the motor end-plate. The latter is therefore to be regarded as a portion of the muscle-cell which retains its proto- plasmic condition rather than as a portion of the nerve-fibre.

The mode of conversion of the embryonic myotome into the muscle-segment has been described as it occurs in Jlepidosvlren because of the two special safeguards against error which exist in that a11in1a1, (1) the large size of the histological units and (2) the fact that the boundary between outer and inner walls of the myotomes is marked by a clear and unmistakable land- mark in the form of the vacuolar zone constituted by the outer portions of the inner wall myo- blasts. lt now remains to indicate shortly the more important differ- Fm. 112.-—Diagram of a motor ganglion- ‘Incas -111 detaill .Whmh are to be

cell in the spinal cord com,-mans found 111 descriptions of the process

through the substance of a nerve-fibre as observed in others of the lower

8 muscle-cell Ill the Hly0tOlIlC. Ve1'tebrates_

c.f, eoI|t_r3.c.tli.l'¢311:11.)l1l‘i'ls in 1pyoe11')ithelitl1ll‘cell; The chief of these concerns

';,t'(',‘1,’::';‘r’\‘,e_‘fu:;‘e_°” ’ ’”"’ "“"‘“°"'“’ ' ""‘ the fate of the outer Wall of the

embryonic myotome. In Lep7Ido- siren as has been stated the outer wall gives rise to muscle. In the case of Elasmobranchs and Granoids, Balfour stated explicitly that the outer wall of the 1]1yot0I11e similarly takes part in the development of muscle. Many authorities (Hertwig, Rabl, Maurer), however, deny that this is the case: according to them the outer wall plays no part in muscle-formation: it simply breaks up into amoeboid cells which contribute to the dermal mesenchyme. Hence these investigators term the outer wall of the myotome the “ Cutis-layer.” In the case of the Sturgeon, Maurer corroborates Balfour's statement that the myotome is composed of two layers of muscle-elements but according to him the outer layer is simply budded off from the inner and does not represent the original outer wall of the myotome as Balfour supposed. In the Amniota the myotome in early stages is almost square-as seen in a transverse section practically the whole of the wall IV MYOTOMES 205

next the endoderm representing the sclerotome. Cells proliferat- ing from this invade the myocoele and completely fill it up. It is only in later stages that the myotome becomes extended into the normal plate-like form by active growth at its inner (dorsal) and outer edges. Of the two walls of this stage the inner admittedly becomes converted into muscle-cylinders. The outer becomes loosened out into a mass of irregularly shaped cells and these are commonly believed to give rise to dermis. In view of what happens in Lepz'dos7I'rm, where accuracy of observation is so much more easily attained to, it seems advisable not to accept this as absolutely certain.

At the same time it may be allowed that there is no a. ymlori difliculty in the way of admitting that portions of myotome which in one type of Vertebrate give rise to muscle, may in another have ceased to do so, for, as already indicated, a quite similar process of concentration of muscle-development in a localized portion of somatic mesoderm is a fundamental characteristic of the whole Vertebrate phylum.

The series of paired myotomes, each composed of a mass of longitudinal muscle-fibres traversing it from end to end, forms the material out of which is formed the, often extremely complicated, system of voluntary muscles of the adult Vertebrate. The various myotomes as they increase in size become divided up into it may be numerous pieces and these are pushed hither and thither by processes of differential growth until the arrangement of the numerous adult muscles contrasts greatly with the simple longitudinal arrangement of the original myotomes. During the various displacements which it undergoes the individual muscle or fragment of myotome remains in organic connexion with its nerve-centre by means of its motor nerve and the course of these nerves in the adult frequently gives an important clue to the developmental migrations of the particular muscles. _

No attempt will be made here to follow out the evolution of the complicated muscular arrangements of the adult beyond a short sketch of the method in which the muscles of the fins or limbs originate.

The median fin is simply the extension of the body in the median plane and we should therefore naturally expect it to be muscularized by prolongations of the myotomes growing into it. The actual process is clearly illustrated in Fig. 113. In A a muscle-bud is seen to be projecting from the end of each myotome where a median fin is developing-the upper group of buds belonging to the dorsal fin, the lower to the anal. The buds diminish in size towards each end of the series and in the case of the dorsal fin, towards its anterior end, there are a considerable number of abortive buds which never come to anything. The muscle-buds grow into the fin fold and then become out off from the main part of the myotome to form the muscles of the fin as is shown in B. 206 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The paired fins or limbs become muscularized by very similar, segmentally arranged, buds and it is necessary from the outset to bear in mind that this similarity need have no deeper signifi- cance than that the paired fins also necessarily obtain their muscular- ization from the segmentally arranged myotomes. The process as it occurs in the pelvic fin of the shark Spinaw is illustrated by Fig. 114. In the 20 mm. embryo (A) the fin rudiment is seen as a longitudinal ridge and a series of myotomes in the neighbour- hood of this ridge are seen. each to be forming at its lower edge two projecting muscle - buds. These sprout out into the limb rudiment, assume an clon gated form and then become separated off from the myotome (0). Each bud now splits into two layers a dorsal and a ventral and each of these undergoes histological differentiation and be- comes a bundle of muscle-

fibres—one of the radial

muscles of the fin: so %\ K that four radial muscles are derived from each

- _ , , in otome a dorsal and a Fm. 113.—-Muscular-ization of median mi 1!! Lepm'.I.).s'tcM.S‘. y _ .’ ( __ . _ . (After Schmalhausen, 191.2.) Ventral fro!" each of the

_ _ t _ two original buds. Such A, 1.3 mm. ; I3, :21 mm. The nmselu-buds and, in the lower . I 1 _ 1 _ . _. ‘ . figure, the llP.‘l'\'t'H (-.mu1e(_-ted with thmn are shown in black. 18 t’ l(-’ P1 Occbs In its “hum

outlines.

The existence of a disturbing complication of i this simple scheme is indicated by the adult arrangements,in as much as it can be shown that a single motor spinal nerve ('i.e. the nerve belonging to a single myo- tome) is related to more than the four radial muscles to which alone we should expect it to be related were the account which has just been given complete. This discrepancy is brought out particularly clearly by physiological experiments. Careful stimulation of a single spinal nerve very commonly causes three consecutive (dorsal or ventral) radial muscles to contract instead of only two, and in some cases apparently at still greater number. This seems clearly to indicate that the end- organs, in other words the muscle-fibres, belonging to a particular motor nerve or myotome are in the adult not strictly confined within the limits of the two pairs of radial muscles corresponding to that motor nerve or myotome.

To those who believe in the organic continuity of muscle-cell and nerve-fibre from an extremely early stage of development the idea obviously suggests itself that a shifting of some of the constituents from one muscle-bud into its neighbours takes place 1v MUSCLE-BUDS 207

during development. According to Mo1lier(1893) and Brena (1899) such a, process actually occurs. Broad enaetoxnoses or bridges make them‘ appearance connecting the varmue redial muscle rudiments

Fm. 114.——-Illustrating the omen-11132-izatior: of the pelvic Jin in .%;pirm.a-. (After Brmm, 1899.)

A, 20 mm. (70 llim-eOI1t'1‘I!1 SPgl:.8.}‘, B, .25 mm. (74 m..~:.)'. (.1. re mm. ; 1), :52 mm. The myotoxnes en- indicated by Arahiu-n11nu:rals. The muscle-bode are shown in black, those within the tin rudnueut being nulnberotl with Romzm nuxnerole. The no: VE?'t]‘!lllk:-l are shown with double contour.

with their neighbours near their proximal ends. These connecting bridges persist for :2. short time and then disappear. According to the authors mentioned they are the expression of a. cellular interchange taking place between neighbouring muscle rudiments. 208 EMBRYOLOGY OF THE LOWER VERTEBRATES 011.

E. Miiller (1911) believes the connecting bridges in the case of Acant‘/mlas to be special developments of a syncytial network which lies between the buds from the commencement: he fails to find in this animal any evidence of shiftin g of muscle-cells along the bridges. The matter appears to stand in need of further investigation. Already within the group of ]_+]l-asinobranchs we find modification of the typical mode of Inuscularization of the fins outlined above. In the case 01' the most anteriorly placed muscle-bud of the pectoral fin of’ Spvinaa: the bud resolves itself into its constituent cells which separate before giving rise to muscle-cells. Again at tl1e anterior and posterior limits of the pectoral limb musculature in .Pm'stz'w/‘us and Torpedo the compact, stage of the '1inuscle-bud is eliminated entirely and the cells which muscularizc the fin are budded off‘

Flu. 115.—-Diagram to illustrate the arrangement of mesoderm segments in the head- region of a. young Elaslnobranch embryo. (From a drawing by Agar.)

ut, otocyst; xplc, splanchnoeoele; 1., u, -v, w, occipital myotomes; 1, ‘.3, 3, anterior myotomes; I, II, ctc., visceral clefts; *, “ Fourth” myotome of van Wijhe.

separately from the myotome, wandering from their place of origin into the limb rudiment and there settling down (Braus).

Amongst Vertebrates outside the group ‘of Elasmobranchs such modification appears to be the rule. Thus in Acipenser and apparently in Lacerta, typical muscle-buds arise singly from the myotomes concerned; In Teleosts Harrison finds muscle-buds in the pelvic fin but a diffused origin in the pectoral. In Lung-fishes and Amphibians the origin seems to be again diffuse and the same appears to be the case in Birds. .

MESODERM or TI-IE HEAD-REGION.-——Tl1e1‘e are two important

characteristics of the head-region of the Vertebrate ultimately connected with the muscular system: (1) loss of flexibility, associated with the evolution of brain and skull and (2) special muscularity of the wall of the alimentary canal, associated with the presence of important movable skeletal structures enclosed in the substance of the visceral arches. These peculiarities find their expression (1) in the tendency to suppression of the myotomes of the head- Iv MESODERM or HEAD ‘ 209

region and (2) in the retention, to a greater extent than in the trunk, of the muscle-forming capacity of that part of the mesoderm which lies ventral to the myotomes.

The mesoderm of the head-region shows the least amount of modification posteriorly where its relation to the mesoderm of the trunk is still clear. In the occipital region—the region between the otocyst and the occipital arch, which may be taken as the binder limit of the skull———-We find a series of typical (“ occipital ”-——~ Fiirbringer) myotomes, the mesoderm ventral to which takes part in the lining of the splanchnocoele just as in the trunk-region. This series of occipital myotomes seems clearly to be undergoing a process of reduction. It is largest in such. comparatively primitive forms as Elasmobranchs. Again during ontogenetic development the series commonly shows progressive reduction. In Spinao; for example seven occipital myotomes make their appearance, but as development goes on the anterior three (t, u, 'v)1 break up and dis- appear, the fourth (Iw) does so incompletely, while the last three (as, 3/, 2) develop into definite muscle-segments though of small size. As each anterior myotome disappears those behind it become shifted forwards so that its place becomes occupied by the myotome originally behind it in the series. It will be realized that there is thus introduced a serious source of possible error which has to be carefully borne in mind in observations on the development of the oecipital region where the identification and correct reference of individual myotomes to their place in the series is of importance.

Anteriorly the series of occipital myotomes is prolonged forwards past the otocyst by a mass of mesoderm (" in Fig. 115) which was regarded by van W'ijhe (l883)—-who may be said to have laid the foundations of modern work upon the segmentation of the mesoderm of the head-—as the equivalent of a single (“fourth ”) myotome. It has already been indicated that the series of occipital myotomes is undergoing reduction from its front end backwards and it seems on the whole more probable that van Wijhe’s “ fourth ” myotome in the Gnathostomata is to be interpreted not as a single myotome but rather as the degenerate remnant of a series of myotomes. The number of myotomes originally present in this series does not appear to be capable of decision with any degree of certainty. Possibly it was very considerable and Froriep finds even in ontogeny (Torpedo) that during early stages (Stage “ D” of Balfour) as many as six distinct segments are recognizable in the region in question~—in other words that the series of myotomes commences not with t but with n, the anterior members of the series disappearing in turn as development proceeds. A point of

1 The hind end of the series-~-the occipital arcli——-being taken as a fixed point while the front end varies, Fiirbringer has introduced the convenient method of

designating the individual occipital myotomes (or their nerves) by the terminal letters of the al ihabet-——the last being 2, the one next in front 3/, and so on. The myotomes behin the occipital arch are counted as belonging to the trunk and are

designated by numerals 1, 2 and so on (cf. Fig. 220). VOL. II . P 210 EMBRYOLOGY OF THE LOWER V ERTEBRATES CH.

interest is that the anterior limit of this series of recognizable segments agrees approximately with the anterior end of the definitive notoehord.

In front of the “fourth” myotome of van Wijhe we find What appear to be fairly typical third and second myotomes, each con- tinuous ventrally with the wall of the pericardiac portion of the splauchnocoele. Of these myotome III gives rise to the External Reetus muscle and II to the Superior Oblique. At the front end of the series we have the first orpremandibular or oculomotor myotome, peculiar in that it is fused with its fellow across the mesial plane and that it no longer shows any connexion with the splanchnocoelic mesoderm. It gives rise to the four eye-muscles supplied by the Third cranial nerve-«the Superior, Internal, and Interior Rectus, and the Inferior Oblique.

We have so far dealt only with the myotomes but the lateral or splanchnocoelic mesoderm is also continued well forwards into the head—region. Its more ventral portion forms the lining of the pericardi-ac cavity, while its more dorsal portion becomes traversed by the visceral pouches or clefts. The splanchnocoelic mesoderm ventral to myotomes II and III comes to form a stalk-like connexion between the myotome and the pericardiac wall (Fig. 115). This .sta1k is hollow in the case of myotome II- and lies in the mandibular arch: in the case of myotome III it is solid and lies in the hyoid arch. In both cases the wall of the stalk gives rise to the muscular apparatus of the particular arch——-—in the one case the masticatory muscles and in the other the hyoidean musculature which is destined to attain to such a development in the mammals as the musculature of the face.

The splanchnocoelic mesoderm corresponding to the myotomic mass behind myotome III (* in Fig. 115) is said to give rise to the musculature of the branchial arches. As the myotomic mass in question shrivels up during development, and the occipital myotomes move forwards to take its place, these myotomes come to overlie the splanchnocoelic mesoderm which gives rise to the branchial muscles. Consequently as will be realized the position of myotomes t, u, and v in relation to clefts III, IV, and V as shown in Fig. 115 is secondary, the myotomes having moved forwards before the formation of these clefts.

The above sketch has dealt with the cephalic mesoderm of Elasmobranchs but a similar scheme of development with minor variations in detail holds for other Vertebrates. Upon the whole it may be said that with upward progress in the evolution of the Vertebrata the segmentation of the mesoderm in the binder part of the head becomes more and more obscured. Right up to the highest forms however traces of it persist. In Fowl embryos of about the third day of incubation the series of obvious myotomes may often be seen to be prolonged forward (see Fig._ 236) by faintly visible blocks agreeing in size and exactly in series with the myotomes. IV MYOTOMES OF HEAD 211

These blocks may be indistinguishable in ordinary thin sections but quite distinct in stained preparations of the whole embryo. It will require strong evidence to justify the refusal to give them the inter- pretation that at once suggests itself——that these slight condensations of the rnesenchyme are as it were the ghostly remnants of once existing myotomes which in Birds have ceased to become functional.

An important side issue of their presence to be borne in mind is that the slightly greater resistance of the more condensed portions of mesenchyme must necessarily exercise pressure upon the soft surface of the rapidly growing brain and produce a modelling of its surface which may be adequate to explain at least some of these appearances of segmentation of the brain-region which are included

under the term neuromery. The blocks in question extend well forwards——-in the specimen

figured (Fig. 236) there are four distinguishable anterior to the middle of the otoeyst and they may be taken as additional evidence in favour of there being not one but a.number of myotomes repre- sented in the region of van Wijhe’s “ fourth ” myotome.

It is of interest to note that in the Lampreys the blurring of the segments immediately posterior to the third of van Wijhe’s series seems not yet to have come about and there is an undoubted simple “ fourth ” myotome (Koltzoff, 1901). We may justifiably associate this with the low degree of cephalization in these creatures which has involved a persistence of, or more probably a reversion to, an apparently archaic condition of this myotome and its immediate successors in the series.

The relations of segments 1, II and III to the eye-muscles have been worked out in a number of Elasmobranchs and similar conditions have been described in Reptiles and Birds. Our knowledge of the holoblastic Vertebrates in this respect is still fragmentary. In the case of Lepidosiren and Protopterus the eye-muscles develop out of compact masses of mesenchyme in which it is impossible to recognize definite segments (Agar, 1907) while on the other hand in Uemtodus (Gregory, 1905) these segments make their appearance much as in Elasmobranchs.

Before leaving this part of the subject it should be pointed out that not all morphologists are convinced that segments I, II and III are actually serially homologous with the undoubted mesoderm' segments or myotomes of the trunk-region: the blurring of the mesoderm arrangements between them and the admitted myotomes, and more especially their late appearance in ontogeny, at a time when the anterior members of the occipital series have already degenerated, are brought as evidence against the more generally accepted view. The present writer does not feel inclined to attach great weight to these objections. (1) The break or blurring of the series immediately behind III seems adequately explained by the disappearance of functional muscles in this region and (2) the relatively late appear- ance of myotomes I to [II is explicable by the fact that the 212 EMBRYOLOGY OF THE LOWER VER'l‘EBRA'l‘ES CH.

functional muscles derived from them are connected with the eye- ball an organ which becomes complete and functional only at a 1'el:i.t.iw1y late pm'iod of development. p llY].‘0l3l{ANClIIAL on .lTvPoeLossA1. MUsoUJ.A'rUnE.——-In addition to the musculature already indicated the Vi‘-.rtel)raLe head possesses on its ventral side a system of hypobranchial muscles which have the appearance of a prolongation forwards of the longitudinal. muscles of the ventral body—wa1l. This hypohranchial musculature as a matter of fact does arise in ontogeny as a prolongation forwards of the anterior trunk and occipital myotomcs, as is well shown by Lepiclos-irren or .P’I"0t()7It6'}’?I.-.8’ Agar, 1907). About stage 29 the ventral ends of myotomcs 3/, z and 1 are seen to be growing out at their Ventral ’ ends into a long slender prolonga- tion (Fig. 116). These -processes grow outwards in front of the pronephros and undergo complete fusion at their tips. The fused apical portion .c.h soon separates from the parent myotomes and grows forwards, on each side of the pericardiac cavity, until it reaches the hyoid arch. It now spreads ventrally until it meets its fellow below the pericardiac cavity. The common mass so formed be- comes converted into a sheet of longitudinal muscle-fibres, attached posteriorly to the shoulder girdle and anteriorly for the most part to Fm. 116. - |)ors_al view of anterior myo- the hyoid arch (coracohyoid muscle, tonnes of a Protopterus of stage 29. Fig 117’ cm._ hy) the branchial (Alter Agar, 1904.) _ . _ ’ . . «uh, ('()I‘a('.ul:_\uitl muscle‘ N notochord' ;n.f arches bfalng reduced In the fishes muscle-bud Lo pectoral liinh,; )1-TL, 1)r«-iii:-plir-(is. In (l.ueSt10n' As Inuscle goes on with its development the an- terior boundary of the portion belonging to myotome 1 becomes marked by a connective-tissue intersection, while in some specimens a similar intersection appears to demarcate 3/ from 2.

In other Vertebrates the hypobranchial or hypoglossal musculature appears to originate in the same way~——difl'erence occurring only in the number of myotomes which take part. Five appears to be the most usual number (Scyllvlmn, Corning : Teleosts, Harrison).

ELECTRICAL ORGANs.—-—’l‘he conspicuous sign of a muscle becom- ing active is that it changes its shape: an inconspicuous accompani- ment of this change of shape is the production of a slight electrical disturbance. In the case of most electrical organs we have to do with portions of the muscular system in .which the function of contraction has been reduced to a subsidiary role or abolished IV ELECTRICAL ORGANS 21 3

entirely, while the production of electrical disturbance has become predominant.- We have here an excellent example of the principle of “substitution of functions,” which is constantly at work during evolution, the previously predominant functions of organs becoming subsidiary or falling into abeyance and being replaced by functions which were previously subsidiary.

The development of the ‘electrical organ can be conveniently studied in the case of the Skate (Ram) of which the most complete description has been given by Ewart (1888,1889, 1892). In this animal the electric organ forms an irregularly spindle-shaped body which lies embedded in the lateral muscles on each side of the tail region. It varies in size in different species and is distinguishable to the naked eye from the muscle by its more gelatinous appearance.

FIG. 117.-—-Side view of skull and myotomes of ];ep27dos-r'7'en., stage 38. (From Agar, 1907.)

Cartilage dotted, myotomes indicated by outlines, nerves black. u.n.p, antorbital process; uud, cups, auditory capsule; b'r.n, brachial nerve ; ¢..-. ,»h.n., nerve to dorsal portion of constrictor of pharynx ; cm‘. hy, coracohyoid muscles; by, hyoid arch ; I: _l/)l0{].'1t, hypoglossal nerve; M.y, M.z, M.1, M.2, ete., myotomes; -m.u.m'I, Mecke1's cartilage; -m1..~'. cups, nasal capsule; um-.. rm-ch, occipital arch; occ. rib, occipital I'll); ml, pectoral girdle; gum/, quadrute-; g/, 2, lIi!['\'¢-H2 l. 2, 1:, on-.. spinul llo'l'\'I'.\'; 2’. br, branch from 3 to hrachial nerve.

On examining transverse sections through ‘the tail it is seen that the electric organ occupies the place of the middle one of five super- imposed portions into which the muscle is divided. And this clearly suggests, as Babuchin first pointed out, that the electric organ is morphologically part of the muscular system. That this is actually so is placed beyond dispute by the facts of development. In an embryo of R. batvls about 7 cm. in length the position of the future electric organ is indicated by a slight modification of the muscle fibres, inasmuch as some of these (Fig. 118, B) show a tendency to assume the shape of a club, the anterior end of the fibre being slightly thickened. In contact with this thickened end is a mass of protoplasm crowded with nuclei. This represents the motor end- plate which has assumed a terminal position.

In a slightly older embryo (Fig. 118, C) the club-shaped fibre of the preceding stage has become further modified, the anterior end 214 i«;Menro1.ocY or THE LOWET’. VERTEBRATES on.

being now still thicker and

'" the whole fibre. having assumed the shape of a mac(-.. In the expanded head a portion of the mace the cross striation is becoming closer, while in the slender handle the striation is becoming blurred and in the portion next the head is almost disappearing. The end-plate forms a very definite layer of uniform thick- ness covering the truncated an- terior end of the mace. It is crowded with large nuclei and to it pass nerve-fibres which show a regular dichotomous branching. In the fibre shown in Fig. 118, D, taken from the same 10 cm. embryo, the head of the mace is still more ex- panded as compared with the stem. The main portion of the head, in which the muscle striation had become closer, now forms a thick plate bent into a

FIG. 118.—-—Development of the electric organ of Ruin, Innis. (After Ewart, 1888.)

A Ml‘-1 '3 WU"! 41" "ml-ryes1i;:l1t-lyrwnr T um. in lvn<,:t.h: (I and I") r__rom an emlnryo about 10 cm. in le“‘n'“|§ 19. l'I'mn a .\'[u'*(_'.lIll(!ll :nlmul- H7 I'm. in lI"Il_‘.."t-ll. ml, :ul\-¢-.ol:u' laym; ::.I, o-lea-1|-it.-. layer; cl, e1€ClIl‘0l<'9'lllll«'ii I"!--./", nerve-lib:-es; .-.l, stI‘iat.ed 1.-uyvr: t. \'«-s!igi:nl 1'eIn:u'n.x' of n111.s'(:lu9.-libre.

— ~w~ IV . ELECTRICAL ORGAN S 215

saucer shape with its concavity posterior and composed of numerous closely packed lamellae. It forms what is termed in the fully de- veloped organ the striated layer. On its anterior face the striated layer is covered by the end-plate, now known as the electric layer, while its posterior face is covered by a thick layer of richly nucleated protoplasm which, from the deeply pitted character of its posterior surface, is known as the alveolar layer. From this passes back- wards the main part of the muscle-fibre which shows symptoms of degeneration especially in the portion next the alveolar layer where it becomes vacuolated. Whether the alveolar layer represents, as

‘seems probable, a localized thickening of the sarcolemma is not

clear from the descriptions.

ln the fully developed condition (Fig. 118, E) the muscle-fibre has become converted into the functional electroplax (Dahlgren, 1908) or electrical unit. What was the head of the mace in earlier stages is now expanded to form a broad thin circular disc, lying perpendicular to the long axis of the ,body—-the stem of the mace having degenerated into an apparently insignificant and functionlcss vestige (Fig. 118, E) or having disappeared entirely. The electro- plax is formed of the striated layer which is almost flat except round its edge where it is bent in a tailward direction. It is com- pletely ensheathed in syncytial protoplasm, that on its posterior face forming the alveolar layer, probably nutritive in function, that on its anterior face forming the electric layer. Into the latter there pass the numerous end-twigs of the nerve-fibres, the superficial (Le. headward) layer showing a characteristic fibrillation of the protoplasm in a direction perpendicular to the surface (nervous layer--Ewart) in contrast to the deeper portion in which the proto- plasm is granular and nucleated (nuclear layer—Ewart). The tail- like vestige of the posterior portion of the muscle-fibre is directly continuous with the striated layer. With the latter it represents the contractile portion of the original muscle-fibre, while the ensheathing protoplasm whether electric layer, or alveolar layer, or sheath of the tail-like vestige, is probably to be regarded as repre- senting the superficial portion of the sarcoplasm.

As the muscle-fibres pass through the above-described modifica- tions, the connective tissue between them increases in quantity and becomes condensed between the electroplaxes in such a way that each electroplax becomes enclosed in a disc-shaped compartment. The walls of this fit close to the electroplax round its edge while, on the other hand, the anterior and posterior walls are separated, especially the latter, by a wide space from the face of the electro- plax. This space is occupied by connective tissue with sparsely scattered cells and a jelly-like appearance. That on the anterior side is traversed by the very numerous nerve-fibres which branching dichotomously pass towards the electric layer, while that on the posterior side is traversed by blood-vessels.

During the earlier stages of development the electric organ 216 EMBRYOLOGY OF ’.l‘TlE LOVVER VERTEBRATES on.

increases i.n size, partly by the addiug- on to it of new electroplaxes formed at its periphery, but the marked growth which takes place in the Ol.‘;___f;LIl later on is due to actual growth of the individual units which form it. Thus comparing a skate of 180 cm. length with one of 45 cm. the indi\-'idual ele(3t1'Oplaxcs are found to have increased in size practically in the same proportion as has the body as a whole.

The above description deals with the development of the electric organ as it takes place in Rwia liatis. In other species of skate the process appears to be similar as regards its main features, but it is interesting to notice that the relative expansion of the front end of the muscle-fibre to form the electroplax is much less pronounced in cer- tain species than is the case in R. bcmis.

Of the species so far investigated R. radi- ata shows the least ad- vanced stage of evolu- tion. In this species (Fig. 119, A) the elec- troplax is, as in vari- ous other species (e._q. R. celrcularis and R.

fullomca, Fig. 119), in the form of a cup rather than a disc. In 18. radiate the wall of the cup is very thick and retains throughout

life only slightly modi-

Flu. ll9.—~--Illustrating the adult condition 01' the electro- fled muscle stnwture plax in ltaia. vrmliata, (B) R. Cirrzl/II-'/‘is, (C) Ii’. 1 it . 1 ' . ' fI.tNm4..LI_!(!,, and (D) R. b(I.ti.s'. (After Ewart, 1892.) 16 _ e ec no -ayer 13 relatively feebly devel-

oped, the thick alveolar layer is represented by hardly modified sarcolemma and the tail is only comparatively slightly degenerate.

The skate has been taken as the basis for the description of the development of the electric organ since the phenomena concerned have been particularly clearly worked out in this fish. In the Torpedoes the electric organ develops from muscles in the region of the visceral arches by very similar stages. As regards the electric organs of Teleosts om; knowledge is still very insiiflicient. _. In Mormyrids and in Gymnoms they are clearly modified portions of the lateral muscles as in the skate; in Ast')'0sc0]m.s (Dahlgren) they are believed to be derived from eye-muscles; while in ]l[a.lople'rm~us though generally believed to be modified skin glands they are believed by Dahlgren and Kepner (1908) to___ be more probably of muscular origin. Iv LATERAL MESODERM 217

LATERAL MESODERM.————’l‘he lateral mesoderm forms the lining of the splanchnocoele. Its superficial layer persists throughout life as the coelomic or peritoneal epithelium, while its deep surface produces by proliferation abundant mesenchyme cells which forma con- nective-tissue backing to the epithelium. The development of muscle- fibres, which is so characteristic a feature of the coelomic lining in the dorsal or myotomic region, is here to a great extent suppressed, this portion of the mesoderm no longer playing any part in the muscularization of the body-wall. It still however takes place in restricted areas, smooth or striped muscle-fibres being developed in those portions of the mesoderm which invest particular organs such as heart and blood-vessels, alimentary canal with its appendages, oviduct. ‘The development of the musculature of the heart will more suitably be treated in the chapter dealing with the vascular system. As regards the muscles of the gut-wall we have little detailed knowledge, what there is being related mainly to the musculature of the skeletal elements contained in the visceral arches.

In the larva of Lepidosiren the important point has been estab- lished by Agar (1907) that the sheath of muscle which forms the con- strictor of the pharynx is of double origin, its ventral and larger portion being a development of the splanchnic mesoderm covering the pharynx, while its dorsal portion arises as an outgrowth from one (3/) or more of the occipital myotomes. The fact that muscular tissue derived from myotomes may join the splanchnic muscle to form part of the muscular sheath of the alimentary canal is of importance (1) by impressing upon us that an apparently homo- geneous muscular apparatus may really be heterogeneous —muscular- ized from two quite distinct sources, and (2) by indicating the possibility of splanchnic musculature being replaced by myotomic or conversely. Obviously the muscular sheath just mentioned might, by reduction of one or other of its component parts, become purely myotomic or purely splanchnic. As will be gathered later (Chap. VII.) the point is an important one from its bearing upon the discussion of certain problems of morphology.

RENAL 0RGANS.—-In triploblastic Metazoa the function of excreting nitrogenous waste products is commonly carried out by tubular organs to which Lankester (1877) gave the name nephridia. Under this term were included the excretory tubes of Chaetopods, Molluscs, Rotifers, Trematodes, Turbellarians and Vertebrates. Subsequent research soon brought to light an important structural difi"erence as regards. the inner ends of these nephridial tubes in different groups of animals. In certain groups the tube possesses at its inner end an open funnel or nephrostome 1 which leads from the coelome into the cavity of the tubule, while in other groups the inner end of the tubule is without any coelomic funnel but is on the other hand provided with an arrangement of flame-

1 Goodrich terms such funnels “ coe1omostomes” and uses the word nephrostome in a special restricted sense. 218 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

cells. 'l‘he appreciation of this difference gave rise, not un- naturally, to a suspicion —-which would now appear to be unfounded——-that under Lankester’s name nephridium were included excretory tubes of two morphologically distinct types and the use of the word nephridium was often restricted to the one of these types in which the coelomic funnel was present.

Later researches brought out the fact that in some cases certain Polychaete worms——-the excretory tube may possess both flame-cells and coelomic funnel. And finally the hypothesis was developed———by Meyer and especially Goodrich (l895)—~—that the nephridial tube and the coelomic funnel were originally quite distinct organs with separate openings to the exterior. On this view the primitive excretory tube or protonephridium (Goodrich) was provided with flame-cells at its inner end, while apart altogether from it and opening independently to the exterior was the coelomic funnel which formed the primitive exit for the reproductive cells. In the course of evolution there came about a fusion of the two structures, the

._ coelomic funnel becoming as it were grafted on to the nephridium

and in many cases shifted up the wall of the tubule right to its inner end. Such‘ a compound organ (N ephromixium— Goodrich) might retain for a time both flame-cells and coelomic funnel-—-as in the Polychaetcs alluded to above—or the flame -cells might, as is more usual, disappear leaving an excretory tube possessing at its inner end a coelomic funnel which shows no trace of its morpho- logically independent origin. To support the hypothesis which has just been outlined there is; brought in the evidence of embryology which testifies (see Vol. I. p. 158) that the main part of the excretory tube is developed as an ingrowth of the ectoderm, while the coelomic funnel arises as an outgrowth of the mesoderm. «

This hypothesis has met with very general acceptance not merely with regard to the excretory organs of Annelids alone but also as a theory of the morphology of excretory tubes in general. As, however, the writer of this volume takes up a somewhat different standpoint it will now be necessary to' state shortly what that standpoint is.

The Word nephridium will be used in the original sense as meaning an excretory tube whether possessing flame -cells or a coelomic funnel at its inner end.

Physiologically the open funnel and the flame-cell appear to be associated primarily with two different sets of spaces. The funnel is associated with coelomic spaces and it serves to transmit to the exterior the products of the lining of such spaces—-fluid, excretory, or reproductive. The flame-cell is associated rather with the meshes of the (mesenchyrnatous spongework :1. it serves to filter off from these spaces watery fluid containing excretory salts in solution. The activity of the “flame” is in direct relation to the pressure of fluid Within these spaces: if the pressure is.lowered by making a minute puncture in the body-wall the movement at once ceases——to IV RENAL ORGAN S 219

commence again when pressure is restored. This association of flame-cells with the spaces of the mesenchyme is seen in the more lowly forms in which they occur and it is therefore justifiahle to regard it as primitive in spite of the exceptional cases in which flame-cells occur in coelomic cavities.

The question of the relative antiquity in evolution of coelomic funnel and flame-cell is one which cannot be decided with certainty, depending as it does in tur11 on the unsolved question as to whether coelome or mesenchyme was evolved first. Looking to the occurrence of mesenchyme in Coelenterates (ag. the Alcyonarians), in animals in which there is not as yet any closed-off coelome, the balance of probability seems to be on the whole in favour of the flame-cell having originated first, in other words in favour of the Original nephridium being of the type called by Goodrich protonephridium. The fact that existing excretory tubes of this type arise from the ectoderm is also an argument for its antiquity, as it seems natural to suppose that priniitively excretory products were got rid of at the outer surface of the body.

In the primitive ancestral form the genital cells, formed by the lining of the enterocoelic pouches, would reach the exterior through the protostoma or primitive mouth but as evolution proceeded and the coelenteric pouches became separated from the enteron to form a closed coelomel another mode of exit would have to be evolved. The natural mode of such exit would be by rupture of the coelomic wall at its weakest spot. Such Weak spots would be provided at points where the cavity of the nephridial tube came into proximity with that of the coelome. At such points rupture would take place and the tendency would be for such a temporary rupture, at the time of maturity of the genital cells, to be replaced by a permanent ‘-’ opening from coelome into nephridium. This permanent opening would be the coelomic funnel (coelomostome, nephrostome in the original sense).

The coelomic funnel, though originally developed‘ to transmit the genital cells, would necessarily also serve as an exit for super- fluous coelomic fluid, and the fluid so transmitted would necessarily serve incidentally to flush out excretory matters passed into the lumen of the tube by the activity of its walls and would thus fulfil the function originally fulfilled by the fluid drawn in by the flame- cell. The function of the flame-cells being in this way otherwise provided for they would tend to disappear.

The nephridial tube thus came to transmit (1) the reproductive cells and (2) the highly poisonous excretory products. There is, as it appears to the writer, ample evidence that under such circum-

1 The argument involves as will be seen the assumption that the coelome was in its evolutionary origin enterocoelic. This assumption appears to be justified by the numerous cases in which the coelome so arises in ontogeny.

'3 An actual case where such a temporary rupture, brought about at parturition, has come to be a fixed character of the species and develops independently of mechanical rupture, is seen in the “ median vagina” of certain Marsupials. 220 EMBRYOLOGY OF THE LOWER VERTEBRATES CI-I.

stances the tendency of subsequent evolution would be to separate from one another the paths to the exterior of the genital cells and of the poisonous excretory products respectively. It might fairly be anticipated for physiological reasons that there would be such a tendency but that it actually exists is demonstrated by the facts of comparative anatomy and embryology. Over and over again we find cases where such separation has undoubtedly come about. For example in the evolution of the (lasteropoda the right nephridium has lost its excretory function and come to be merely a genital duct. In Vertebrates there are several familiar examples of parts of the renal system which have to do with transmitting genital cells becoming separated of?’ from those which retain a renal function.‘

The present Writer then’ believes the balance of probability to be in favour of the evolutionary origin of the type of nephridial tube commonly met with in coelomate animals, possessing a coelomic funnel or nephrostome at its inner end, having come about in the manner outlined above. The essential difference between the view here outlined and that developed by Goodrich is that it rejects the idea that evolution has brought about a more and more intimate connexion between originally independent genital funnel and nephridial tube as opposed to physiological probability. On the contrary it regards the funnel as having opened into the tube at the time of its first appearance, the progress of subsequent evolution having been in the direction of separating genital funnel and nephridial tube and not of uniting them. Even in the ease of Polychaete worms the arguments against interpreting the anatomical arrangements in difi"erent genera as illustrating evolutionary sequence in the reverse order to that believed in by Goodrich seem unconvincing and insuflicient to counterbalance the weight of physiological probability.

In the case of a tube leading from the coelome to the exterior the two ends are almost of necessity mesodermal and eetodermal in their nature respectively. Consequently the fact that the “nephromixium” has such a twofold origin in ontogeny does not appear to the present writer to constitute evidence of any particular weight that it actually arose in phylogeny by the fusion of two pre- existing independent organs. As regards the proportion derived from the two layers the probability would be that the specially excretory portion was originally ectodermal——excretory products being naturally got rid of by the outer surface—and that the portion specially concerned with the getting rid of coelomic products would be mesodermal—-arising as a bulging of the coelomic lining.

Accepting as a working hypothesis that the nephridial system of tubes with their nephrostomes arose in the manner outlined above, it is important to bear in mind how greatly the system would be influenced in its subsequent evolution by the establishment of

”.'E.g.'«' the separation of the Miillerian duct from the kidney system or the separation of the renal collecting tubes from the Wolffian duct. Iv , N EI’HRIDIAL' ORGAN S 221

circulating mesenchyme or blood. This would render possible the shortening up of the nephridial tubes and the more definite localiza- tion of the excretory tissue. Whereas the original flame—cell type of excretory apparatus was d.ifl'use——the flame-cells being scattered throughout the mesenchyme sponge-work———as it is still to be seen in the more lowly organized forms -it would now become compact, the waste-products being brought to it by the movements of the circulating blood.

N EPHRIDIAL ORGANS or V Ea'rEBnA'rEs.——Before passing on to the details of development of the renal organs in Vertebrates it is necessary to notice one or two points of general importance regard- ing the morphology of these organs within this particular phylum, and also to define precisely the sense in which certain technical terms will be used.

In the first place the kidneys or renal organs of Vertebrates are built of tubules each of which is a nephridium according to the original definition of the term.

The conclusion, already arrived at, tllat the ancestral Vertebrate ~

possessed a completely segmented coelome, carries with it the further conclusion that in all probability a pair of nephridial tubes originally opened to the exterior from each segment. A characteristic feature however of the Vertebrates (with the exception of A7nph'£0:1c'us)‘is. that the nephridia open not directly to the exterior on the surface of each segment as in a typical Annelid but into a longitudinal duet which passes back along each side of the body and communicates at its hind end with the cloaca. The whole series of nephridial tubes on each side of the body is known as the archinephrosl and the duct as the archinephric duct.

In the embryos of Vertebrates development takes place from the head end backwards. We should therefore expect the nephridial tubules to appear in regular sequence from before backwards. It is however highly characteristic of the Vertebrate that the tubules, instead of developing in this regular sequence, develop in three batches one behind the other———an anterior, a middle, and a posterior. These constitute respectively the pronephros, mesonephros, and metanephros (Lankester, 1877). In many of the lower Vertebrates there is no separation between mesonephros and metanephros, the two forming a continuous structure which acts as the‘ functional kidney. Such a type of renal organ consisting of the series of tubules corresponding to mesonephros together with metanephros may conveniently be termed the opisthonephrosfi

Of the four types of kidney just mentioned the first-——the pro- nephros-—forms the functional kidney in larval Vertebrates. It is well seen in the larvae of Crossopterygians, Actinopterygians, Lung- fishes, and Amphibians, while, as Sedgwick first pointed out, it is

1 Archinephron, Lankester (1877). l’rice’s term Holonephros is also frequently

used in the same sense. _ _ ’ In analogy with the use of the word opisthosoma In the group Arachnida. 222 EMBRYOL()GrY OF THE LOWER VERTEBRATES CH.

reduced in forms with richly yolkcd eggs where the development is not larval. Its reduction or disappearance in the last-mentioned forms may probably be due to the facilities afforded for getting rid of excretory products by simple dill'usion from the blood circulating on the surface of the yolk-sac into the surrounding medium.

The opisthonepliros f0_rms the functional kidney in the adults of most if- not all anairlnilatic Vertebrates.

Distinct mesonephros and metanephros are found in the Amniota --the mesonephros being functional during the later embryonic period and in the Reptiles during the first few months after hatching,

FIG. 120.———Rena1 organs of the Frog (Rana temporaria) as seen from the ventral side after the ventral wall of the splanchnocoele and the portion of the alimentary canal contained within it have been removed. (After Marshall, 1893.)

A, 12 mm. tadpole. ; 13, -10 mm. tadpole; C, frog at time of metamorphosis. A, dorsal aorta; Am, nor! in root; Iv‘, fatty body; gl, glomerulus; up, ()piStlHnlupl|l'()s; -)1», pI'ull(*.p]n'ns_

while the metanephros forms the definitive kidney of the adult, its excretory activities being reinforced during the first few months in the case of Reptiles by the still functional mesonephros. _ THE P.RONEPHROs.—-—A typical functional pronephros is well seen in a frog tadpole of about half an inch in length (Fig. 120, A, gm). It consists of a massive organ lying dorsal to the anterior portion of the splanchnocoele on each side. It consists mainly of a much con- voluted tube the anterior portion of the archinephric duct, and into this there open three segmentally arranged pronephric tubules also, except the anterior one, much coiled and twisted. While the organ as a whole is retroperitoneal, vie. outside the coelomic lining, there exists an opening leading from the splanchnocoele into each tubule ———the nephrostome (Fig. 121, ns). Duct and i3'l1l'_)l'lleS are lined with cubical, or almost columnar, epithelium and in the neighbourhood IV PRON EPHROS 223

ot' the nephrostome the cells become pigmented and carry powerful flagella. At the lip of the nephrostome the lining epithelium of the tubule is continued into the flattened epithelium lining the sp1anch- nocoele which is richly ciliated in its immediate neighbourhood.

The archinephric duct is continued back from the pronephros along the coelomic roof to open at its hinder end into the cloaca.

The pronephros has very characteristic relations to the blood- vascular system. The tubules serve to transmit to the exterior the fluid secreted by the coelomic epithelium and a patch of this epi- thelium, lying on the roof of the splanchnocoele at its mesial side and facing the nephrostomes, has its secretory activity much exag- gerated. This specially secretory epithelium has its area increased by bulging into the splanchnocoele, the bulging portion enclos- ing a vascular skein connected - with the aortic root. This bulg-‘X I/, ing structure is known‘ as the glomerulus (Figs. 120 and 121, gl). The anterior convoluted part of the archinephric Z. . duct and the tubules " opening into it have other relations to the vascularsystem,fortheir surface is bathed by the blood of the posterior cardinal sinus which

FIG. 121.—~Transverse section through a 12 mm. Tadpole at the level of the pronephros. (After Marshall, 1893.)

forms a system of ir-

, gl, glommulm; mf, mlv‘sl)ln0; I, lung; li, liver; M, medulla regulal Spac.eS between oblongata; .\, uotm-luml. us, nephrostome; oos, oesophagus; them. T1118 d011b1e up, operculum; gm. })I‘0lu-pllros.

relation to the blood-

system is doubtless correlated with the double function of the organ. It serves in the first place to get rid of watery fluid secreted by the coelomic epithelium, and with this function the glomerulus with its aortic blood supply is concerned; secondly it has to extract poisonous waste products from the circulating blood, and this is done by the wall of the tubule acting on the venous blood which bathes its surface.

DEVELOPMENT or THE PRONEPHROS IN HYPoGEor111s.—}[ypo- geophis, a member of the Gymnophiona, will be taken as an example of the mode of development of the pronephros for the following reasons:

(1) In this as in other Amphibians the pronephros still becomes an actively functional organ. Consequently the probabilities are in favour of its developmental processes having departed less from the 224 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

ancestral methods than in the case of those Vertebrates (e._q. E1asmo- branchii, Ainniota) in which the organ is modified to the extent of being reduced to a functionless rudiment.

(2) Its histological texture is comparatively coarse and the general structural arrangements in the embryo are so distinct as to eliminate to a great extent risk of observational errors.

(3) Its development has formed the subject of a particularly careful and complete investigation (Brauer, 1902). '

D —--a.n.a7. FIG. 122.-——Early stages in the development of the pronephros of II?/pogeop/ufs. Each figure represents a longitudinal section, so arranged as to pass outwards through the

uephrotoines, cutting them across, and viewed from the dorsal side. (After llraucr, 1902, slightly siniplilii-cl.)

A, from an cinln-yo with 15 ll\I_:.~4u(lm'lIl seginents; B, 12 .~".«'-g.:IlIvIIl..\' ; (‘, 16 segments; D, 27 scginmiits. u..n.vl. ill'('lllIlH1Dll!‘l('. duct: ‘nu, ]n‘tIlll-}.’.l‘l(' Lulmla-. Tlw Honnui ll_'.El1l‘I'.\‘ are placed in the lll‘.]blll‘P(‘(’)8lt’.S.

The first signs of the pronephros make their appearance-—in embryos with about 9 or 10 mesoderm segments——in the form of bulgings outwards of the outer or somatic Wall of the nephrotome of segments IV and V. These outward bulgings are the rudiments of the pronephrie tubules. A third soon appears in segment VI (Fig. 122, A, of. also Fig. 123, A, pn). The three rudiments grow actively i.n length pushing their way tailwards along the body just external to the nephrotomes. They come to be in close contact and presently fuse to form a rod-like structure (Fig. 122, B) which continues to extend backwards towards the tail and becomes tubular through developing a cavity secondarily, in its interior. This, at first solid, rod-like structure (Fig. 122, C, a.n.d) is the rudiment of IV - - PRONEPHROS 225

the _a.rchinephrie duet, which thus owes its origin to the fusion

FIG. 123.——Deve1opment of pronephros of Hg/pogeopluls as seen in transverse sections. (After Brauer, 1902.)

A, embryo with 22 segments; B, with 29 segments; C, with 44 segments. A, dorsal aorta; end, endoderm; gl, glomerulus; mm, lateral mesoderm; mc, myocoele; my, myotome; N, notochord; no, nephrocoele; ns, nephrostome; p.c, peritoneal canal; pn, pronephric tubule; sac, spinal cord; sol, sclerotome; splc, splanchnocoelo.

together of the outer ends of the tubule rudiments belonging to segments IV, V, and VI.

'VflT. TT 0 226 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Additional tubule rudiments to the number of about 8 arise in order behind those first laid down. They arise in exactly the same way "as the first (Fig. 122, B, Vll) but as the archinephric duct rudiment has already grown past their point of origin they become joined on to the duct by their outer ends undergoing sec(mda'ry fusion with it. Each tubule rudiment grows actively in length so that it eventually forms a much-coiled tube connecting the nephro- eoele or coelomic cavity of the nephrotome with that of the duct.

In the meantime the nephrotome is undergoing important changes apart from the tubule rudiment. Its cavity, the nephro- eoele, from being a mere slit with its floor and roof in contact, becomes widely dilated and it becomes cut off from the dorsal part of the segment which forms the myotome and sclerotome (Fig. 123, O). The nephrotome also becomes gradually constricted off from the lateral mcsoderm but in this case the separation either never becomes completed (Fig. 123, 0, 11.0) or if it does so, is merely tempt»rary————communication being soon re-established at the point where the constriction took place. The nephroeoele is thus, even in the fully developed pronephros, in open communication with the splanchnoooele by a more or less narrow channel the peritoneal canal (Fig. 123, C, 12.0), the splanehnocoelic end of which forms the peritoneal funnel.

As the nephrotome and tubule go on with their development there arise characteristic relations with the blood-vascular system. An intersegmental branch from the dorsal aorta passes to each nephrotome, causing its floor to bulge into the nephroeoele (Fig. 123, C, gl) and form a conspicuous projection——-the glomerulus——— which later on fills up most of the nephrocoelic space. From the glomerulus the vessel passes (as the was ejferens) into a network of blood-spaces lying between the coils of the tubule and belonging to the posterior cardinal vein.

The fully formed pronephros of Ilypogeopltis is composed of about a dozen segmentally arranged units each developed in the way described. It is to be noted however, that the last three of these units never become fully developed and further that behind the last as well as. in front of the first unit of the functional pronephros each segment has its typical nephrotome though this never proceeds with its development. In other words the pro- nephros of Ifypogeopluis possesses at its anterior and posterior ends a number of units more or less reduced or vestigial.

It is also of interest to notice certain variations which occur in connexion with the relations of the tubule to the nephrotome. In What may be termed the typical arrangement the nephrostome opens from the dilated part of the nephroeoele (Fig. 124, A). Frequently however it has become shifted on to the constricted peri- toneal canal (Fig. 124, B). When it does this there are apt to arise very misleading appearances as illustrated by the accompanying figure, whereby on the one hand the tubule appears to lead directly from iv PRONEPHROS 227

s the splanchnoeoele, the chamber containing the glomerulus appearing

to be a sid_e branch (Fig. 124, C‘), or on the other hand the pro- nephric chamber appears to form the dilated end of the tuhulewhile the peritoneal canal appears to form a side branch (Fig. 124, D).

In connexion with what has been said it is important that the student should get clear in his mind from the beginning (1) that the cavity into which the gloinerulus projects (known as the cavity of the Malpighian body in the more highly evolved types of kidney) is simply a more or less completely separated off portion of the



J

FIG. .l.24.—-Illustrating variations in the relations of neplirocoele, tubule and peritoneal canal in the pronephros of 1/3/pogcoplufs.

a..n..d, archinepln-ic duet; 'n.«_', l](‘.])hI‘OCO(‘l(‘.; ns, nephrostome ; p.f, peritoneal funnel; 1., t.uhu1e.

coelome (nephrocoele) and that neither it nor the peritoneal canal is to be regarded as a portion of the tubule, and (2) that the actual tubule commences at the nephrostoine or opening leading into it from the nephrocoele. The word neplirostoine throughout morpho- logy nieans an opening leading from coelome into n.ephridiuin. It is necessary to accentuate this because in many embryological writings the term nephrostoine, or nephrostomal canal, is applied to the peritoneal canal which is not an opening leading from coelome into nephridiuiii but simply a communication between the splanch- nocoelic and the nephrocoelic portions of-_ the ‘coelome. A clear appreciation of these points is of help in facilitating the comprehen- sion of a difficult chapter in Vertebrate morphology.

.q.n.d. 228 EMBRYOLOGY OF THE_LOWER VERTEBRATES CH.

clan other Verte- brates possessing a functional pronephros the appearances seen in early stages are readily reconcilable with those described above for IIyp0_qe0-

plmls, and we ma.y
 a take it that, apart

from variations in detail, this represents the normal mode of development of the organ. Cnossorrnmrerr. —- In Crossoptery- gians, so far at least as Polyptew-us —— the only member of the group investigated- is concerned, the first rudiments of the pro- nephric tubules are in the form of pro- jections which pass outwards and back- wards from the exter- nal side of each of the anterior nephrotomes (Fig. 125, A). The number of these tubule rudiments pre- sumably varies, seven being seen in one specimen and nine in another. Apparently the tubule rudiments become fused at their outer ends to form a solid mass—-the rudi- ment of the archi-

Fm. 125.-—'l‘ransverse sections of Pol}/1;te1'us-—stages 20, 23, nephric duct. In the and 28—passing through the rudiment of nephrostome Stages Shown in Fjg-_ B which is seen projecting outwards from the wall of -. tlie nephrocoele. ‘ 126’ A and B’ five


tubule rudiments are

A, dor.»-:11 not-ta: Purl, coelomo-: rm. --nt-e-rit°‘ca\'it.v:_'m.u, !'ll:}'O(_'.-l'H‘l(‘2 Seen Passing at their N, n()t',()(_-lmrtl ; M12. In-1')l|I'()t:m-.l¢'-. ; ,-_v'. 2', Iflnil 4-rum l!.{n'dIlI:ll \'I*Ill ; R)./(‘-, . split re1')1'osmnti1i;: sphmchnocoele. Outer end-S In to the IV ’ PRONEPHROS 229

duct rudiment. As development goes on however the tubules belonging to nephrotomes I, III,- and IV, those labelled A, C and D in the figure, become reduced in size and finally


A B C D - E FIG. 126.——Dorsa.l view of p|'mIr:pln‘0s of l’nlyp?eru.s- at stages 20, 23, 24+ , 25 untl ttl)I_n,I.t ‘Z8.

(1..'n..d, amlniiu-pln'ic. «luct. 'l‘lu- tubule rudiments are indicated by letters, the nephrotomes by l{0m:u1 numerals. '

disappear, while B and E on the other hand increase in length and become the functional tubules. The anterior end of the archinephric duct becomes gradually modelled out of the solid rudiment ‘already referred to in the way indicated in the figure.

FIG. 127.—Pa.rt of a longitudinal v('-.1‘tic:tl section through the series of nephrocoeles in the pronephros of I '0/ypI¢.'I‘H.5‘——8tIlge 24 + .

a,.'n..d, arcliiuephric duct; eat, ectoderm; end, undoderm; no.3, nephrocoele “ B"; M. F, nephrocoele “ F."

After it has assumed its definitive tubular form this front part of the archi.nephric duct commences to grow actively in length, it becomes thrown into complicated coils and forms a large fraction of'the entire bulk of the pronephros in its early functional stages. 230 EMBRYOLOGY OF THE LOWER VERTEBRATES

FIG. 128. ——Renal organs of the right side of a Protopterus larva of stage 34. (From a reconstruction by M. Robertson.)

a.n.d, archinephric duet; op, opistho- nephric tubules; pn, pronephros. The capital letters indicate nephrostomes and the figures metotie * mesoderzn segments.

  • “ Metotie ”=poste-rior to the otocyst.

CH.

In its later functional stages the pro- nephros reaches relatively enormous bulk, occupying the whole thickness of the body-wall, but in these later stages the two tubules become much elongated and coiled as well as the duct itself.

The nephrocoeles belonging to the various nephrotomes which develop tubules form a series of closed cavities lying in a row one behind the other (Fig. 127, mzb’, 97.0.11’). They are for a long time. in Polg/p/erus, the only coelomic spaces which are Widely open (Fig. 125, B). As development goes on the nephrocoeles connected with the func- tional tubules (B and E) become more and more dilated, their wall becoming thinner as they do so, and the lloor bulging into the cavity to form the glomerulus. Eventually the cavity of the nephrocoele becomes continued ven- trally, a split spreading downwards to form the splanchnocoele, into which’ the nephrocoele opens freely. The portions of splanchnic Inesoderm to which the glomeruli are attached, 'i.e. the floors of the original nephrocoeles, become folded in towards one another, as the splanchnocoelic cavity dilates, to form the dorsal mesentcry so that the glo- meruli are eventually borne by the mesentery one on each side.

Meanwhile the nephrocoeles belong- ing to the tubules which atrophy gradually shrink up and disappear, and as they do so the two large functional nephrocoeles increasing still more in size meet and their cavities as well as their gloineruli become continuous. N o definite constrictions (peritoneal canals) are formed between nephrocoeles and splanchnocoele, unless possibly during late stages, but the dorsal portion of the more posterior nephrocoele becomes cut off from the splanchnocoele by another method-—the free edge of the glomerulus coming to fuse with the somatopleure so as to form a floor to the nephrocoele. Iv PRONEPHROS 231

This posterior nephrocoele is still in wide communication with the splanchnocoele indirectly by way of the anterior nephrocoele.

DIPNOI.——In Lepidosiren and .P7'0t0pte7"u.s1 the fully functional pronephros of the larva possesses usually two tubules (Fig. 128, B and D). These are the surviving members of a series of tubule rudiments extending through at least the anterior 4-7 segments but probably extending much further back. The tubules which become fully developed are normally “ B” and “ D ” 'i.e. those corresponding to the second and fourth mesoderm segments. Thus the second tubule does not correspond with the second tubule in the fully developed pronephros of Pcilyptems. The tubules appear to originate (cf. Fig. 129, A, B) as in H3/pogeoplmls except that the outgrowths from the nephrotomes are solid as in I’0lypte'rus and such is the case also with the archinephric duct rudiment.

The nephrocoeles of the two main pronephric tubules undergo fusion as in Polypterus so as to form a large proncphric chamber on each side. This is continuous with the pericardiae portion of the splanchnocoele and the two glomeruli as usual become fused together to form a compound glomerulus? lu Lepiciosrrowt the fusion of the pro- nephric chambers takes place before the appearance of the glomerular rudiments. These appear first on the floor of the continuous cavity (Fig. 129, C, gl) and very soon undergo fusion themselves. By dif1'erential growth the root of attachment of the glomerulus becomes gradually shifted towards the mesial plane and dorsally (Fig. 129, D and E) so that it comes to hang down into the pronephric chamber or nephrocoele from a point in close proximity to the dorsal aorta.

The pronephric chambers are at first perfectly continuous with the splanchnocoele which spreads outwards from them. Later on the. pronephros becomes greatly enlarged and bulges across the sp1anchno- coele until it comes in contact with the mesodermal sheath of the oesophagus. Fusion then takes place (at the point marked with * * in Fig. 129, E) between the surfaces in contact so that the glomerulus comes to be enclosed in a secondary pronephric chamber, which however remains freely open to the splanchnocoele at its hinder end. The glomerulus becomes firmly slung diagonally across this chamber by its tip undergoing fusion on the ventrolateral side of the chamber with the mesoderm investing the pronephros.

In Uemtodus (Semen, 1901) the pronephros probably develops in a manner similar to that described in the case of the other two Lung- fishes. The organ in its first stage is a solid projection of the meso- derm the appearance in section being similar to that figured for Lepidosiren. The portions of the rudiment corresponding to the individual tubules are in such close apposition as to be at first indistinguishable (as is often the case in the other two Lung-fishes):

1 A large part of the investigations upon which this account is based were carried out by Miss Muriel Robertson in the University of Glasgow during 1904. 2 The word glomus is often used for such a compound glomerulus. 232 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

it is only when nephrocoeles begin to appear (in the regions of the

firm. as shown in trzu verse sections.


Fm. 129.-—-Development of the proneplwos in Lepz'u’z;'

A, stage 21; B, stage 21 ; (3, same 24+. u.'n.¢1, zu-chincphric duct; and, uudoderm; ant, enterlu cavity; gl, g1ouwrulu.~:; I.m,1ate.ml n1usmlm'In; -my, myutmne; N, notochord; ow, nephrocoule; pn, pmnephric tllblllv; s.«.:, spinal cord ; .~.-a-I, -*.-c.lex-oton1e.

fifth and sixth segments) that the segmented nature of the rudiment IV PRONEPHROS 233

becomes apparent. The fully functional pronephros has two tubules on each side, corresponding to the segments above mentioned: it may be presumed that these are the survivors of a once greater number, though there is no record of other rudiments having been actually observed.

FIG. 129A.——Development of the pronephros in Lepidus'£-re-n as shown in transverse sections.

D, tags 30: E, st:u,:e 31+. A, dorsal aorta; a.n.d, archinephrie duct; end, emloderm; gl, glamo- rulus; li, liver; -my/, myotome; N, notochord; nc, nephrocoele; oars, or-sopliagus; pn, pronephric tubule; p.'v.c, posterior vena cava; splc, splanehnocoele.

ACTINOP'l‘ERYGII.———Tl1e acquirement of a thorough knowledge of the development in the more primitive members of the group—the ganoids———is an essential preliminary to the proper comprehension of the development of the more highly evolved Teleosts but-un- fortunately our knowledge of renal development in the qanoids is still far from complete. 234 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The tubule rudiments appear to arise in normal fashion, as out- growths of the lateral Wall of the nephrotome. These outgrowths show the familiar variation of being sometimes hollow sometimes solid. Thus in Amia according to Felix (1904) the anterior three rudiments are hollow pockets while those farther back are at first solid.

Tubule rudiments make their appearance from segment III to segment Xlll but here as elsewhere only relatively few of these complete their development and are to be found in the pronephros at the height of its functional activity. Thus in a six-day Acipenser larva Jungersen found six functional tubules vvhile in Amid Felix finds only a single tubule functional. In the latter case the tubule

opens from a large pronephric chamber apparently formed by the fusion of at least three nephrocoelcs. The tubule belongs originally

to the most anterior of these and corresponding to it there is present a single open peritoneal canal. Later on this becomes replaced functionally by another peritoneal canal situated farther back. In Lepidostcus the functional pronephros has at least three tubules each with its nephrocoele (Felix, 1904).

As in the case of the Lung-fishes the dorsal part of the splanchno- eoele in the pronephric region becomes floored in by the approxi- mation of the mesial surface of the pronephros to the lateral surface of the oesophagus (cf. Fig. l29A, E) so as to form a secondary pro- nephric chamber. In Lepwldosteus this forms a widely patent cavity with which the first nephrocoele becomes completely merged and which remains ventrally in continuity with the main sp1anchno- coele by a narrow richly ciliated tubular channel. In Acipenser the first nephrocoele undergoes a similar modification while the remain- ingl five are fused with one another but isolated from the splanchno- coe e.

TELEOSTEI.-—The development of the renal organs has been worked out in detail in the case of the genus Salmo by Felix (1897). In this case the myotomes are already separate from the more ventrally situated portions of the mesoderm at a very early stage. The first rudiments of the pronephros are in the form of a series of somewhat conical, segmentally arranged, solid projections from the median edge of the lateral mesoderm towards the mesial plane. These projeetions—five in number (segments 3-7) in a 26-day Trout—-are probably to be regarded as nephrotomes which have been precociously separated from the myotomes, if indeed they ever were continuous. These five nephrotomes soon come into intimate contact so as to be no longer distinguishable. They now together form a continuous mass of mesoderm the so-called pro- nephric fold. The dorsal and outer portion of this mass becomes nipped off to form the anterior portion of the archinephric duct except at one point where a connecting isthmus remains to form a tubule. The mesial portion of the mass becomes the wall of the single pronephric chamber. Iv PRONEPHROS 235

The whole mass is at first solid, the cavity of duct, tubule, and pronephric chamber, developing secondarily.

'l‘he cavity of the pronephric chamber is for a time continuous with the split-like splanchnocoele, but it soon becomes constricted off from it and forms a completely closed cavity. Bearing in mind the segmented condition of the pronephric rudiment in its first stage of‘ development and the process of fusion of successive nephro- coeles which takes place in Ganoids, we may conclude that the pronephric chamber of the Teleost probably represents a number of nephrocoeles fused together. The single pronephric tubule is very possibly the same member of the series as that which occurs in Amie although this has not yet been actually determined.

A remarkable peculiarity found within the group Tcleostei is that in a few genera (e.g. Fierczsfer, Zoarces, Lepadogaster) the pronephros retains its renal function throughout life (of. Guitel, 1901, 1902).

AM1>H1n1A.——In Amphibians other than Gynmophiona the pro- nephric rudiment first becomes visible-as a solid swelling of the somatic mesoderm at the level of the anterior mesoderm segments (Rana segments 2-9, ’l’m'ton alpestrm'.s~ 1-6, Mollier). Though at first no segmentation is _to be detected by the ordinary methods of observation in this swelling it is reasonable to interpret it as representing morphologically a series of closely apposed or fused nephrotomal projections like those of 1-"I3/pogcophis. This pronephric rudiment gradually becomes demarcated off from the rest of the mesoderm by a split which becomes apparent on its ventral side at first laterally and then spreads inwards.

The rudiment new forms a thick flap (of. Lung-fishes, Fig. 129, A and B) hanging down on the outer side of the mesoderm, and continuous with the somatic mesoderm along its dorsal and median edge. Segmentally arranged coelomic splits make their appearance along the line of attachment of the pronephric flap and these we may interpret as incipient nephrocoeles. The split already mentioned as demarcating the pronephric rudiment ventrally spreads round its median edge, so as to detach it completely from the (nephrotomic) mesoderm except at Segmentally arranged points where a connect- ing isthmus remains as the nephrostomal end of a tubule. The pronephric rudiment now undergoes a kind of modelling process similar to that occurring in Crossopterygians and Lung-fishes, its outer portion being gradually cut off from behind forwards so as to form the archinephric duct, while the part nearer the mesial plane forms the recurrent portion of the duct with the tubules connected with it.

The whole rudiment is at first solid. The earliest coelomic spaces to appear are the nephrocoeles and from these split -like extensions spread outwards in each tubule rudiment, while towards the outer margin of the rudiment the continuous longitudinal cavity of the archinephric duct develops.

Of the tubule rudiments, as usual, only a few become functional-— 236 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

in Anura commonly 3, in Urodeles commonly 2 (in Amphqluma 3 according to Field). Probably here as elsewhere the number is really a variable one. As the tubules develop they show active increase in length so that they become much coiled and the same applies to the part of the archinephrie duct lying in the pronephric region.

It is only when they first appear that the nephrocoeles show a segmental arrangement: later on they become merged in the general splanchnocoele. Along the inner wall of the dorsal portion of this cavity, 13.3. the portion which represents the fused nephrocoeles, the glomerulus develops as a continuous laterally projecting fold of splanehnie mesoderm. Usually the portion of the body cavity con- taining the glomerulus becomes for a time incompletely shut off from the rest to form a secondary pronephrie chamber as in Lung- fishes, the mesoderm covering the lungs undergoing fusion with that covering the bulging surface of the pronephros. The secondary

pronephric chamber may in turn be subdivided by the edge of the”

glomerulus fusing with the mesoderm covering the pronephros.

MEROBLASTIC Vslrrsnnxrns.-As a r11le, in the Meroblastic Vertebrates the pronephros never becomes a functional organ, and correlated with this it shows a reduction in its structure. Possibly, as already indicated, this may be due to the presence of the large yolk-sac with highly vascular surface in contact with the external medium, which will facilitate the getting rid of excretory material by diifusion outwards.

ELASMOBRANCHII.-——1n Elasmobranchs the ventral ends of certain of the anterior mesoderm segments, usually commencing with segment VII, become dilated to form vesicular cavities (van Wijhe, 1889) which are probably to be interpreted as nephrocoeles. The tubule rudiments appear as thickenings of the somatic wall of these nephrocoeles which grow outwards and being in close apposition form at their outer ends, apparently by fusion, a solid continuous pronephric swelling. The tubule rudiments make their appearance in sequence from before backwards.

Different workers vary in their statements as to the number of rudiments in different forms [Sop/llrium, 5———Riickert, 3~—van Wijhe; Pristiurus, 5--Riickert, 4—Rabl, 3—van Wijhe; ]t’a/ta clwvata, 5——van Wijhe; R. alba, 8——Rabl; Torpedo, 7——~Riickert (Fig. 130)] from which we may conclude safely that the number of tubule rudiments is very liable to variation both as between different species and different individuals. This variability may be taken in correlation with the fact, observed by van Wijhe, that in P1"'ist'i'u/rus dilated nephrocoeles made their appearance from segment I to segment XIV, gradually diminishing in size towards the end of the series, although tubule rudiments appeared only in 3 segments. Both phenomena indicate that the pronephros in Elasmobranchs as in other groups has under- gone reduction from a once much greater anteroposterior extension.

In a comparatively late stage the tubule rudiments develop their 1v PRONEPHROS 237

lumen. The pronephric swelling extends backwards into the archi- nephric duct. N 0 glomerulus develops but segmentally arranged branches of the dorsal aorta appear on the right side corresponding in number and degree of development with the pronephric tubules. These give rise either one of them (Riickert, van Wijhe) or by fusion together (Rabl) to the root of the vitelline artery but are termed by Rabl pronephric arteries.

The pronephros undergoes rapid degeneration and eventually

pothlipg is; left of it but the coelomicfunnel of the Miillerian duct see ‘e ow .

SAUROPSIDA.—-In the fowl pronephric tubule rudiments develop to the number of about 12, in the form of solid outgrowths of the somatic mesoderm at the level of the nephro- tomes although, except in the case of the most anterior, the mesoderm is not yet segmented at this level at the time when the rudiments appear. In at least some cases segmental dilatations of the otherwise split-like coelome occur op- posite the tubule rudiments and are no doubt to be interpreted as the nephro- coeles of the corresponding tubules. The first tubule rudiment makes its appearance in embryos with 8 or 9 segments, its position showing consider- able variation (usually segment 4, 5 or 6).

The successive tubules appear in , _ _ rapid succession——almost synchronously. 1‘ “,§§~",‘?i“g’1’.’t’ At about the 19-segment stage the Torpedo. (After Riickert,1888.) myotomes become separated from the a,vn,d,archinep]1x-ic duct; z,,.,, mom. nephrotomes, the latter remaining in "0-'-<01: rm-1. etc-. pronephric tubules. continuity with the lateral mesoderm ‘""’ "“"‘b‘”"“‘ ‘““‘ and their cavities (nephrocoeles) with the s lanchnocoele. About the same starre the baekwardl ro'ectin tip cl‘) each tubule rudiment undergoes tiusion with its dligcegsor llgl the series and thus gives rise to a continuous longitudinal rod—like structure——the rudiment of the archinephric duct (Felix, 1904). The archinephric duct in its anterior portion thus would appear to develop i.n a manner essentially the same as that found in the Grymnophiona and so many other of the lower vertebrates.

The hinder end of each tubule rudiment, as well as the archi- nephric duct itself, is at first solid. The definitive lumen makes its appearance (about 20-segment stage) secondarily in the form of dis- continuous chinks which gradually become continuous and spread backwards.

Pronephric glomeruli develop in the Bird though at a late stage 238 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

when the pronephros is already degenerating. They were discovered first by Balfour and Scdgwiek (1878) in the Fowl where they vary in number from about 3 to about 7. They may, a.s so commonly occurs in pronephrie glomeruli, undergo a less or greater amount of fusion with one another and also with the anterior glomeruli of the opistho- nephros. The whole pronephros in the Bird undergoes rapid atrophy and by the sixth day of incubation has usually in the Fowl com- pletely disappeared except the glomeruli which may still be detected for a day or two longer.

in the Reptiles also a rudimentary pronephros makes its appear- ance but degeneratcs without becoming functional. The nephro- tomes or protovertcbral stalks, at first solid, develop a patent cavity or nephrocoele. ]n a varying number of segrncn ts (in Lizards 6-8, commencing with segment V) proncphric tubule rudiments develop as outgrowths of the somatic wall of the nephrotome after the ordinary fashion and fuse together at their outer ends to form the archinephric duet.

THE ARCHINEPHRIC DUcr.——-As has already been indicated, it is characteristic of the Vertebrate that its ncphridial tubes no longer open directly to the exterior, but that, on the contrary, they open into«a longitudinal duct on each side -—the archinephric duct———which in turn opens into the alimentary canal towards its hinder end. The first steps in the evolution of the archinephric duct have passed beyond our ken and to decide as to how they came about we have to balance probabilities on a basis of somewhat scanty embryological and anatomical data. Two obvious possibilities present themselves ——(1) that the row of segmentally arranged nephridial openings came to be sunk beneath the general surface in a longitudinal groove and that this groove became covered in to form a longitudinal duct, and (2) that the external opening of each tubule became shifted back- wards so as to open into its successor in the series and so give rise first to a common opening with it and later to a common longi- tudinal duct (Fig. 131) in the way exemplified by the posterior kidney collecting tubes of male Elasmobranchs. On which side the balance of probability lies will be apparent on considering the developmental facts so far as they are known to us at present.

It will be recalled that in Hypogeophas-, according to Brauer, the anterior portion of the archinephric duct arises by a number of pronephric tubule rudiments bending tailwards at their outer ends and undergoing fusion together. The fused portion forms the duct rudiment and it proceeds to extend backwards by independent growth until eventually it reaches and fuses with the wall of the cloaca. It is only a small portion of the duct close to its anterior end which is formed by the direct fusion of tubule rudiments-——the tubules farther back growing out and fusing secondarily with the already formed duct.

If we turn to other Vertebrates we ~find considerable evidence for believing that Ifypogeophis presents to us a mode of development IV ARCHINEPHRIC DUCT 239

of the archinephric duct which. is relatively primitive. In a number of Vertebrates there appear to be distinct traces of the formation of the front end of the archinephric duct by fusion of the outer ends of tubule rudiments in a manner essentially the same as that which holds for fly/pogeoplznls. As will have been gathered from the pre- ceding pages this is the case with such different groups of Vertebrates as Elaslnobranchs, Crossopterygians, Lung-fishes, Reptiles and Birds.

Fm. 13l.—--Diagram illustrating a possible mode of evolution of the archinephric d11ct.

A, the coelomiu eompm-tment-s are bulging t-I)\\'{l.l‘(l!~‘. the neplwidial tubes; 13, the colnpartinentis hax-‘e come to open into the nephridial tubes and the flame-cells have disappeared ; C, l), the externa‘. openings of the nu-'-phridia are beconiing shifted backwards so as to give rise to the archinephric duet E, the archinephric duct is completely formed and commtmieates with the enteron through one 01 the segments retaining, or revert-ing to, its primitive enter-o(2oelic. connexion. a.n.d, 8.!‘('.llinHpln'i(' duct; c.f, eoelomic funnel; (uwl, coeloniie cavity; cl.u, cloncal opening of arc.hinephI'ic duct: ect ectoderm; end, endorlenn; _/'.r_-., flame-cell ; n, nephridial tube.

If we are justified in looking upon this mode of formation of the duct in ontogeny as relatively primitive, i.t obviously affords strong support to the second of the two above-nlentioned hypotheses as tc the evolutionary origin of the archinephric duct: the bending back

of the tubule rudiments would then be interpretable as a develop- mental reminiscence of the backward shifting of their external open-

ings which took place during phylogeny. 240 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

.The independent backgrowth of the remainder of the duct in Hypogeophis is probably to be regarded as a case of accelerated or precocious development to allow the anterior tubules to become functional at an early stage of development before those farther back have developed.

As regards the ontogenetic development of the main part of the duct in other Vertebrates we find the most divergent statements and it seems clear that this divergence can only be explained by the actual facts not being always the same.

In the Sauropsida it is admitted that the main part of the duct is formed as in Hg/pogeophvls by independent backgrowth. Amongst the Anamnia the same is said to be the casein Elasmobranchs by Balfour and by J-labl, and in Al;/tes according to Gasser, but other authors describe two other methods of formation as occurring.

The first of these is found in Elasmobranchs according to van Wijbe, Beard, Riickert and others. According to these investigators, the archinephric duct makes its first appearance as a longitudinal ridge - like thickening projecting inwards from the ectoderm. This becomes split off as a solid ectodermal rod which develops a cavity secondarily and forms the archinephric duct. Such a

mode of development would be of great morphological interest

[as it would lend decided support to the View that the archinephric duct originated in evolution as an ectodermal groove———it being a

common ontogenetic modification that what is morphologically a groove develops ontogenetically in the form of a solid ridge-like ingrowth. It has however to be borne in mind that there exists a serious source of possible error in making observations upon the archi- nephric duct in early stages. The duct lies between ectoderm and somatic mesoderm——the two cell-layers mentioned fitting close round it. During the various processes to which the embryo is subjected preparatory to being cut into sections the ectoderm usually separates slightly from the mesoderm, and the archinephric duct tends to adhere firmly to one or other of these layers. This is the case more particularly at its tip, where it is pushing the ectoderm and meso- derm apart as it grows back and is therefore in particularly intimate contact with them. It is exceedingly difficult in studying sections to distinguish with certainty between such intimate contact and actual organic continuity. In cases where the hinder part of the duct is adherent to the ectoderm an appearance is produced which simulates closely a development by splitting off from the ectoderm.

As a matter of fact C. Rabl’s very careful investigations (1896) fail to confirm the ectodermal origin of the duct in Elasmobranchs and upon the whole in the writer’s opinion there does not appear to be any longer justification for accepting it as actually occurring.

The other mode, by which the extension of the archinephric duct backwards has been described as taking place in the Anamnia, is that the duct becomes split off from the.underlying somatic meso- derm. It is necessary again to bear in mind the caution expressed Iv ARCHINEPHRIC DUCT 241

above but making full allowance for this it seems impossible to escape the admission that in many forms (Petromg/zon, Lung-fishes, most Amphibians, Teleosts and probably actinopterygian Ganoids) the duct is prolonged backwards by a process of this kind.

It being accepted that in a ‘number of Anamnia a large part of _the archinephric duct arises in development by being split off from the mesoderm, we are faced by the problem how this mode of develop- ment is to be correlated with the mode of development by fusion of the outer ends of tubule rudi- ments. It may be suggested that what has happened is that the development has been accelcrated—--as often happens ——by skipping over the early stages._ The mode of develop- ment in question may have been derived from the more primitive mode by the omission of the separate tubule stage and the passage at once to the stage in which the tubule ends are fused into a continu- ous structure.

In some cases however the primitive mode of development has undergone a further modi- . fication. This is exemplified a by P0l.7/Ptwus (Graham Ker1'a Fm. 132.——'1‘ransverse section through Polyptems 1907) where the hinder portion of stage 23 at level of cloacal opening.

Of the duct appears l}O be a.n..d, opening of archinephric duct into cloaca; cl, for]-“ed conversion ope-.i|i1Ig of clo:u-u lo exte.ri01:; end, nlinn.-utlzu-_\-' I(‘.'ll1{ll of the series of nephrotorneso :_;l(:;.l‘l]l‘lll-‘Ill-, my/, m__\o1ome, .\,1|utoc-lund. .~.«-, spllllll These are not segmented but

form a continuous structure which becomes converted directly into the archinephric duct. .

In whichever way the archinephric duct completes its extension backwards, it eventually comes to open into the cloaca. This is, in the great majority of Vertebrates, described as coming about by fusion of the previously freely-ending tip of the duct with the cloacal wall. It is obvious that such a process cannot correspond with what happened during evolution as the duct must have had its posterior aperture throughout in order _to perform its function.

It is possible that a clue to the evolutionary origin of the com- munication between archinephric duct and alimentary canal is given by Polypterus. It has already been mentioned that in this animal the hinder part of the archinephric duct arises by bodily conversion of the row of fused nephrotomes. Fig. 132 shows that the opening of archinephric duct into the alimentary canal presents a striking

VOL. II R 242 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

resemblance to the primitive communication of mesoderm segment with enteron, and it is suggested that it actually is this primitive communication which has remained patent while in all the other segments it has disappeared.

The two archinephrie ducts open at first separately into the cloaea, one on each side. In some groups of Vertebrates however their terminal portions become gradually approximated and eventu- ally fused together into an unpaired dorsal vesicle which may undergo various modifications. In Elasmobranchs it forms the urinogenital sinus which bulges forwards and on each side becomes prolonged into the sperm-sac. In Lung-fishes it forms the cloacal eueeum: in Teleostei the urinary bladder.

It is noteworthy that in the adult Lung-fish the communication of the kidney duets with the caecum is close to the posterior opening of the latter, so that a small amount of shifting would cause these ducts to open into the cleaca independently of the caecum.

This suggests a possible evolutionary origin of the allantois. It is conceivable that a caecnm similar to that of Lung—fishes arose by a fusion of the terminal portions of the kidney d11cts"mmtml, instead of dorsal, to the alimentary canal and that the ducts then came to be emancipated from the caecum which remained as a ventral diverticulum of the cloaca to form the allantois. We have no definite evidence as to the evolutionary origin of the allantois and it is well to bear in mind the possibility here indicated in addition to the simpler and perhaps more probable hypothesis that the allantois was from the beginning simply a bulging outwards of the ventral cloacal wall as it is actually in ontogenetic development.

DEGENERATION or TIIE PRONEPHROS.—-—The role of the pronephros

as the functional renal organ is usually confined to comparatively

early stages in development and at the end of this period, when its function is being taken over by the opisthonephros, the pronephros commences to undergo characteristic degenerative processes which normally culminate in its almost complete disappearance.

In the frog (Marshall and Bles, 1890) these processes become apparent in the tadpole of about 20 mm. in length. The archi- nephric duct becomes more or less obstructed behind the pronephros and as fluid continues for a time to pass into the tubules the latter become greatly distended in places, their lining cells assuming a cloudy appearance, the cell boundaries becoming indistinct and their inner surfaces losing their smooth outline and becoming ragged. The whole organ shrinks in size, becomes invaded by leucocytes, the nephrostomes close, one after the other, and by the end of the first year the whole organ with the adjacent portion of the archinephric duct has practically disappeared. 3

MULLERIAN DUc'r.—-—Throughout the series of gnathostomatous Vertebrates, with the exception of the teleostomatous fishes, the oviducts are admittedly homologous. They-—the Miillerian ducts- are above all characterized by the fact that they open freely into the IV MtiLLER1AN DUCT 243

splanchnocoele at their anterior end by an open funnel (ostixmn iubwe). There exists in some of the more archaic fishes what appears to be distinct evidence that the Miillerian duct has been evolved out of the tubules and duct of the pronephros and it will therefore be convenient to consider this evidence now.

The Elasmobranchs are the fishes in question. I11 Torpedo (Riickert, 1888) as the pronephros degenerates its tubules become reduced to the three hindermost. Of these three the two posterior degenerate while the other—tubule E-;pcrsists and its enlarged nephrostome becomes the coelomic funnel of the Miillcrian duct. Other workers (e.g. van Wijhe and Rabi), working on other J£lasmo- branchs (P'r'isti'u/rus), trace back the coelolnic funnel of the Mullerian duet also to an opening derived from the pronephros and nephro- stomal in its nature, but they believe the opening to he formed not by the persistence of a single enlarged nephrostomc but rather by the fusion of three or four nephrostoines together. That it is morpho- logically a single nephrostome is however rendered more probable by what we now know regarding the development of the pronephros in those of the more archaic fishes in which it develops as a functional organ. It will be recalled, for example, how in 1’0l-3/pterrus tubule E (like B) becomes enlarged as compared with A, U, and D. A pre- nephric tubule enlarged in this manner in correlation with purely excretory needs would provide an obviously adequate beginning for the evolution of a funnel for the transmission of the eggs like that at the front end of the Miillerian duct.

While the funnel of the Miillerian duct is nephrostomal in origin the main part of the duct is developed in the Elasinobranchs (Semper, 1875; Balfour, 1878) from the arehinephric duct. The latter undergoes a process of splitting from before backwards into a dorsal and a ventral tube, the latter being at first a solid thickening of the ventral wall of the arehinephric duct. Of the two tubes so formed the ventral is continuous with the pronephric funnel, while the dorsal carries the openings of the kidney tubules farther back in the series: the former becomes the Miillerian duct, the latter persists as the functional duct of the opisthonephros (Fig. 133, C, W.d).

This mode of development is satisfactorily explained by the assumption that the relatively archaic fishes in which it occurs are repeating the process by which the Mullerian duct arose in evolu- tion. Such a splitting of an originally common duct into two, so as to separate the routes by which two different products reach the exterior, is probably of frequent occurrence in evolution. Good examples are seen in the splitting of the common genital duct of hermaphrodite gasteropods (e.g. the ordinary snails) to form a separate oviduct and vas deferens. It appears then justifiable to accept as a working hypothesis that the Miillerian duct arose in evolution by being split off from the arehinephric duct and that its coelomic funnel is a persistent pronephric funnel.

Turning to Vertebrates other than Elasmobranchs, well-marked 244 EMBRYOLOGY OF THE LOWER VERTEBRATES

CII.

differences are found to exist between the phenomena as described for different groups and even for members of the same group by


P“ --as. D 0 S 2 1' ~ : 1 Md. ‘ I

.1

I U.l‘l.d. W.al. .-J‘ ~ -. A B C - C/.

Fm. l33.—-Arrangement of archi- nephric duct, etc., in embryos of l’7'istizmts. (Based on Rab1’s figures.)

A, male 17 mm. ; B, female 19 mm. ; 0, female 27 mm. a.n.d, archinephrie duet; cl, cloaca; M.d, Muller-ian duet; as, coe- lomic opening of Mullerian duct; gm, pronephrie nephzostome; W.d, duet of opisthonephros.

different observers. While some of these may be due to observations being pushed to within the limits of prob- able error it is impossible to avoid the conclusion that great differences do actually exist in the details of develop- ment of the Miillerian duct.

It is possible on general embryo- logieal principles to arrive at an idea of the kind of variations which might be expected to show themselves from the supposedly primitive mode of de- velopment.

1. The Miillerian duct might con- tinue to arise in an unmodified manner by splitting from the archinephrie duet, its funnel being a persisting nephro- stome.

1].. In correlation with the fact that the one derivative of the archi- nephric duct (duct 01' the opistho— nephros) is I‘equi1‘e(l to be functional at a very early period, while the other (Miillerian duct) does not function until adult life, there would be a tendency for the two duets no longer to keep exactly abreast in their development but to become separated, the Wolifian duct developing relatively earlier, the Miillerian relatively later. T 0 enable this to take place, the primitive stage in which the two ducts were still one would tend to be more and more curtailed until it was eventually elimin- ated and the two duets were independent from the beginning.

III. The independently arising Miillerian duct might retain the mode of extension backwards by intrinsic growth, eventually reaching and fusing with the wall of the cloaca.

IV. Its separation from the somatic mesoderm might take place relatively

later than its extension backwards so that it would arise in develop- ment eomparatively late, as a longitudinal ridge or fold gradually separating off from the mesoderm from before backwards. Iv MITLLERIAN DUCT 245

A survey of the phenomena as described for the various sub- divisions of the Vertebrata shows that as a rule they may be fitted without difiiculty into one or other of these types of modification.

Thus in the Amphibia some, especially of the older observers, described the extension of the Miillerian duct as taking place by splitting off from the archinephric duct as in Elasmobranchs, others as being due to independent intrinsic growth, still others as a process of folding or splitting off from the splanchnococlic epithelium.

()ne of the most careful modern accounts (H. Rabl, 1904) based upon?

the phenomena observed in the relatively primitive Urodela (Salmmmdm) states that the funnel. is the persisting “second” nephrostome of the pronephros and that the portion of duct behind this arises as a thickening of the coelomic epithelium—-the cells first assuming a columnar shape, then becoming arranged in several layers to form a ridge projecting into the subjacent connective tissue, and finally becoming split off as a solid rod. Only the anterior portion of the Mnllerian duct is formed .in this way, the rod-like rudiment so formed proceeding to grow back independently to form the hinder part of the duct.

In Reptiles and Birds the ostium is described as originating as a pit in the eoelomio epithelium, which we may look on as a delayed and modified nephrostome, and the extension backwards as taking place by independent growth. The cvagination of the epithelium to form the pit is, as is usual in such cases, preceded by the epithelium becoming somewhat thickened.

The mode of origin of the Miillerian duct has not yet been worked out in detail in the Ganoids and Lung-fishes. In ordinary fishes (Teleostei) the conditions are peculiar and will be dealt with along with the development of the ovary.

As regards the further development of the Miillerian duct, it should be noted that its completion and opening into the cloaca is commonly delayed till a comparatively late stage-——-often till a period but shortly before sexual maturity.

Though primarily rctroperitoneal the Miillcrian duct comes, with increasing growth, to bulge into the splanchnocoele, pushing inwards the peritoneal lining which comes to surround it as a sheath con- taining muscles, blood-vessels, etc. Its lining epithelium becomes glandular and specialized to minister to the nutritive and protective needs of the egg in ways which differ in the different groups.

Various modifications make their appearance in later stages. Very frequently the coelomic opening becomes shifted by the addition to the tube of a secondary extension formed from the peritoneal lining. In Elasmobranchs this shifting is towards the mesial plane and except in a few species leads to complete fusion so as to form a single median opening for the two oviducts. Again the hinder ends of the Miillerian ducts are in many cases approximated and they too may fuse to form a terminal unpaired portion.

In the case of the Birds the right oviduct lags behind in 246 EMBRYOLOGY ()F THE LOWER VERTEBRATES CH.

development from about the eighth day of incubation; it never opens into the cloaca, and it persists in the adult as a functionless vestige.

There is considerable probability that the genital p0reS——paired openings leading from the hinder end of the splanchnocoele directly into the urinogenital sinus (Cyclostomata) and through which the gametes pass out——are to be looked on as Miillerian ducts in the last stage of reduction, the whole duct having disappeared except its hinder opening. Whether there is any evidence bearing on this in their ontogeny is not yet known.

The Mullerian duet goes through the early stages of development in the male as well in the female. It usually however never opens into the cloaca and it soon becomes reduced to a vestige. This may persist to a greater or less extent as an individual variation or as a normal characteristic, 12.1]. in the male El-asmobranch or Lung-fish well-marked vestiges remain in the adult, and so, still more markedly, in some of the Aniphibia such as the Bufonidae and some of the Gymnophiona.

OI’ISTIIONE1’liROS.——-Here again l3rauer’s excellent account of the development in II”:/pogeoplmls (1902) may be taken as a basis of our description. The opisthonephros in this amphibian is composed of segmentally arranged units extending from segment 24 to segment 100. Each unit is identical in composition with those of the pronephros, consisting of a tubule and a chamber (Malpighian body) containing a glomerulus and communicating with the splanchnocoele by a peritoneal canal. As in the case of the pronephros, each unit arises in development from the nephrotome or protovertebral stalk, the tubule rudiment being in the form of a diverticulum of the lateral or somatic wall of the nephrotome, the blind end of which comes in contact and fuses with the wall of the duct secondarily. Again as in the case of the pronephros, the nephrotome becomes completely separated from the myotome. It also becomes constricted off from the splanclmocoelic mesoderm, incompletely in some cases, a narrow communication——the peritoneal canal ~-- remaining open between the nephrocoele and the splanchnocoele, but more usually completely. In this latter event a new peritoneal canal is developed secondarily in place of that which has been obliterated, a diverticulum growing out from the wall of the nephrotome which meets and fuses with the splanchnoeoelic epithelium.

.There are differences in detail between the development of pronephros and opisthonephros, e.g. the tubule rudiment makes its appearance relatively later in the case of the latter—at a period after the nephrotome has become constricted off from the splanchnocoelic mesoderm. A further difference lies in the fact that there takes place in the opisthonephros a great increase in the number of its tubules——secondary, tertiary, etc. tubules being added to those of the original series. These arise in characteristic‘fashion. An outgrowth arises from the posteromedian portion of the nephrotome and Iv OPISTHONEPHROS 247

becomes constricted off as a small round vesicle with thick wall, composed of tall epithelial cells, and a small lumen. This is a secondary nephrotome. It remains for a time without change but eventually behaves very much as the original (primary) nephrotome, one wall becoming invaginated to form a glomerulus, and pocket-like outgrowths giving rise, one to a tubule rudiment, the other to a peritoneal canal. An important difference in detail is seen in the behaviour of the duct, which sends out a tubular projection of con- siderable length to meet the secondary tubule. This outgrowth arises from the duct some distance behind the point where the primary tubule opens into it.

The secondary nephrotome in turn buds off a tertiary nephrotome which again behaves as before and its tubule is met by a projection from near the tip of the outgrowth of the duct which has already developed in relation to the secondary nephrotome. Consequently secondary and tertiary tubules open into the arehinephric duct by a common collecting-tube formed of this outgrowth.

Apparently new generations. of subsequent nephrotomes may go on being formed in a similar fashion each from the preceding one until there may be as many as eight in a single segment, all of them, except the primary, opening into a common collecting-tube.

The degree of development reached by the opisthonephric units is different in different parts of its length. They attain full develop- ment in the manner above described in the region of segments 50- 100. In the region in front of this (segts. 30-50) the secondary nephrotomes and their derivatives never become functional and their rudiments degenerate. Still further forward (segts. 24-29) even the primary units as a rule degenerate without completing their develop- ment.

Apart from differences in detail it is clear that the primary units of the opisthonephros present the most striking resemblance to those of the pronephros and the evidence that they are serially homologous seems convincing.

Normally there is a gap of a few segments between the hind end of the pronephros (segt. 15) and the front end of the opisthonephros (segt. 24) but Brauer found that even in these segments there makes its appearance a distinct nephrotome, with the vestige of a glomerulus, although it does not proceed with its development. Consequently the units of pronephros and opisthonephros (primary) are to be regarded as members of a once continuous series. That this series once extended back beyond the present limits of the opisthonephros is indicated by the fact that distinct nephrotomes are present in segments 101-104, but as was the case in the intermediate zone between pronephros and opisthonephros these do not proceed to develop tubules.

ELAsMoBRANCHII.—-—It will be convenient now to consider shortly the development of the opisthonephros in the Elasmobranch fishes as they have provided the material for a large proportion of the most 248 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

important work dealing with the morphology of the Vertebrate kidney. It was in the opisthonepbros of Elasmobranchs that Sedgwick (1880) made his classical discovery—- which forms the foundation on which our presentéday knowledge rests-—that the nephridial tube of the Vertebrate is a development of the coelomic Wall, of that part of it which we now call nephrotome or proto- vertebral stalk. Since the date of Sedgwick’s work the op1stho- nephros of Elasmobranchs has formed the subject of detailed studies by Riickert, Babl, van Wijhe, and other well-known investigators: Owing to the lower end of the myotome in these fishes becoming displaced in a lateral direction, through the accumulation of mesen- chyme between it and the mesial plane, the protovertebral stalk becomes rotated outwards so as to assume a nearly horizontal position (Fig. 13-5-I, A, rat), the originally dorsal end of the stalk becoming now

0. Rab], 1896) ; B, variation observed in ’I'o'rpedo (after Hiiekert, 1888).

A, dorsal no1't;n.; a.n.d, :u-iuhineplnic duet: act, ect.od¢'.-r-m; my, n'l)'Ol30lllI'!: wt, nephrotmm-; splc, splunclnlocoele; t, tubule rudiment.

external, and the originally external side coming to be ventral. The duct (a.n.d) thus comes to lie ventral to the nephrotomes instead of being on their outer side as was the case originally. The nephro- tomes become isolated from the myotomes by their ends next the myotomes breaking up into mesenchyme. The result is that the nephrotomes now form a series of blindly ending pocket-like pro- jections of the coelomic epithelium which curve outwards dorsal to the duct.

Each pocket has an epithelial wall and it is noticeable that the somatic portion of the Wall is markedly thicker than the splanchnic, the cells of the former being taller and more columnar in shape. As development goes on it is found that the thicker more columnar celled portion of the wall of the pocket extends for some distance on to its dorsal wall, and this is interpreted by Riickert and Rab] as meaning that the somatic epithelium is spreading inwards towards the mesial plane, replacing splanchnic epithelium as it does so. In View of what we know regarding the development of other groups it seems more reasonable to explain the appearance as being expressive Iv OPISTHONEPHROS 249

of outward growth on the part of the somatic wall of the nephro- tome——-the terminal portion becoming the tubule rudiment. Riickert (1888) figures a remarkably interesting variation which he came across in Torpedo in which the separation of nephrotome from myo- tome had been delayed. In this (Fig. 134, B) the nephrotome still forms a distinct stalk continuous with the myoteme and the tubule rudiment is visible as a pocket-like projection of its somatic wall, agreeing exactly with the assumcdly primitive type of tubule rudi- ment as it occurs in the pronephros of one of the lower holoblastic Vertebrates. To correlate this specimen with the normal condition all that is necessary is to imagine the portion of the stalk next the myotome to have disappeared by becoming resolved into mesenchyme. The rest of the stalk together with the tubule rudiment would then remain as a curved blindly ending pocket‘ the tip of which would represent the tip of the tubule rudiment. This curved pocket-like structure increases in length, its tip comes into contact with, and later fuses with, the dorsal wall of the- duct and it is in this way converted into a short tube opening at its inner end into the splanchnocoele and at its outer into the duct. The tubular structure so arising does not retain its simple tubular shape but undergoes the series of changes shown in Fig. 135. Its cavity dilates in the middle to form the definitive nephrocoele, the cavity of the Mal- pighian body (mb); its splanchnocoelic end becomes relatively narrow to form the peritoneal canal (p.c)9: its outer end becomes also relatively narrow and it is this outer portion which undergoes an immense increase in length and becomes the functional tubule.

Opisthonephric rudiments appear in the fashion above indicated throughout the greater part of the length of the body where the splanchnocoele is present. They commence behind the pronephros (about the 8th or 9th segment) and extend back to the cloaca or a few segments posterior to it. In the latter case the postcloacal rudiments do not come to anything. Their occurrence is to be looked on as a reminiscence of a period when the alimentary canal and splanchuocoele extended farther tailwards. It is to be noted also that the group of tubules at the front end which subserve a genital function in ‘the male similarly appear only as transient rudiments in the female.

The portions of the opisthonephros which perform an active renal function increase much in bulk and this, as elsewhere, is brought about not merely by the great increase in length of the individual tubules, but also by the addition of numerous new tubules, each with its Malpighian body etc., of the second, third and so on, order. Probably (Balfour, 1878) these arise by a process of budding of the nephrotomes of a similar type to that which occurs in Hypo-

1 Care should be taken to avoid the not uncommon error of referring to the whole of this structure as the “ tubule-rudiment.”

’ Attention has already been drawn (p. 227) to the undesirability of applying the misleading adjective “ ncphrostomal ” to this canal. 250 EMBRYOLOGY OF THE LOWER VERTEBRATES CH‘. geoph/is though in the case of the Elasmobranchs the details seem to be more obscure and the descriptions are conflicting.

The male Elasmobranch is an excellent example of a Vertebrate in which the nephridial system is responsible for the function of conveying to the exterior both the renal excretory materials and the reproductive cells and we find a well-marked tendency to separate


A f l _. \ H B /, Pf -J Inc. M ‘pr C ’ m 5


Pf-J

Flo. 135. -——Illustrating the later development of a segmental unit of the opisthonephros in male Pristmrus. 'l‘he ligure is in each case a view from the mesial side.

(After C. Rab], 1896.)

A, 15th unit of 17 mm. embryo; 1!, 15th unit of 2225 mm.; C, 15th unit of 25-3 nun. ; 1), 25th unit from same embryo as C. m.b, Malpigluau body; p.c, peritoneal canal; p.f, peritoneal funnel; t, tubule.

each opisthonephric tubule leads has already

the routes of those two products to- wards the exterior. This separation is brought about by the shifting back- wards of the openings of the collecting- tubes of the posterior, purely renal, part of the opisthonephros, so that instead of being spaced out along the course -of the duct they come to be coincident with its opening into the urinogenital sinus. This backward shifting is most pronounced, and it also makes its appearance earliest in ontogeny, in the most anterior of the tubules in question. It is accompanied by a fusion together of the terminal parts of the collecting-tubes into a continuous ridge-like projection of the dorsal wall of the duct, in which the individual lumina are for a time greatly reduced or even completely obliterated. Eventually, as a rule, the ridge splits up and the terminal parts of the collecting-tubes regain their indi- viduality forming a group of distinct tubes, varying in number in different forms from about 4 (Spmax) to about 15 (Acant/w'a,s), and converging so as to open close together into the urine- genital sinus. In some cases the split- ting apart is not ‘complete and more or fewer of the tubes may be united to- gether to form a longitudinal “ureter.”

The mode of origin of the Mal- pighian body—— the definitive condi- tion of the nephrotome ——- from which been indicated. It

is for a time rounded in form (Fig. 135) but eventually one portion of its wal1—varying greatly in position———comes to bulge inwards to form the glomerulus containing a loop of blood-vessel.

The peritoneal canal during development lengthens out consider-

ably (Fig. 135, D) and becomes narrower.

This narrowing is most

marked in the posterior third of the opisthonephros and in this we Iv OPISTHONEPHROS 251

see what is probably the expression of a general tendency for the portion of coelome containing the glomerulus to become more and more completely isolated from the main splanchnocoele as the renal unit becomes more and more highly evolved. Eventually, in the adult of the majority of Elasmobranchs, the peritoneal canal becomes completely obliterated, but in a considerable number of others 1 this happens, if at all, only towards the anterior and posterior ends of the opisthonephros so that the greater part of the organ retains open peritoneal funnels throughout life. Bles (1897) has made the interesting suggestion that there is a physiological correlation between the persistence of open peritoneal funnels and the absence of abdominal pores —— secondary perforations of the wall of the splanchnocoelc in the neighbourhood of the anus which make their appearance, at a late period of development, in various Elasmobranchs and other Vertebrates.

URODELA.——The third type of development of the opisthonephros amongst the more primitive Vertebrates is found in the Amphibians, especially in the Urodeles. The excellent account given by Fur- bringer (1877) still forms a thoroughly adequate basis for the description.

The Amphibians possess, as has already been shown, a large and highly developed pronephros amply suflicient for their excretory needs during early periods of development. In correlation with this there is marked delay in the development of the opisthonephros, the myotomes having already become separated and their stalks or nephrotomes breaking up into mesenchyme before the opisthonephric units make their appearance. The rudiments of these units-—-the ne];hretomes——become reconstituted in the midst of the mesenchyme as solid cellular strands which may retain their metameric arrange- ment (Amplmluma —— Field, 1891; anterior segments in Trrvlton, Amblg/stoma, etc.) but usually have completely lost it. Each of these nephrotome rudiments is a solid strand of cells which curves out- wards dorsal to the duct. In the anterior region where, as is specially clear in ’l’mIt_on, the inner end of the strand is for a time continuous with the lining of the splanclmocoele, the general arrangement is clearly the same as that of the Elasmobranch (cf. Fig. 136, A, with Fig. 134). The splanchnocoelic end of the nephrotome disappears for a time while the main portion develops a cavity in its interior and becomes converted into a vesicle with epithelial wall lying immediately dorsal to the duct (Fig. 136, B). This vesicle becomes elongated in a mediolateral direction (? by active growth of its outer wall) and then assumes a characteristic curvature first f\- and then in-like in shape (Fig. 136, C). The mesial end of the un gives rise to the Malpighian body, the remainder to the actual tubule, its outer end undergoing fusion with the wall of the duct (Fig. 136, D). The tubule grows rapidly in length and is forced into complicated

1 E.g. Cestracion philippi, Rlwina squatina, Scyllvium. camZcula,S. stellarr, {’r1Istiu'r'us mclcmostomus,‘ Spinax miger, Acanthiws vulgaris, Scymmos lwlclmla.-—(Bles, 1891). 252 EMBRYOLOGY OF THE LOVVER VERTEBRATES

CH.

coils and windings as it does so (Fig. 136, E) while the Malpighian body dilates and its dorsal wall becomes invaginatcd to form the glomerulus. _

As a rule the primitive con- tinuity of nephrotoine with the splanchnocoelic lining disappears in the Amphibian as already i11di- cated, but it becomes re-established by a peritoneal canal developing secondarily (Fig. 136, D) as an out- growth, arising in Urodcles usually from the neck of the Mialpigliiaii body and in .Anura from a point farther down the apparent tubule, which grows towards and fuses with a thickening of the coelomic epithelium. Such displacements of the communication between Mal- pighian coelome and splanchnocoele are probably of a similar nature to those mentioned in the case of the pronephros of H3/pogeoplz/is (see p. 226 . - o

)In those parts of the opistho- nephros which are actively renal in function, z'.e. the hinder portion in Urodeles and the greater part of the whole length in _Anura, there takes place great increase in bulk, associated with the development of generations of subsequent tubules. Such secondary, tertiary, etc. tubules make their appearance amongst the mesenchyme in the form. of cellular strands which resemble closely— both in their original appearance and in the series of

Flu. 136.——'.l‘ransv¢-rse st-ut.ioi1s showing changes pass th_r0ugh various" Stilgcs in the «levelopment of ———those from which the primary

the opistlionepbros. 1877)

A, Triton alpestris: B, 14 mm. ; C, D, Salamandra maculata, 17 mm. ; E, Salamandra mxwulata, 21 mm. ; F, Sa.la.ma/ndm mac-ulata, 25 mm. A, dorsal aorta; a..n.d, atchirlfipllric duct: 9. gonad: gl, glomerulus: n, nephrotome; nc, nephrocoele; p.c, rudiment of peritoneal canal; splc, splanchnocoele; t. tubule; tl $2, I3, primary, secondary, and tertiary tubule rudiments.

(After Fiirbringer,

Salmmndm maculata,

elements arise (Fig. 136, F, £3). Eventually the secondary tubule comes to open into the primary tubule, the terminal section of which thus forms a collecting-tube com- mon to both, while the tertiary tubule similarly comes to open into Iv OPISTHONEPHROS 253

the secondary. As the various generations of tubules go on with their development, undergoing the same histological differentiation and increasing enormously in length, they become inextricably mixed up together to form the compact fully developed opisthonephros of the adult.

Eventua.lly, in the Urodele, the duct is slightly displaced out- wards so as to leave a distinct gap between it and the opisthonephros across which pass the terminal parts of the collecting-tubes. In the male Urodele the openings of these become, as a rule, shifted back- wards to the hind end of the duct as in Elasmobranchs.

The Amphibia alone among tetrapod Vertebrates retain the relatively primitive feature of possessing open peritoneal funnels in the adult, and they can be excellently demonstrated with their actively moving flagella by examining the slender anterior portion of the excised and still living kidney of a female Urodele in normal salt solution under the microscope. In the anterior genital portion of the opisthonephros of the male they as a general rule (not in Spelerpcs, Spengel) remain however obliterated. _

In the Anura (N ussbaum, 1886) the peritoneal canals at an early stage of larval life lose their connexion with the Malpighian body or tubule and establish a secondary connexion with the blood spaces between the tubules, thus affording a route by which the fluid in the splanchnococle is returned to the blood, analogous to that provided by the lymphatic system in higher Vertebrates.

AMN1OTA.———In the Amniota the opisthonephros of the Fishes and Amphibians is represented by the mesoncphros and metanephros-— and it will be convenient to consider the mesonephros first.

MESONEPHROS OF BIRDS:--AS has already been pointed out one of the marked differences between Amplmloams and the Craniata is that in the latter segmentation is no longer apparent at any stage of development in the ventral or splanehnoeoelic region of the meso- dcrm. The Amniota show a further accentuation of this difference inasmuch as the loss of mesodermal segmentation has extended so far towards the dorsal side as to involve the region of the nephro- tomes. In the early embryo of the bird the nephrotomie part of the mesoderm has the form of an unsegmentcd mass—the intermediate cell-ma.ss——showing more or less distinct traces of being composed of a somatic and a splanchnic layer continuous with the correspond- ing layers of the splanehnoeoelic mesoderm and of the myotome. Although the intermediate cell-mass no longer consists of discrete nephrotomes, traces of its primitive segmental nature persist in its connexions with the segmentally arranged myotomes and in the fact that its connexion with the lateral mesoderm is not continuous in a longitudinal direction.

As regards the mode of origin of the actual mesonephric units differences exist, as was shown long ago by Sedgwick (1880), which are of much interest owing to the fact that the less modified mode of development found at the front end of the series is readily correlated 254 EMBRYOLOGY OF THE LOWER VER'l‘EBRATES OH.

with that which is found in the Anamnia, while the more highly modified mode of development occurring posteriorly is equally readily correlated with what happens in the Inetanephros of the Amniota. In the a.nterior region (approximately segments 12-15) the inter- mediate cell-mass is compact, rccognizably two layered, and the split which separates the two layers may he obviously continuous with the splanchnoeoele (Fig. 1317, A). It separates at an early stage from the myotome, but it remains continuous at intervals with the lateral

Flu. 137.——Sections illustrating the development of the xnesonephros in Birds. . (A and B, after Sedgwick, 1881 ; C, D, E, after Schreiner, 1902).

A, 22-segment chick at level of the 15th segment; B, 34-segment r_-hick at level of 13th or 14th segment (combined from two sections); 0, 45-segment duck at level or :2‘.Hh so-;;u1~-rut; I), 45-sogrm-.nt duck at level of 25th segment; E, 45-segment duck at level of 24th :-u-_-,-nu-nt. .—l, dorsal aorta; u..'u-.d, archinephrieduct; glam, glomerulus; no, nephrocoele; nr, nu-pin-otmm-.; pm, pm-itonuul (::u'1ul; p.c.-v, posterior cardinal vein; splc, splanchnoeoele; t, tubule rudiment.

mesoderm. The intermediate cell-mass becomes closely apposed to and very soon directly continuous with the duct by a narrow isthmus in each segment——the tubule rudiment (Fig. 137, A). Ventrally, 7J.e. near its junction with the splanchnoeoele, the split between the two layers of the nephrotome dilates and forms a definite nephrocoele which opens into the splanchnoeoele by a wide peritoneal canal (Fig. 137, B, 19.0). The tubule rudiment develops a lumen leading from nephrocoele into duct} and the dorsal wall of the nephrocoele becomes

1 The opisthonephric duct in the Anmiota is known as the mesonephric or Wolfian duct as its function 13 restricted to draining the mesonephros or “ Wolffian body.” IV _ MESONEPHROS 255

invaginated into the cavity to form a glomerulus (glam) which may become much enlarged so as to extend right out into the splanch- nocoele.

As the process of development is traced back into the second region of the mesonephros (stretching approximately from segment 16 to 19 or 20) a distinct modification becomes apparent. The inter- mediate cell-mass in this region becomes loosened out into mesen- chyme, and amongst this loose tissue what may be termed the definitive nephrotomes make their appearance secondarily in the form of roundish condensations of cell elements. Each of these becomes more and more sharply marked oil from the surrounding mesenchyme, its cells assume a radial arrangement, and presently a small rounded cavity appears in the centre. This cavity dilates and the result is a hollow vesicle with a wall composed of a single layer of cells————the definitive nephrotome.

In the third, hinder, region of the mesonephros, extending from about segment 20 or 21 backwards, the -process in the Fowl, though not in the Duck, has undergone the further modification that the intermediate cell-mass is from an early period completely isolated from the peritoneal epithelium. The peritoneal canals have here completely disappeared, except for faint vestiges, the cells of the peritoneal epithelium still showing here and there traces of the same arrangement as they have further forwards where they are passing into a peritoneal canal. Apart from this separation from the peritoneal lining, the process is similar to that already described. Here also the intermediate cell-mass becomes separated out into loose mesenchyme in which the definitive nephrotomes make their appearance secondarily.

An important feature in the above described processes of develop- ment is the obliteration of the primitive segmentation of the nephro- tome region. When the definitive nephrotomes become visible, and so bring into view the metameric segmentation of the mesonephros, a further modification becomes apparent in that the mesonephric segments, except towards the front end of the series, are more crowded together than are the_ primitive mesoderm segments as represented by the myotomes (Sedgwick, 1880). Thus in the Duck Schreiner (1902) found in the region of myotome XX, 4 or 5 meso- nephric rudiments, in that of myotome XXV-7, in that of XXVI -—-9, in that of XXVII as many as 13.

As development proceeds, the mesonephric elements become still more crowded together inasmuch as from segment 21 or 22 back- wards “ subsequent” nephrotomes make their appearance in the mesenchyme. These closely resemble in appearance the primary nephrotomes, with which they are at first in close proximity if not in actual continuity, and they develop in succession one over the other, each series forming a vertical row over its primary nephrotome. The number of subsequent tubules is greatest posteriorly where there are commonly four to a segment. 256 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The later development. of the individual nephrotome of the meso- nephros takes place in the Birds along lines exactly similar to what takes place in lower forms such as the Amphibia. The tubule rudiment originates as an, at first solid but later pocket-like, outgrowth of the lateral wall of the nephrotome (Fig. 137, C, t). The tip of this presses against the mesial wall of the duct and, as the tubule grows in length, fusion takes place and the lumina of duct, tubule rudiment and nephrotome—which together form a characteristic m-shaped structure as seen in a transverse secti0n——bec0me continuous (Fig. 137, D and E). The portion of the m nearest the mesial plane represents the nephrotome in the strict sense, tie. the forerunner of the Malpighian body, and has assumed a watch-glass shape, its dorsal wall being involuted into the cavity as the rudiment of the glornerulus (glam).

The further development of the mesonephric unit, Which need not be followed out in detail in this book, consists in (1) the immense growth in length of the tubule, which leads to its becoming in- extricably intertwined with its neighbours, (2) the histological dill'erentiation of its wall, and (:5) the differentiation of the Malpighian bod .

lt should be mentioned that where the tubules are much crowded together they do not all establish a communication with the duct in the typical manner above described. Some, even of the primary tubules, come to open into neighbouring tubules. In the ease of the subsequent tubules, some open into the duct in the typical fashion, others open into neighbouring tubules, while the majority become con- nected with pocket-like outgrowths from the duct. These outgrowths are greatly developed in some birds (of. Duck, Fig. 138, B), becoming much elongated and taking the form of branched collecting-tubes into each of which open a series of subsequent tubules (cf. H3/pogeoplmls), the whole condition distinctly foreshadowing arrangements presently to he mentioned in the metanephros.

The mesonephros acts as the renal organ only for a short period during the early stages of development. In the Fowl it begins to develop about the end of the second day of incubation, it reaches its maximum about the 7th or 8th day, and almost immediately thereafter begins to show signs of degeneration as the renal func- tion becomes concentrated in the metanephros. The mesonephros never completely disappears though it ceases to be of any importance as a renal organ: its persistence is correlated with the fact that this portion of the opisthonephros has already in the forerunners of the Amniota important functions connected with reproduction. Its modification in relation to these functions will be gone into later.

METANEPHROS OF BIRDS. —- The continuous mass of mesen- chymatous tissue representing the nephrotomes or protovertebral stalks does not cease at the binder limit of the mesonephros at segment XXX: it is continued on through segments XXXI, XXXII, Iv METANEPHROS 257

XXXIII. and XXXIV to the level of the opening of the duct into the cloaca. The nephrotomal tissue in the segments mentioned remai'ns for a time passive but eventually it gives rise to the definitive nephrotomes of the metanephros. The metanephros is therefore ontogenetically as was indicated long ago by Sedgwick (1880) in its origin simply a tailward continuation of, the niesonephros. In the terminology used in this hook it consists of the greatly enlarged posterior segment or segments of the opisthonepliros. The develop- ment of the metanephros is inaugurated by the appearance of the rudiment of the

ureter or meta— \ai\82l8-9l.u/ nephric duct. This

arises as an out-

growth (Fig. 138, H,

M’) from the dorsal

wall of the nieso-

nephric duct near , its posterior end.

The outgrowth ex-

tends in a dorsal

direction and then

spreads out at its _. tip, pi‘ojecting very mn

slightly .ta1lwards 95 ' M I 2, | M l 9, , ,0 , ,, 1 ,, . ,3 i but growing much more actively in a headward direction along the outer side of the hinder or n,leba'n0Phrlc POI.‘ Fm. 138.-—ReL-onstructed outlines of hind end of mesoneplitic 171011 Of the n91)l1r0* duct and ureter in Bird embryos as seen from the left side. tomal mesenchyme. (After S<="I'ei"*=r» 1902-)

latter becolnes A, duck embryo with 48 segiiieiits; B, duck eHll)I‘)() with 50 st-5.-,iin-nts;

secondarily (about 0, duck embryo, lO'75 nini.; I), fowl viubi-_\o, l3'.’» mm. )Il.’It.,l1N‘SO-. _ _ in-phros; W7‘, ureter. 'llie Arabic iiiinierals iiidicate the position ot the end 0t the the nlPSO(l6I‘m segnii-nts.

day) marked off’ by

a distinct break from the mesonephric portion. About the same time the dorsal wall of the actively growing ureter begins to develop pocket-like outgrowths (Fig. 138, D). These increase in length, branch repeatedly, especially the hinder ones, and become collecting-tubes. As this takes place the nephrotomal mesenchyme becomes condensed into small portions, one of which ensheaths the growing tip of each branch of the collecting-tubes. In these terminal caps of mesenchyme definitive nephrotonies gradually come into view, similar to those of the mesonepliros. In other words the definitive nephrotome is at first a mere rounded cellular mass. This develops a lumen and, as the latter dilates, assumes a vesicular form, and finally the actual tubule makes its

VOL. II S 258 EMBRYOLOGY OF THE LOWER VERTEBRATES GIL.

appearance as an outgrowth which fuses secondarily with the tip of the collecting tube. The Malpighian bodies begin about the ninth day to develop their special characteristics in a manner similar to those of the mesonephros. An important point to notice is that the metanephros differentiates from behind forwards instead of in the opposite direction as does the mesonephros.

About 24 hours (the exact time varies greatly) after the first appearance of the ureter the part of the mesonephric duct between it and the cloaea becomes incorporated in the cloaca so that inesc- nepbric duct and ureter come to have independent openings into the oloacal cavity.

As the metanephros goes on with its development it comes to be situated in great part dorsal to the Inesonephros but it will be under- stood that this topographical relationship is secondary. At first it is completely posterior to the mesonephros. Even the ureter is in its lirst stage localized about segment X XXIV (Fig. 138, 'B) and its extension forward as far as segment XXV or even farther is purely secondary. It will be noticed in Fig. 138 how exactly the ureter in its first beginnings resembles one of the pocket-like outgrowths of the duct which in the mesonephric region develop into collecting-tubes, and it seems scarcely possible to avoid the conclusion that the meta- nephros of the Fowl is simply the enormously hypertrophied nephridial apparatus of a single segment, the ureter being a greatly elongated collecting-tube with an immense number of subsequent tubules opening into it.

OPISTIIONEPIIIZOS IN OTHER GROUPS OF VER'l‘EBRA'l‘ES

CROSSOPTERYGII.—-—0ur knowledge of the early stages of develop- ment is still fragmentary being based upon three specimens of I-’ol3/pterus (stages 32, 33 and 36) obtained by Budgett (Graham Kerr, 1907).

In the youngest of these stages a number of the opisthonephric units have made their appearance in the form of rounded cell masses arranged segmentally in the mesenchyme ventral to the myetomes, those which are best developed possessing a distinct lumen.

In the specimen of stage 33 these rudiments have become elongated forming thick curved masses, one end of which is closely applied to, or even fused with, the dorsal wall of the duct. The lumen is restricted to the end farthest from the duct, which represents the definitive nephrotome, while the part which extends towards the duct-—the tubule rudiment——is solid.

In the 30-mm. larva described by Budgett (1902) the opistho- nephros commences about 4 segments behind the pronephros and stretches through about 39 segments with from two to five Mal- pighian bodies and tubules in each segment. On the right side of the body 18 of the Malpighian bodies——rou_ghly one to each segment -—communicated with the splanchnocoele by a nearly straight peri- 1V OPISTHONEPHRQS 259

toneal canal. The earlier developmental material does not suffice to show definitely whether or not, as is probable, this peritoneal canal is a secondary connexion with the peritoneal epithelium. The peritoneal funnels exist only for a time during larval life : in specimens 90 mm. in length they had disappeared. In 0a.lr(.7mIchth3/s (Lebedin- sky, l895) the peritoneal canals have been found still persisting in a larva of 15 cm.

AUTINOPTERYGIAN GANon>s.-—-Here again the definitive nephro- tomes appear a.s solid masses of cells arranged segmentally. The gap separating them from the pronephros is in the more primitive Sturgeons about 3 or 4 segments, in the more highly evolved Amia 16 or 17 (J ungersen, 18923-4). 'l‘he rudiment grows in length, develops a lumen secondarily, joins on to the duct by its lateral end while its mesial end dilates to form the Mzilpigliiaii body——all in the usual fashion. At a late period—after the Malpighian bodies have already assumed their characteristic features-——they develop peritoneal canals as outgrowths from their walls which meet and fuse with the peritoneal epithelium secondarily. Later on the peritoneal canals become again obliterated and appear to be absent in the adult except in the case of Amia.

TEI.E()S'[‘EI.——I11 the Teleostean fishes, as is indeed the case to a certain extent in all the members of the Teleostomi, the 0pistho- nephros is delayed in development in correlation with the prolonged functioning of the pronephros. According to Felix (1897) in the Trout the first opisthonephric units or definitive nephrotoines begin to make their appearance about '70 days after fertilization as rounded clumps of cells, in the centre of which a small lumen appears. These lie immediately dorsal to the duct, in the connective tissue trabeculae which at this stage of development traverse the cavity of the interrenal vein. These rudiments appear lirst about the middle third of the duct and gradually spread backwards, those in front being segmental in position while those farther back are no longer segmental and fuse together into irregular masses. Each rudiment grows actively in length and goes through the usual series of changes before joining up to the duct.

To the primary units just described are added secondary and tertiary units. These develop exactly as do the primary except that in the case of the tertiary set the tubule may fuse either with the duct directly or with an already developed tubule.

As the tubules increase enormously in length they become inex- tricably entangled together extending even across the median plane so that the substance of the two kidneys becomes continuous through the interrenal trabeculae. It is further characteristic of the tele- ostean kidney that there takes place in it a great development of round-celled pseudolymphoid (Felix) tissue. This forms a packing tissue between the tubules and appears to be formed by proliferation from the walls of the interrenal venous spaces.

The opisthonephros extends back for a short distance behind the 260 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

cloaca. This postcloacal portion drains into a pocket-like prolonga- tion which grows back from the duct, usually on the right side only.

A remarkable peculiarity has been observed in certain Teleosts (I3e11a.(log(1ste7-, (fruitcl, 1901) in which, correlated with the persist- ence of the enlarged glemcrulus of the pronephrcs, the Malpighian bodies of the opisthcnephros have, at least in the adult, completely disappeared.

l)IPNo1.—ln the Lung-fishes the development of the opistho- nephros closely resembles that in Ampliihia. ln Leynldosi/ren and 1"r0to;ntm'u.s' the units appear as rounded, at first solid, masses inde- pendent alike of myotome and of peritoneal epithelium. In Promp- rrrus they commence about segment 14-18 but in some specimens they ll.]_|])('.:l.l‘ to be represented by slight eondensations of Incsenchyme right forwards as far as the hind limit of the pronephros. The rudi- ments extend hack to about segment 36 73.6. to about one segment in front of the cloaca. They are roughly segmental in position and remain so during the greater part of larval life. Each rudiment gives rise to a typical Malpighian body and a tubule which joins on to the duct secondarily. There does not appear to be any trace of peritoneal canals developed although they are for a time present in Ccmtorlus.

The development of the primary units is followed by the develop- ment of subsequent ones but the origin of these has not so far been worked out.

In Protoptm-w.s, though not in I167)/I:I]0r5‘?lT67l/, the hinder ends of the kidneys become continuous across the mesial plane and this fused portion becomes gradually marked off conspicuously by its pale colour the cortical region of the paired kidneys becoming crowded with amocbocytes laden with melanin which settle down there and give it a coal-black appearance.

REP'I'ILIA.--In the Reptiles we find, as we should expect, that the process of development follows upon the whole the same lines as in Birds but at the same time shows various features in which the con- dition remains more primitive. Thus in Lacnrta Schreiner (1902) finds that, except in the hinder portion of the opisthonephros, the units arise directly from typical nephrotomes or protovertebral stalks. These become isolated from the peritoneal mesoderm and then from the myotoine. Each develops a lumen and becomes vesicular and its‘ lateral wall gives rise to an outgrowth which becomes the tubule rudiment and fuses with the duct. N o peri- toneal canals are developed, though vestiges of these may appear——a vestigial peritoneal funnel appearing as a slight projection from the splanchnocoele into the ventral end of the nephrotome (Lacerta), or the latter remaining for a time connected with the peritoneal lining by a solid stalk representing the peritoneal canal (Angwis).

In the posterior segments the nephrotomes are no longer distinct: they form a continuous mass of mesenchyme stretching uninterruptedly from segment to segment. In this, cellular conden- IV RENAL ORGANS 261

sations occur which give rise to definitive nephrotomes and these also are no longer strictly segmental, there being about 2 to each segment from segment 25 to 3 .

The definitive nephrotomes pursue the normal course of develop- ment. The first to appear are towards the ventral edge of the nephrotomal tissue but later other subsequent units appear in succession more dorsally. .

GENERAL l\1()R1’I{()LO(}Y or THE RENAL (_)acANs or VEm‘E— RRA'1‘ES.——'l‘l1e main problem connected with the morphology of the renal organs is that which deals with the serial homology of its con- stituent elements. Lankester (1877) clearly implied this homology when defining his terms archinephros etc. while, looking at the natter from a more strictly embryologieal standpoint, Sedgwiek

l88l) formulated the View that pronephros, mesonephros and meta-

iephros are simply successive portions of a single elongated ancestral excretory organ possessing a d11ct and segipentally arranged, serially iomelogous, tubules.

In discussing this archinephros theory it is necessary to bear in nind the following points :-—

(1) The names pronephros, mesonephros and metanephros accord- ng to their original definitions signify three sets of renal structures 'orming a succession along the length of the body in a tailward lirection :-—(a) an anterior or headward Set, (b) a middle set and (c) l. posterior or tailward set respectively. It is inadmissible by the

erms of the original definition to use them in any other sense and

.0 do so is bound to lead to confusion.

(2) In addition to the anteroposterior series of renal units there nay develop a sequence of elements within the same body segment-- 5.6. the development of the primary unit may be followed by the groduction of a series of subsequent un-its, secondary, tertiary, luaternary and so on, probably derived originally from the primary lephrotome by (5. process of budding. The extent to which such subsequent units may develop differs greatly in different animals and 11 different segments. In the pronephric region there are commonly 10118, in the opisthonephros of Hy/pogeoplmls there may he as many as awenty in a segment, while it is possible that the metanephros of the Bird is to be looked on as a gigantic mass of subsequent tubules aelonging to a single segment.

It is obvious that in comparing renal elements of different parts )f the series care must be taken that the comparisons are made Jetween elements of the same order, and it is further obvious that a langer to be guarded against is involved in the theoretical possibility )f the suppression of the tubules of one order—say the primary ,ubules—in some particular region.

The comparison of mesonephros with pronephros involves then

hese two fundamental questions 

(1) Does the mesonephros contain a set of units of the same order is those of the pronephros ?—i.e. in this case p7'7Jmcw~y elements and 262 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

(2) Are these elements serially homologous throughout the length of pronephros and mesonephros?

Froni the facts of development as stated earlier in this chapter it is clear what the answer to these two questions must be. lt has been shown that in 11;:/pogenplm and other forms the first tubule to appear in each segment of the opisthonephros arises as a direct out- growth from the nephrotome exactly in the same way as the pro- nephrie tubule: it is clearly then a primary tubule, and its Mal- pighiau body, arising directly from the main part of the stalk, is also primary. The evidence then seems conclusive that in II;/po- /;z’n}2lz;-z3.~= the pronephric and opisthonephric tubules form a homologous series, and naturally if this is true of I[‘(/})U.(/e0I)]’I1.s it is, in all proba- bility, true of other Vertebrates.

Yet the View has been strongly advocated and is still held by many morphologists that there is no precise homology between the units which build up pronephros and opisthonephros. Riickert, van Wijhe, Field, Semen, Boveri, Felix, have been among the more important protagonists of this view. They have brought forward such arguments as the following :

(1) While the pronephric tubule arises as an outgrowth of somatic mesoderm, the mesonephric is derived partly from somatic and partly from splanchnic.

(2) The pronephric tubules arise relatively early and in continuity with the archincphric duct, the mesoncphric tubules arise much later and in discontinuity with the duct.

(3) The glomerulus of the pronephros is unsegmcnted and lies in the general splanehnocoele: that of the mesonephros is segmental and lies in a special chamber the cavity of the Malpighian body.

These arguments however do not appear any longer to have the weight which formerly attached to them.

(1) The evidence of Hypogeop/nfs that opisthonephric tubules arise as outgrowths of the somatic wall of the nephrotome just as do the pronephric tubules seems quite convincing.

(2) In II3/poge0p7m.'s all the pronephric tubules except the first three join up to the duct secondarily precisely as do the opistho- nephric tubules. Further the precocious completion of the archi- nephric duct is a physiological necessity, in view of the early function- ing of the pronephric tubules, and this in turn involves as a necessary consequence that the tubules behind those which first function become joined to it secondarily.

(3) The glomerulus of the pronephros is segmental and the pronephric chambers are also segmental at first in some of the more archaic forms and the unsegmented condition is purely secondary.

Another line of argument is directed not against the view that pronephros and mescnephros are built up of serially homologous units but rather against the strict homology of the functional parts of these units. Thus it is stated that in the’ region of the pronephros in addition to the main tubules there occur rudiments of other 1v RENAL ORGAN S 263

tubules which resemble more closely those of the mesonephros and similarly that in the region of the mesonephros, in addition to the ordinary tubules, there occur vestiges of another set of tubules resembling more closely those of the pronephros. Consequently, of the set of potential tubules (primary, secondary etc.) which is repeated in each segment, it is not the corresponding member which becomes the functional or main tubule in the pronephric and opisthonephric regions respectively. 'l‘o the present writer the various observations which have been brought to support this argument do not appear to be anything like so convincing as the very clear evidence afforded by Jly/pogeoplm: and he consequently holds that in the present state of our knowledge there is no adequate reason to refuse to accept the precise homology of the first-appearing (“primary”) tubules of the opisthonephros with those of the pronephros.

The idea of the primitive continuity between mesonephros and metanephros is less open to attack than that between the pronephros and the anterior (mesonephric) portion of ‘the opisthonephros. Apart from the evidence of embryology we find in various of the lower vertebrates (Elasmobranchs, Urodeles) an elongated opisthonephros in the adult which shows in the clearest possible manner an incipient stage in the differentiation of the organ, into an anterior genital region and a posterior renal region, of precisely the same kind as we believe to have taken place in the Anmiota.

Further we have seen that in actual ontogeny the tubules of mesonephros and metanephros arise from an at first perfectly con- tinuous mass of nephrotomal mesenchyme. As regards the minor problem whether one or more primary tubules still persist in the metanephros among its immense mass of subsequent tubules there is, as yet, no adequate evidence.

Accepting then the idea of the arehinephros as a sound theory of the primitive condition of the renal system of Vertebrates we may sketch out the probable course of the modifications which have come about in its development somewhat as follows.

Primitively its tubules developed——in accordance with the develop- ment of the body-segments generally—in regular sequence from before backwards.

The disappearance of segmentation in the ventral portion of the coelome enabled the early-formed tubules-——those towards the head end—to drain the whole length of the splanchnocoele. Correlated with this these tubules became greatly enlarged and their eflicieney greatly increased.

This high development of the anterior tubules to drain the whole splanchnocoele enabled them to cope with the entire excretory needs of the developing animal for a prolonged period and the tubules behind them in the series being unnecessary were either delayed in their appearance or ceased entirely to develop.

Thus a gap arose separating oil‘ the precociously developed tubules as the pronephros. Within the pronephros itself there was 264 EMBRYOLOGY OF THE LOVVER V ERTEBRATES CH.

a tendency for functional activity to become specially marked in certain tubules these becoming enlarged in comparison with the others. The increase in size of pronephrie tubules was accompanied by increase in the size of their glomeruli, which con- sequently came into contact and fused together.

As the pronephric tubules drained the whole splanchnocoele the peritoneal canal leading to their nephrocoeles became wider and wider until at last they ceased to be marked off from the rest of the splanclmocoelc.

The opisthoncphric tubules the renal functions being still fora time undertaken by the proncphros -developed in regular sequence from before backwards. With the acquisition of new outlets for lluid in the splalichnocoele, such as abdominal pores, or connexions with lymphatic or blood vessels, the peritoneal canals leading from it into the nephrococlcs (Malpighian bodies), in which the secretion of coelomic fluid was specially concentrated, became gradually reduced and finally disappeared, there being no longer any physio- logical need for them.

Witliiii the series of opisthonephric tubules, the excretory function became more and more concentrated in the segments nearest the cloacal opening. In these segments the opisthonephros increased in bulk owing to the specially active budding processes which gave rise to successive generations of subsequent (secondary, tertiary, quaternary and so on) tubules. ,

'l‘he final stage in this process was reached in the Birds, where renal activity ‘became concentrated in a single segment close to the cloaeal opening. In this segment an immense hypertrophy of the opisthonephric elements took place, successive generations of tubules being added on in front. Thus the opisthonephrie mass belonging to this segment came to extend hcadwards dorsal to the anterior portion of the opisthonephros (mesonephros) and became the definitive kidney or metanephros.

Olucm or THE N EPHRIDIAL DUcTs.—As already pointed out the nephridial tubes in eraniate Vertebrates open primitively into a longitudinal archinephric duet-the presence of this duct being the most conspicuous feature which differeritiates the renal system in Vertebrates from the presumably ancestral condition as exemplified by Annelids, where the tubules open separately upon the external surface.

Two possible ways in which this duct may have originated in evolution have already been indicated and it has also been indicated that on the whole the balance of probability seems to be in favour oi the view that it came into being through the backward shifting of the external opening of each tubule till it beca.me coincident with the next behind it.

Those who_ take this view usually assume that the archinephric duct originally opened posteriorly upon the outer surface of the body and that its opening became secondarily shifted into the Iv NEPHRIDIAL DUCTS 265

cloaca. lint as already pointed out there is no embryological support for t.his view. Everywhere the archinephric opening is at first within the endodermal part of the alimentary canal and this suggests that the communication of duct with cloaca has come about in some other way. The evidence of 1’oly/ptems suggests as already indicated that the opening into the cloaca represents the persistent primitive communication of a mesoderm segment with the enteron. It is quite conceivable that a secondary communication between archinophric duct and gut may have come about in this Way, in correlation with the pronephric -part of the archinephros reaching the actively functional condition at a period when the mesoderm segment at the level of the anus had not yet been completely separated from the endoderni. Once this secondary opening was estab- lished it would be a natural consequence for the post-anal portion of the nepliridial system to atrophy and disappear.

The hypothesis indicated in this descrip- tion derives the nephridial apparatus of the Vertehrata from an ancestral condition resembling that characteristic of Annelids —-—the main difference being that in the Vertebrates the nephridial tubes open into a longitudinal duct which at its hinder end communicates with the alimentary canal. It is of great interest then to find even within the rou of the Annelida clear expressions 0% the? tendency for the nephri- ‘Fm’ .13-9'-Diagmm of am

_ _ posterior end of the body of dlal l}l1b0S l';O open 1l'll)O such 3. duct. Allolobephom (I-n£i1;(tv.'. as seen best marked case of this known up to the “Om tl1°}‘°1‘-W -7‘i‘l° to SNOW present appears to be thatof the Earthworm the re1“h°m °i thc Mal

, _ organs (shown in black) All0l0b0])}L0?'a cmtzpae (leS(3I‘lb6(l R088. according to Rosa (1906).

(1906). Here (Fig. 139) in the posterior portion of the body the nephridial tubes lead into a longitudinal duct which fusing posteriorly with its fellow opens into the alimentary canal on its dorsal side and near its posterior end. In other words in this particular case an arrangement precisely like that of the vertebrate has been evolved out of an ancestral condition in which segmentally placed nephridial tubes npmuml independently upon the outer surface. s

In regard to theorigin of the typical metanephric duct or ureter as seen for example in a Bird there are two obvious possibilities. If the mctancphros represents a number of nephridial segments its special duct may have originated by such steps as are represented by the adult conditi_on in male Urodeles and male Elasmobranehs tie. by the openings of the collecting-tubesinto the original duct becoming displaced backwards. Or on the other hand if the metanephros 266 EMBRYOLOGY OF THE LOWER V ERTEBRATES CH.

represents the greatly enlarged tubule system of a single segment the ureter would probably have arisen simply by the enlargement of the collecting-tube of that segment. When one studies the facts of development as now known (see p. 258 and especially Fig. 138) the balance of probability appears to be decidedly in favour of the second of these hypotheses representing the method by which the ureter has actually arisen in phylogeny. '

TIIE (a‘r0NAD.——Thc great mass of the cells which constitute the body of a Vertebrate or any other of the higher Metazoa are specialized for the performance of particular functions in the ordinary life of the individual, and, correlated with this specialization, such cells have lost the power of giving rise to reproductive cells or gametes. The main mass of the body constituted of such specialized cells is known technically as the soma. At one or more points in the body there remain however patches of cells which have not uncle.1'gone this specialization for ordinary vital functions an d which retain the power of giving rise under favourable circumstances to gametes. The sum total of such cells con- stitute the gonad. The word gonad is commonly used in a loose sense as an equivalent of ovary or testis but it should be borne in mind that each of these organs contains a large proportion of immigrant tissues ——connective tissue, blood, nerves and so on——-which are strictly speaking part of the soma.

The problem of greatest general importance attaching to the development of the gonad of Vertebrates is that which concerns the origin of the.cells (gonocytes) which constitute it.

And the interest of this question rests especially on the fact that in certain invertebrates the germ-cells have been traced back to blastomeres specially set apart at early stages of segmentation. All the probabilities seem to indicate that such a process if it occurs in the animal kingdom at all, is of a fundamental character and that indications of the same process may be confidently looked for in other groups.

The most however that we seem to be justified in asserting to be definitely established for Vertebrates is that genital cells are derived from the mesoderm of the coelomic wall. Apart from the actual facts of observation such development of gonocytes from coelomic lining fits in well with general morphological ideas. It is clear that we must believe that in the simplest diploblastic ancestor of the Vertebrates the gonocytes were derived from epithelial cells. It is also clear that, on the view that the Coelomata passed through an Actino- zoan-like stage during their evolution, we must regard it as probable that during that stage the gonocytes were situated, as in existing Actinozoa, in the endodermal epithelium lining the pockets between the mesenteries~ ~an epithelium which, on that view, is represented by the endoderm of the enterocoelie pouch of an Amphioxus embryo and by its derivative the coelomic mesoderm of an adult Amphioxus or other Vertebrate.

In Amphioxus the gonad of the adult shows special peculiarities 1v GON AD 267

which mark it off from all other Vertebrates. Bearing in mind however that the general arrangement of the mcsoderm of the adult Amphioxus, which also shows striking peculiarities, is preceded by a condition in ontogeny which there is reason to regard as more nearly primitive than occurs in any other V ertebrate—~-the possibility at once suggests itself that this may also be the case with the gonad. Consequently it becomes important to enquire what are the early conditions of the gonad in Amp/w'owu3 and whether it is reasonable to interpret the conditions in the more typical vertebrates as being modifications of those illustrated byAm,pIwl09c'us.

The earliest so far recognized stage of the gonad (Boveri, 1892; Zarnik, 1904) consists of a thickened portion of coelomic epithelium at the ventral end of the mesoderm segment 03.6. in the region where at an earlier stage the segmented part of the mesoderm was continuous with the portion which loses its segmentation. The thickening lies close to the headward boundary of the segment and within its ventral angle. As the segment has already become nipped oil‘ from the lateral mesoderm it is not possible to say from actual observation that the thickening belongs to the splanchnic rather than the somatic wall though this is probable from the condition in the more typical vertebrates. The genital thickening is repeated over a number of segments (from about the 9th or 10th to about the 34th or 35th--Zarnik).

There are then three important points to be gathered from the study of the origin of the gonad in AI/:1,plmI0a:'as:-~ -

(1) It arises as a thickening of coelomic epithelium ’i.e. it shows the mode of origin characteristic of coelomate animals in general,

(2) It arises close to the boundary of segmented and unsegmented mesoderm, and

(3) _It arises on the dorsal side of that boundary.

ln the more typical Vertebrates the ovary or testis first becomes clearly recognizable as a rule in the form of a longitudinal ridge—— the genital ridge-——-which runs along the dorsal wall of the splanchno- coele on each side, at a varying distance from the line of attachment of the dorsal mesentery, and projects into the splanchnocoelic cavity. The genital ridge commonly extends over a greater antero-posterior extent than does the functional gonad later on--6.9. in the Salmon of the 185th day it extends from about the level of the fourth trunk myotome back to behind the anus (Felix). '.l‘he restricted portion of the ridge which is destined to develop into functional ovary or testis is termed by Felix the gonal portion to distinguish it from the portions in front (progonal) and behind (epigonal) which remain sterile.

The relatively great anteroposterior extent of the gonad during early stages in its development is probably to be regarded, along with the greatly elongated condition in the adult of some of the more archaic Vertebrates, as evidence that at one period of evolution the gonad extended throughout the whole length of the splanchnocoele. 268 EMHRYOLUGY OF THE LOWER VERTEBRATES CH.

As development goes on the genital ridge‘ increases in depth and is new termed the genital fold. Tliis is composed of peritoneal epithelium covering a supporting and, later on, vascular core cl’ meson- chymatous connective tissue.

The rudiment of the actual gonad in the strict sense consists of a thickening of the peritoneal epithelium covering the genital fold- the germinal epithelium. This thickened germinal epithelium may extend over both mesial and lateral surfaces of the genital fold as in most Amphibians, Reptiles and Birds or it may be restricted to its lateral (I(:lz.t/2,y/up/uls S? , most 'l‘eleosts) or median (Elasmobranehs except in very early stages, Icltthy/opI:.'z's (5) surface.

(_)f the more primitive holoblastic Vertebrates the Amphibia are the only group on which detailed observations on the origin of the gonad have been recorded. We shall accordingly summarize the early stages in the development of ovary and testis in this group and where possible in its more primitive subdivision the Urodela.

Fig. 1-10 illustrates the earliest stages of the gonad so far identified in Urodeles, as described by Schapitz (1912) for the Axolotl. Fig. A is taken from an embryo in which the protovertebral stalk or nephro- tome is not yet completely restricted oil’ from the inyotolne. On its outer side is seen the rudiment of the arehinephrie duct. The stalk is continuous ventrally with the. lateral or splanchnocoelic mesoderm. In its inner portion certain of its cells (tag. the two adjoining cells in the figure in which the nucleus is shown in a darker tone) are begin- ning to show recognizable indications of nuclear and cytoplasmic features which are characteristic of the gonad later on. It will be borne in mind that the wall of the protovertebral stalk is morpho- logically part of the coelomic wall to which therefore these gonad cells also belong. In the sections shown in B and C the mass of cells showing these peculiarities has become more and more distinctly marked 011' from the lateral niesoderm (mes) and may now be spoken of definitely as the gonad. In the stage illustrated by D the lateral mesoderm is seen to be spreading inwards towards the mesial plane ventral to the gonad and it is beginning to show here and there a distinct split separating its somatic and splanchnic layers. In the later stages (E, F, G) this split becomes a patent cavity-—the splanch- nocoele (spIc)——and the gonad is seen to lie on the dorso—lateral side of this, separated from the actual cavity by the somatic layer of peritoneal epithelium. In the last stage figured (G) the gonad is causing a slight bulging of the peritoneal lining into the splanchno- eoele: this bulging is the incipient genital ridge (gm).

During the earlier of the stages illustrated the gonocytes gradu- ally acquire the superficial histological characters of germ-cells. The cell-body is larger than that of the other cells, it remains full of yolk particles, and in the spaces between the latter are to be seen fine granules of dark pigment. The nucleus is elongated or lobed in shape, the chromatin distributed in fine particles so that the nucleus as a whole stains less deeply than do the nuclei of other cells, and IV GONAD 269

large round nucleoli are present, frequently corresponding in number

With the lobes of the nucleus. , The embryonic gonad during the stages which have been described

Fin. 140.——Origin of the gonad in Am-b/_'I/.s'l(_mI(.l. (After Selmpitz, 1912.) A, 7-8 (lays after fertilization ; B, 1.0 days ; '3, 12 days ; D, 18 days ; E, 17-18 days ; F, 19 days ; G, just after hatching. .

u.-ml, :ll‘('llill|‘])ll[‘i(' duct; 1], ;.';mI:u| ; g.r, 3.-“u-nitul 1-i:l;_;-n-; nu-s, Intvml nu.-soul:-rm: -my, m_vntr_mw: .s-pie, spinm-lnu)uoo-la‘. .-\1.- -“ in Fig. I) is st-o-n Inn‘ «of HM‘ hc:l\'i|,\' _\H|l<|‘<l vvlls \\'llil'll .-m- int.«-rim-It-ml by smut.-

ats a.eeessur_y gu|i0eyt;e.~'.

is not as a. rule continuous from end to end. On the contrary it consists of 'l.S(')l£Ll)(3('l pleces and these HI numux-' <'-.:1.ses Show distinct traces of metaineric arrmigenient, the pieces being directly opposite 270 EMBRYO-LOGY OF THE LOWER VERTEBRATES CH.

the mesoderm segments. The discontinuity becomes less marked in the later stages but even in an 18-day embryo Schapitz found the gonad still consisting on one side of the body of metamerically arranged blocks while on the other it had become a continuous strand, except for a single small isolated piece posteriorly.

From what has been said it seems clear that the gonad of the Urodela is a derivative of the coelomie wall lying close to the boundary between the segmented and the unsegmented (lateral) portions of the mesoderm. As in early stages it consists of blocks with a roughly segmental arrangement it would appear to lie on the dorsal or nephrotomal side of the boundary mentioned. There is no apparent reason for declining to interpret this early segmented stage of the gonad as a persistent trace of a primitive segmental arrange- ment like that of Awtplmloams.

'I‘he tendency for the segmented condition to disappear in the typical V ertel:»1'-ates is adequately explained by the gradual dorsal- ward encroachment of the unsegmented splancbnocoele. The boundary between segmented and unsegmented (lateral) mesoderm has altered much in position during the course of evolution, and there is no adequate rea.son to suppose that this boundary is not still a lluctuating one and if it is so we may expect varying traces of the original segmented condition to present themselves during develop- ment.

'l‘he gonad has been described as being paired throughout but it may be mentioned that various observers have noticed an unpaired condition at one or other period during the early stages of develop- ment. This appears to be adequately interpreted as a secondary fusion similar to that occurring between the right and left opistho- nephros in a Teleost or in 1-’9'ot0_pte'ru.s rather than as a primary condition.

We have traced back the gonad to its first recognized beginnings in one of the relatively primitive holoblastic Vertebrates. Before passing on to its farther development it has to be noticed that there exists a considerable volume of evidence pointing to the existence of additional germ-cells which arise independently of the coelomie lining and some of which migrate into the germinal epithelium and may give rise eventually to functional gametes. It is not proposed to describe this evidence as it has not as yet, in the present writer’s opinion, reached the stage of being convincing. It does not appear to have been satisfactorily demonstrated that the supposed extra gonocytes are really gonocytes at all rather than somatic cells. What is needed to provide such a demonstration is a careful study by skilled cytologists of the nuclear features of these cells, to determine whether there are any definite nuclear characters (such as Boveri discovered to be present in the gonocytes of Ascaris me_qalo- cephala) showing them to be beyond doubt gonocytes and affording a means of tracking them down in their supposed migration. Mere shape and staining capacity of the nucleus as a whole, or presence of IV GON AD 271

nucleoli, do not seem sufliciently definite characters as these are probably directly related to volume and metabolic activity of the cell. Cytoplasmic features—<>f which much use is made in this con- nexion~—such as richness in yolk or roundness in shape are also unreliable. As regards the first of these, the study of the develop- ment of embryos rich in yolk brings out clearly the fact that the cells in particular tissues do not, by any means, all keep pace with one another in their developmental processes. Individual cells lag behind, and one of the commonest characteristics. of such cells is that the yolk stored up in their cytoplasm remains unaffected for some time after that in the neighbouring cells has been completely used up. Obviously in such a case richness in yolk, even when occurring along with greater size due to less active division, does not constitute evidence of any weight as regards difference in morphological nature between the heavily yolked cell and those round about it. Again there is reason to believe that yolk may be stored up secondarily in particular cells or portions of tissue of a developing embryo as a preparation for future needs quite apart from the actual germ-cells.

As regards approximation to a spherical shape, it should be remembered that there is a usual tendency for irregularly shaped or branching cells, such as those of ordinary mesenchyme, to assume temporarily a rounded form at the period during and about mitosis. Such cells are apt to assume an appearance misleadingly like that of young germ-cells. .

The various features above indicated occurring together are sufficient to give a characteristic appearance to the cells in the main gonad but they form hardly definite enough criteria to prove that cells elsewhere are germ-cells in face of the strong probability that the Whole mass of germ-cells in the body are of a common origin.

GENITAL Rrncn AND GENITAL FoLn.——The genital ridge was left as a slight bulging inwards of the peritoneal epithelium covering in the gonocytes. As development goes on the ridge becomes con- verted into a prominent fold ~ -the genital fold. The peritoneal epithelium at first passes continuously over the surface of the strand of gonoeytes but soon a change comes about in their relative positions the gonoeytes coming to be incorporated in the thickness of the epithelium which may now therefore be spoken of as germinal epithelium. The gonoeytes are to be seen first along the free edge of the fold (Fig. 141, A) and this during subsequent development swells out greatly and forms the functional ovary or testis, while the proximal portion acts merely for suspensory purposes. The gonoeytes increase in number by mitotic division but are also reinforced from small apparently indifferent cells lying between them (Fig. 141, 0, gal). We may take it that these small cells are in all probability to be interpreted as cells of the original gonad which have lagged behind in development, though it is naturally difficult from mere observation to make certain that they are not ordinary peritoneal cells. At a particular stage in development (between 26 and 33 mm. in 272 EMBRYOLOGY or THE LOWER VERTEBRATES on.

Rana teInporm"ia~~—Bouii1) the total number of gonocyteslin the gonad undergoes a remarkable reduction, e.g. from between 200 and

Flu. 141.-I)eve1opnient of the gonad in Amphibia as seen in transverse sections. (A, atter Schapitz, 1912 ; B—E, after Bouin, 1901.)

A, lar\-'n01'Axu10t| (.~'hnlII3/stowm m.ea:ica.na) ; B, Rama te-nzpm~m~z'.a, 20 mm. tadpole; C, 24 mm. tadpole; 1), 33 mm. tmlpolu; E, 35 mm. tadpole with hind legs C0n1plett*.ly developed. fol, nucleus of follicle- cell; 0-}; Kfinmll "Old: {I-8. Wnititl Stlfllltli Kit‘, ££0110('}'tfi: gel, transitional stages showing conversion of apparently lmlifl‘ex-out cells into gonocytes; mes, inesenchyme; som, somatic mesoderm; spl, splanchnic meso<l<-i'ni; 3/, yolk.

250 in a frog tadpole of 26 mm. to between 37 and 46 in a tadpole of 33 mm. (Bouin). During this process individual gonocytes IV GEN ITAL ORGAN S 273

degenerate and in many cases they appear to be shed into the splanchnocoele leaving behind them the spaces or follicles in which they lay walled in by indifferent cells. The meaning of this pheno- menon is obscure but a suggestion is made regarding it below.

A period of active mitotic division of the gonocytes new sets in which leads to the formation of solid masses of gonocytes p1'0_iecting down into the interior of the young genita.l gland (Fig. 141, E). These gonocytes are the ancestral oogonia or spermatogonia as the case may be.

Up till now the genital fold has been spoken of as if it were merely a fold of the coelomic epithelium. As a matter of fact the fold very soon becomes invaded along its attached edge by immigrant mesenchyme cells forming a solid connective tissue core or frame- work which serves both a supporting and later, when it develops blood-vessels, a nutritive function to the developing germ—cells. The penetration of nests of gonocytes into this in the form of solid down- growths of the germinal epithelium may be interpreted as represent- ing an ancestral increase in area of the fertile portions of the germinal epithelium -—the increase being originally brought about by the formation of hollow downgrowths into the vascular stroma, and these downgrowths having secondarily lost their cavities. The otherwise mysterious degeneration and shedding of large numbers of gonocytes already referred to may probably be regarded as a means of providing room for the localized parts of germinal epithelium which undergo this active proliferation.

Ul{INOGENl'l‘AL NETWORK. A characteristic feature of the Verte- brate is the set of tubular channels—-—va.sa. efi‘erentia—-which in most of its subdivisions connects the testis with the opisthonephros and is frequently to be recognized in more or less vestigial form in the female sex as Well.

Apart from variations in detail, these channels may be said to pass from the cavity of the testis to the cavities of the Malpighian bodies of the opisthonephros. They are clearly recognizable during early stages of development as solid strands of cells lying within the genital fold (Fig. 141, 1), gs), the cavity in their interior develop- ing secondarily. As regards their first origin the majority of observers state that they are first recognizable at their renal ends and have therefore interpreted them as outgrowths from the coelomic epithelium of the Malpighian body, i.c. from the nephrotome. Other observers seeing them make their appearance gradually in the core of the genital fold and reaching the Malpighian body secondarily regard them as differentiating in situ from the mesen- chyme, while still others have adduced evidence in favour of the cells which give rise to the strands being budded off from the peritoneal epithelium close to the attached base of the genital fold. The disparity between the statements of different observers is most reasonably to be attributed to actual variation in the mode of development. It may be assumed that originally the connexions

VOL. II T 274 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

of the genital portion of the peritoneal epithelium with the peri- toneal funnels, or with the nephrostomes, were in the form of open eiliated grooves or gutters on the surface of the peritoneum, that later on these became closed in to form tubular channels, and that in actual ontogenetic development in the modern amphihia the development from the coelomic epithelium has become obscured except for traces now at one end now at the other.

At their distal ends the cell-strands in the male can be traced gradually farther and farther into the genital fold until they come into immediate relationship with the cell-nests of gonoeytes. Tn the female of Urodcla and Anura. the strands do not spread so far into the genital fold, nor are they, even in early stages, so well developed as in the male.

The fatty body is developmentally simply a portion of the genital l'old which becomes specialized as a store-house of fat. In Anura it is the progonal portion which undergoes this differentiation while in Urodeles and Gymnophiona the rudiment of the fatty body is continued backwards as a ridge along the mesial face of the genital fold throughout its extent.

The fat is stored in the connective tissue of the organ, the fat cells being usually interpreted as immigrant mesenchyme cells which have invaded the rudiment by its base of attachment. It has also been suggested that these fat cells are peritoneal in their origin (Abramowioz, l9l3)~——a suggestion of obvious interest in view of the general tendency in the animal kingdom for potential germ-cells to undergo degeneration in order to provide nourishment for the germ- cells which become functional.

TE.STIS.——-The development of the functional testis out of the genital fold is seen in peculiarly simple and diagrammatic form in the Gymnophiona. Here the strands of the urinogenital network, as they sprout into the interior of the testis, anastomose together along its axis so as to form a central canal——-around which, embedded in the stroma of the organ, lie the rounded nests of gonocytes. Fusion takes place between each gonoeyte-nest and the wall of the central canal and then each nest develops a cavity in its own interior and becomes a hollow ampulla opening into the canal at its inner end.

Various modifications of this simple scheme are to be found. ln Grymnophiona themselves ampulla-formation becomes suppressed except in localized regions between successive vasa efferentia, so that intervening portions of the testis are sterile and form merely thin tubular connexions between the bead-like fertile portions. Again the ampullae vary in shape: they may be elongated and tubular (Discoglossas) or, as in the majority of cases, flattened against one another by pressure. The “ axial” canal again may lie close to the surface: it may become greatly branched, as in most Urodeles, or may form a complicated network as in most Anura.

OVARY.-——In the differentiation of the ovary (Bouin, 190]) the most important points to be noted are the following. As regards IV OVARY 275

the germinal epithelium the most conspicuous feature is of course the immense increase in size, accompanied by the storing up of yolk in the cytoplasm, exhibited by those gonocytes which are to become functional eggs. Synchronously the indifferent cells of the germinal epithelium in their neighbourhood become converted into follicular cells, having for their main function the ministering to the metabolic needs of the growing egg-cells. The intervening portions of germinal epithelium, which do not undergo this modification, retain their germinal character and provide successive batches of eggs in successive breeding seasons.

’l‘he cellular strands of the urinogenital network assume, as in the male, a tubular form, their wall becoming a cubical epithelium. As in the male the ovarian ends of these channels come into close relation with one another and fuse to form a central canal. In the Anura. the fusion together to form an axial cavity appears to be less complete than in the male, a number of isolated central cavities being formed one behind the other. Fusions which take place later lead merely to reduction in the number of these central spaces (in Izfu//2.a from about 12-15 down to about 5-7-——Bouin). With further development, and the functional egg-cells increase in size, the epithelial walls of these spaces become thin and flattened. Eventually they become pressed together and the cavity is reduced to a mere slit. The portions of the tubes lying nearer to the attachment of the ovary become vestigial.

The presence of these axial cavities in the ovary, homologous with the central canal into which the microgametes are shed iii the male, is of great morphological interest. It suggests the possibility that at one time the eggs were shed into this central space and therefore that the condition now holding in the Vertebrata, where the eggs are shed into the open splanchnocoele, is to be interpreted as a reversion to, rather than a persistence of, the primitive method of shedding the eggs.

Leaving on one side the elaboration of histological detail which is not dealt with in this volume, the development of ovary and testis shows in its main features great uniformity throughout the gnathostomatous Vertebrates, and may therefore be dismissed with a few general remarks. Everywhere we see the gonad consist- ing at an early stage of a localized patch of coelomic epithelium in contact with nutritive and supporting mesenchyme: everywhere We see this coming to project into the splanchnocoele as a more or less prominent ridge or fold.

A conspicuous feature is the widespread tendency towards increased localization of the actual functional areas of the germinal epithelium. This is seen on a small scale in the development of cell- nests of gonocytes separated by indifferent or sterile portions: it is seen again in the restriction of fertility to a relatively small antero- posterior part of the genital fold, long progonal and epigonal portions becoming sterile: it is seen again even in the actual differentiated 276 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

testis where a considerable length towards the posterior end may lose its fertility and assume a merely conducting function,‘ or where such sterile portions may be repeated at regular intervals throughout the length of the testis (Gymnophiona).

This concentration of the activity of the gonad may affect its bilateral symmetry. In Elasmobranchs both ovaries may be present and functional (]3we'n1,a7'gus,Notidanusg7“?§s(ms), or one may be function- ally inactive ((}entroph07'us, ’/’r_y_¢/on), or as happens in the majority, one, usually the left, fails to complete its development and is reduced to a more or less insignificant vestigc. A similar reduction of one ovary takes place in many Tcleosts. In the Birds the right ovary ceases its development at an early period and soon disappears entirely in the majority of individuals, although exceptions are of com- paratively frcquent occurrence.

Ovnnr AND (_)vioUc'r or TELEOSTOMA'1‘(_)US Fisnus.-——1n the most archaic of existing teleostoniatous FiShes——tl1e Urossopterygian ganoids——the ovary is in the form of a typical genital fold which sheds its eggs into the splanehnocoele, from which in turn they pass out by a Miillerian duct. Consequently we may take it, in the absence of convincing evidence to the contrary, that the ancestral condition of the ovary and oviduct in the teleostomatous Fishes did not differ from that in other primitive gnathostomes such as Elas1no— branchs, Dipnoans, or Urodeles. A peculiarity however of the Crossopterygian oviduct as compared with that of the other groups mentioned is seen in the reduction of its glandular activity, and this reduction——which finds its physiological expression in the reduction of tertiary egg envelopes——-probably gives a clue to the subsequent evolutionary history of the oviduct within the group Teleostomi, in the majority of which the whole Mullerian duet has apparently been reduced to the verge of complete disappearance.

As regards the ovary itself there has come about secondarily-— perhaps in correlation with increase in number and diminution in size of the eggs——a condition in which the eggs are not set free in the general splanclmocoele but are shed into an ovarian cavity, the wall of which is in complete continuity with that of the oviduct. The ovarian cavity is formed by the walling in of the portion of splanehnocoele which lies along the fertile (usually lateral) face of the ovary. The precise method of enclosure differs in detail in different Teleosts. In some (e._q. I-’e'rca, Gaste7'osteu.s-, Acerdna, Zoaroes) the ovigerous surface of the genital fold becomes invaginated, or overgrown by flaps which eventually meet and undergo fusion (Fig. 142, A): in others (ag. Uyprinoids) the free edge of the genital fold meets and undergoes fusion with the wall of the splanehnocoele (Fig. 142, B).

Which of these two types of development is the more nearly primitive cannot be stated with certainty but the balance of

1 Lepidasiren and Protopterus (Graham Kerr, 1901); 1’olyptorus and probably Teleosts (see below). IV TELEOSTEAN OVARY 277

probability seems on the whole in favour of the first mentioned, for the formation of folds or grooves of the fertile surface of the genital fold, so as to give increased area, is a very usual phenomenon, and the formation of a single longitudinal groove would readily lead to the first—1nentioned condition. 011 the other hand the replacement of this condition by the second is also readily understandable.

The ovary passes without a hreak into the oviduct which is simply the posterior sterile portion of the genital ridge in which a cavity develops .=econdarily——not always continuously-——~from hefore hackwards. The oviduct dilfers greatly in length in different Teleosts: in some (Zowrces, 03/cloptcw/.s) the ovary itself may stretch right back to the genital pore.

Although the above description fits in with the normal con- ditions, there are various Telcosts in which the processes of fusion connected with the ovary do not take place and in which the ovary remains as a genital fold hanging down. into the splanchnocoele,

/ / I a

/ ///////////////W

/


//

I:‘u.'. 1l2.—l)iagranx illustrating the conversion of the genital fold into a closed ovary in the Teleostean lishes.

e.g. in the case of the Salmon fusion of the ovarian edge with the body wall takes place anteriorly for a short distance and again in the posterior sterile region, but the greater part of the fertile region of the ovary hangs free. In such cases the eggs are shed into the splanchnocoele and pass to the exterior by genital pores (compare Cyclostomata, p. 246).

Unfortunately we are still in almost complete ignorance regard- ing the development of ovary and oviducts in the Ganoids. From the little we do know it would appear that in Lepidosteus (Balfour & Parker, 1882) the ovary hecomes enclosed in the same manner as in Cyprinoids (Fig. 142, B). Posteriorly it is continuous with the oviduct as in 'l‘e1eosts generally. In the other Ganoids the ovary retains the form of a genital fold hanging down into the splanchno- coele while the oviduct is provided anteriorly with a coelomic funnel. The position of this funnel, far removed from the front end of the splanchnocoele, is sometimes used as an argument against the homology of this opening with the ostium of a true Miillerian duct, but such an argument carries little weight as we know from 278 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the higher vertebrates that the ostium of an undoubted Mullerian duct is liable to undergo secondary shifting into such a position. Again the fact that the opening lies on the mesial side of the ovary is adduced as an argument in the same sense but in this case We have delinitc embryologioal evidence from 1’0lypte'ru,.s' (Budgett, 1902) that this position is secondary, the early rudiment of the duct lying external to the ovary and immediately ventral to the Wolflian duet as is the case with a typical Miillcrian duct. Consequently there is no suilicient reason to doubt that these oviducts with open ostia in ganoids are really Mullerian ducts.

l’HYI.()GENY OF 'J‘i+JI.1«;0sTEAN 0VmUC1‘.—--Tlie facts of development show clearly that the main part of tho Teleostean oviduct is of the same morphological nature as the ovary with which it is continuous. It arises from the hinder part of the primitive ovary which has become sterile and assumed a merely conducting function.

The main diflieulty connected with the morphology of the organ is that of accounting for the joining up of the part of the oviduct of ovarian origin with the eloaca or exterior. Balfour suggested that this had come about by the oviduct becoming fused with the lips of the “abdominal pores.” As an objection to this was adduced the observation by llyrtl that in filo’/'/zzyrus abdominal pores exist along with oviducts. This objection disappears, however, if we remember that in Balfour's time there were confused together under the same name two different types of aperture-——true abdominal pores and genital pores. Substituting genital pores for abdominal Balfour's view seems still the most feasible. The probability seems to be that the main steps in the evolution of the Teleostean oviduct were as follows :-

(1) The primitive oviduct or Mullerian duct underwent gradual atrophy becoming gradually shorterl until eventually nothing was left but its external opening—~—-the genital pore. This process would doubtless be correlated with the loss of its glandular function and this in turn may have been connected either with the adoption of pelagic spawning, in which special tertiary investments for the eggs were no longer required, or with a special development of primary envelopes within the group. A stage was thus reached which is represented by the condition in Salmo. Of course We do not know whether Salmo has retained this condition or has reverted to it: the latter is more probable.

(2) The portion of splanchnocoele along the ovigerous surface became enclosed so as to form a cavity which served to conduct the shed ova back into the neighbourhood of the genital pore. Anteriorly the ovarian surface abutting on this cavity remained fertile, while posteriorly it became sterile, so that the posterior portion of the cavity performed merely a conducting function (oviduct).

(3) The lips bounding the posterior end of the oviduct from

1 We may see early stages in this process illustrated by the Ganoids Amie and Acipenser. Iv URINOGENITAL CONN EXION 279

being merely in proximity to the genital pore came to be com- pletely fused with the edges of the latter opening which consequently became the oviducal aperture. '

U RINOGENITAL CONNEXION.-—We have already summarized for the Amphibia the course of development of the urinogcnital network —the system of tubes or vasa, efi”erent'ia which connect testis and kidney and which serve as the outlet for the sperm. It is now necessary to glance at some points in the general morphology of this system of tubes. It has already been indicated that at their genital end the tubes become merged together in the axial cavity of the testis. The latter we must regard as morphologically an isolated portion of splanchnococle into which the spermatozoa are shed although it is no longer traceable to splanchnocoele in actual ontogeny. It has also been suggested that the tubular channels were probably originally open grooves of the peritoneal lining which became con- verted into closed tubes as the gonad became isolated from the main splanchnocoele. .

’l‘he vasa efferentia frequently show a tendency, more or less pronounced, to anastomose together into a network. In the Amphibia it is a very general though not invariable rule that anastomosis takes place close to the edge of the kidney, forming the longitudinal “marginal canal” which is conspicuous in most Amphibians. A similar marginal canal is formed in Elasmobranchs.

In taking a general view of the system of vasa cfferentia we find that one of its characteristics is, as in the case of the gonad itself, a tendency to increased localization of its functional portions. Thus during ontogeny in Amphibians the vasa efierentia towards the hinder end of the series become blocked and non-functional, or dis- appear entirely, leaving only those at the anterior end functional. This process reaches its limit in such forms as Alytes or Discoglossus where only two or a single member of the series persist.

Similarly in Elasmobranchs the number of functional vasa efferentia becomes reduced to a few at the anterior end of the series (0cntr0ph07'us 9, Scyll'£'u/m 6, Acantlmlas 4-6, Pristiurus 3, Jllustelus 2-3, Rwia 1). The same happens in Amniota.

In the Dipnoi on the other hand the localization takes place at the hind end of the series, the functional vasa efferentia being reduced to about half-a-dozen (Lepidosiren) or to a single one (Protopterus). '

Another phenomenon which makes its appearance is the simplifica- tion and shortening of the route by which the spermatozoa pass from the vasa efferentia towards the exterior. Primitively the vas efferens opens into an otherwise normal Malpighian body containing its glomerulus and continued into a functional renal tubule. This condition may persist (Rama esculenta, Buj'o), or the glomerulus may disappear (R. temporaria), or finally the whole Malpighian body and its tubule may be shortened and widened and converted into a simple tubular continuation of the vas efferens towards the opisthonephric I I uga 0 o - o n u

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280 EMBRYOLOGY OF THE LOWER -VERTEBRATES

FIG. 143.—Diag1-am illustrating the ex olution of the genital duct of the male Teleost. A, condition in which the sperms were shed mtu the .».planchur)coe-19 and passed outwaids tlu ough t-he nephrostomvs and tubules of the Op1St:h0I1PphI‘0~ ' B, test].

sh'ut qfi‘ from s'planchnocoel_e and_c0rm_numcatmg with the (.'{l.Vlt1t‘S. of certain lfutalpighian bodies scattered throughout. t3he_1ength of kidney and te~ti~. (exemplified by ficzpenser, ebc.): C, t.est1s_ divided into functional (T1) and sterile (1?) portions and vasa elferentia reduced to a few in hinder portion of T, (exemplified by I-91”’1°3_”'¢’n)1 D. "833 efferentla reduced _to a_§mgle one ap hind end of '12 (exemplified by I'ro!uptcrus): E, the communication; between the hind end of T2 and the °P1Sth0n€PhUC duct has become slmphhed (exemplified by Polypterus or young Teleost). :1 «vona-1 (portion of lining of coelome); and, duct» of opist;ho—

nephros ; p.e, peritoneal lining with nephrostomes opening through it. ‘ ' O

CH. 1v URINOGENITAL CONNEXION 281

duet (Dis00gl0ssw,.9, Alg/tes, anterior vasa eiferentia of B07/2b'i7uttor). In A13/tes the single vas efferens with its continuation becomes completely emancipated from the kidney tissue and lies in the adult some distance from the anterior end of the kidney.

The same phenomenon is seen in Elasmobranchs and Amniotes where the opisthonephric tubules connected with the vasa efferentia never reach the length and complicated convolution of the normal tubule and the Malpighian bodies either degenerate (S03/llium., Pris- tzlur/'u.s', Birds) or are eliminated entirely from ontogeny (Skates).

On the other hand the simplification of the route from testis to Wolllian duct may come about in a difi'erent fashion, as is seen in Amia, where the opening of the vas efi"erens has become shifted from the Malpighian body down the course of the tubule, in some cases till it has come to open directly into the duct.

A careful study of the method by which these various modifica- tions eome about during ontogeny is greatly needed.

Application of the general priI1(3ipleS- outlined above seems to afford a probable explanation of the remarkable arrangement in Teleostean fishes, where the testis is continued back into a special sperm duct which opens to the exterior near the opening of the kidney duct (Fig. 143). '

The presence of a urinogenital network along the whole length of the testis in Ganoids (Acipenser, Lepvidostems) justifies the assump- tion that the ancestral Teleost possessed this primitive arrangement of the network. In the case of Poly/pterms the testis is continued back as a duct which opens into the urinogenital sinus formed by the hinder ends of the Wolffian ducts. The duct, however, is not, except near its termination, a simple tube but contains a network of cavities continuous with those of the testis. It is in fact not a simple duct but a portion of the testis which has become sterile.

Similarly in the case of various typical Te_leosts it has been shown (J ungersen, 1889) that the duct is formed by the hinder part of the genital ridge, that it contains for a time a network of cavities con- tinuous with those of the functional testis——-that it is in other words the modified and sterile hinder portion of the testis-—and, finally, that posteriorly it opens into the Wolffian duct.

N ow the method by which the condition met with in Polypterus or in the young Teleost has arisen is probably indicated by what has happened in the two Lung-fishes Lepirioszrevt and Protoptems. In the former the testicular network is reduced to the extent that only about half a dozen vasa efferentia persist at the hind end of the series. In P'rotopte'rus these are still further reduced to a single vas efi°erens which passes from the hinder end of the sterile portion of the testis—“ sperm duct ” of the older descriptions——-into the hind end of the kidney and communicates with the Wolflian duct through the hindermost kidney tubules.

The only further step needed from the condition exemplified by Protoptemos to that of Poly/pterus or the young Teleost is that 282 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the communication between the sterile or duct portion of the testis and the Wolflian duct should come to be by a direct tubular prolonga- tion of the vas efferens instead of by tortuous kidney tubules. That such a shortening up and simplification of the channel from testis to Wolflian duct does actually tend to come about in evolution is demonstrated by the precisely similar series of modifications which have occurred in the Anura at the front end of the urinogenital network. In those not only has the series of vasa efferentia become reduced to a single (anterior) member in such a form as Discoglossus but the kidney tubule interpolated between it and the Wolflian duct has become shortened and widened, so that there exists simply a single tube leading from the testis and continued at its other end into the Wolllian duct.

Giving consideration to these various points it appears to be justifiable to relate the probable evolutionary history of the sperm duct of the Teleost in the following terms :—

I. Primitively the elongated testis communicated with the Wolffian duct by way of ((t) a series of vasa efierentia distributed along its length, and (b) the tubules of tlie opisthoncphros.

I l. The posterior portion of the testis became sterile and functioned merely as a reservoir and duct.

III. The vasa. etferentia became reduced to those connected with this sterile region and finally to the hindermost one of these.

IV. The channel formed by this together with the kidney tubules into which the spermatozoa passed from it became shortened and widened until it reached the condition of a simple tube leading from the hind end of the testis into the hind end of the Wolffian duct.

V. The final stage was reached by the opening of this tube into the Wolfiian duct becoming shifted back until its opening to the exterior came to be independent.

SUPRARENAL ORGANs.——The organ familiar to students of the Amniota and especially of the Mammalia under the name Suprarenal or Adrenal is now generally recognized as being not a single organ but an organic complex formed by the union of two originally separate elements-—the medullary substance and the cortical sub- stance. These two elements arise quite independently i11 ontogeny, the medullary substance being derived from the sympathetic ganglia, while the cortical substance arises in the form of a number of thick- enings of coelomic epithelium on the roof of the splanchnocoele between the two kidneys. That this independence in early stages of ontogeny is a repetition of a condition which occurred during phylogeny is indicated by the fact that in Fishes the two elements are still independent. The names medullary and cortical substance, reierring as they do to a topographical relation which occurs only in mammals, are obviously unsuitable from the point of view of comparative morphology and it is becoming customary to apply other more characteristic names. The medullary substance in mammals and what corresponds with it in other Vertebrates has a very characteristic Iv THE SUPRARENAL ORGAN S 283

chemical or physical reaction, in that it takes on a deep yellow or brown colour when treated with salts of chromic acid. Hence it is convenient, and usual, to apply to it a name expressive of this reaction-— such as Ghromophile (Stilling), Chromafiine (Kohn) or Phaeochrome Poll).

( The cortical tissue has also characteristic features——-in particular the fact that its cytoplasm contains numerous granules of lipoid or fat-like substance, soluble in Ether, Xylol, etc., staining deeply with various Aniline stains, and giving the characteristic black with Osmic Acid. For masses of this tissue the name Interrenal organ may be used (Balfour) which although a topographical term like cortical substance has the advantage of being correct for vertebrates in general during at least the early stages of their development.

Of the more primitive groups of gnathostomatous Vertebrates only the Elasmobranchs and the Amphibians have been studied carefully in regard to the development of these organs and we shall consequently use them as illustrating the general mode of develop- ment which, with variations in detail, holds throughout the groups dealt with in this volume.

ELASMOBRANe111I.1——The Interrenal organs are here interrenal in position through life, forming either one (Sharks) or a pair (Skates and Rays) of elongated bodies lying in the region of the mesial plane and extending for some distance opposite the hinder part of the opisthonephros.

In S’c;2/llvlmn (Poll) the interrenal makes its first appearance (7 mm. embryo) in the form of a number of irregularly distributed thickenings of the splanchnic mesoderm in the region of the root of the mesentery, just ventral to the dorsal aorta. The possibility of metameric arrangement in the very earliest stages does not seem to be absolutely excluded but there is no evidence of this so far. The rudiments are most numerous in the genital region but they occur as far forwards as the hind end of the pronephros and back as far as the cloaca. The rudiments of the two sides, projecting towards the median plane, meet and become continuous, and as antero- posterior fusion also comes about, the rudiment takes the form (10 mm. embryo) of a cellular rod lying beneath the dorsal aorta and above the mesenterie root, and for a time still continuous with the splanchnic mesoderm which gave it origin. For a time there is close apposition, amounting to apparent continuity of tissue, between this red and the opisthonephric nephrotomes lying on either side of it, but it is doubtful whether any special morphological significance is to be attached to this. In embryos of 16-28 mm. in length the interrenal organ gradually becomes separated in a tailward direction both from the coelomic epithelium and the ncphrotomes, and assumes its definitive form.

Only the tailward part of the series of original rudiments

1 The best general account of the development of the Suprarenal organs is that by Poll (1905). 284 EMBRYOLOGY OF THE LOWER VERTEBRATES ml.

completes its development in the way described. The whole series extends through about 25 segments but of these only about the posterior half take part in the formation of the interrenal rod: the anterior ones either atrophy completely or develop into small accessory interrcnals.

The chromophile organs of the Elasmobranch (Swale Vincent, 1897) are small, rounded, segmentally arranged bodies lying ventral to the intercostal arteries-—the anterior few on either side forming a continuous structure which was regarded by the earlier workers as an accessory heart (Duvernoy). These bodies are, as Balfour showed (I878), derivatives of the sympathetic ganglia. In a Scyllium of about 53 mm. the lateral part of the ganglion rudiment begins to show differentiation from the rest, its cells being relatively smaller than those which are destined to become ganglion—cells, and their protoplasm not only staining more deeply with ordinary stains but also developing the characteristic chromic acid reaction. In the Scyllium of 90 mm. the chromophile organ has assumed its definitive rounded form. Intrusive connective tissue forms a sparse stroma and capsule and in the former a capillary network is present. The series of segmentally arranged chromophile masses undergoes much modification in subsequent develop1nent—son1e, particularly at the ends of the series, aborting, others undergoing fusion. The details vary in different genera, the result being a striking variety in the adult arrangements in the various members of the group.

AMPIIIBIA.——B1‘a11er (1902) in his work on the renal organs of H3/pogeophis gives a clear account of the development of the supra- renals.

The interrenals appear as in Elasmobranchs in the form of cellular proliferations of the coelomic epithelium, in this case a little external to the root of the mesentery. These proliferations are paired and segmental in their arrangement, and extend from the region of the pronephros to that of the eloaca. The cellular buds become constricted off from the coelomic epithelium and lie above it as rounded masses embedded in the mesenchyme. As the two posterior cardinal veins approach and fuse the interrenal buds become displaced upwards so as to lie between the cardinal vein and the dorsal aorta. As development goes on processes of fusion take place between the rudiments more especially anteriorly where they come to form an unpaired elongated mass lying below the dorsal aorta and for the most part dorsal to the posterior vena cava (role. the fused posterior cardinals) but here and there passing laterally round the vein to its ventral surface or even piercing it—- the fusion between the two cardinals having been obstructed at such points. In the posterior half of the organ the several rudiments retain their distinctness and lie on the ventral face of the opistho- nephros.

The chromophile bodies develop as in‘ the Elasmobranchs from split off portions of the sympathetic ganglion rudiments. These IV THE SUPRAREN AL ORGANS 285

become shifted in a ventral direction round the dorsal aorta and take up their position in intimate contact with the interrenal bodies, lyiiig in the posterior paired region of the interrenal on its mesial face, elsewhere dorsal or lateral or even completely surrounding the

FIG. l44.—-——I11nstrating origin of the sclerotome as seen in transverse sections through young stages of

A, Amplu'..»..ru.< (for tha :<ulu- n1'vlo'-:n'm-.~'s the slun-4-s h:1\‘e linen a‘x:i_'_,-‘:44-l‘:1t.(=.«l, uml t-he \'vnl1':1l poi-Lion of the ding,-‘1':un is Lulu-n I‘:-om :1 rather _yuun;.-_-mi .~a1.-:;_~,-- llnul the <|m-:-ml): ll, I,rp[4lu.s-ir.-11., stagu -3.1; (1, Polyptcms, .st.=ig«- ‘.23. mt. o-nlt.*ri(- (‘H\'ll)'§ my, 1n_yui-nuu': .\". nutm-Inn-«I; nu, m-phrocmrle; pn, pronephros; 5..-, spinal wml; .-cl, .~u-lemlmm-; sn, subImt'(n:lnmlul me! : Spit‘, 5;aluin-lxmu-¢_w1e_

interrenal. The chrornophile bodies stand out with great; distinct- ness from the interronal by their fine grained"deeply-staining proto- plasm and their larger nuclei.

We thus see that in the Amphibia the originally separate inter- renal and chromophile bodies become during the course of develop- 286 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

ment associated together to form a suprarenal complex of the type seen in the higher Vertebrates. Incidentally the unsuitability of the terms medullary and cortical is accentuated, for here when one of the elements comes to surround the other it is the chromophile which does so -—- precisely the opposite to what happens in the Mammalia.

Tum Sc1.uno'roMs.—-In Amp/z.iowus the sclerotome (Fig. 144 A, .90!) arises as a pocket-like diverticulum of the splanehnic mesoderm just ventral to the myotome. It grows inwards and dorsalwards, pushing its way between the notoehord and spinal cord on the one hand and the myotome o11 the other. until it reaches the mid-dorsal line where it meets its fellow of the opposite side. The epithelial walls of the sclcrotome finally break up into mesenchyme --amoehoid connective tissue cells. The cells derived in this way from the outer wall of the sclerotome apply themselves to the mesial face of the myotome, penetrating in between its muscle cells and forming septa of connective tissue between adjacent myotomes, while those derived from the inner wall go to form packing tissue in the inter- stices round spinal cord and notochord. Over the spinal cord this packing tissue forms a tough protective roof. During this resolution of the sclerotomes into mesenchymc all trace of the original

scgniental character of the sclerotomes disappears.

It is customary -——— although the present writer regards it as questionable whether this is wholly justified——to regard the mode of origin of the sclerotome seen in the developing Amphioxus as representing the primitive mode of development. Upon this assump- tion we may describe what takes place in the_ typical Vertebrates as follows. The breaking up of‘ the sclerotomc into niesonchyme tends to take place at earlier and earlier periods of development——the diverticulum stage becoming more and more transient and eventually disappearing completely so that sclerotome formation comes to be represented merely by a very active proliferation of mcsenehyme cells from the splanehnic surface of the mesoderm ventral to the myotome (cf. Fig. 144 O, scl).

It must not be supposed that the whole ‘of the connective tissue in the body is necessarily derived from the sclerotome. On the contrary it would appear that other regions of the mesoderm also give rise to mesenchyme cells. Thus the inner surface of the splanehnic mesoderm of the gut-wall would appear to give rise to the connective tissue of this region, and the whole of the splanchnoeoelie mesoderm of the postanal region apparently becomes resolved into mesenchyme.

On the whole perhaps the safest position to take up is that of regarding the power of. forming mesenchyme as a general property of the mesoderm, and of regarding the sclerotome merely as expressing a localized concentration of this power, rather than as being the representative of some primitive pocket-likesdivertieulum of unknown

function. IV THE COELOMIC ORGANS, 287

LITERATURE

Abramowicz. Morph. Jahrb., xlvii, 1913.

Agar. Trans. Roy. Soc. Edin., xlv, 1907.

Balfour. Monograph on the development of Elasmobrancli Fishes. London, 1878.

Balfour and Parker. l’hil. Trans. Roy. Soc., clxxiii, 1882.

Balfour and Sedgwick. Proc. Roy. Soc., xxvii, 1878.

Bles. Proc. Roy. Soc., lxii, 1897.

Bouin. Arch. dc Biol., xvii, 1901.

Boveri. Anat. Anz., vii, 1892.

Brauer. Zool. Jahrb. (Anat.), xvi, 1902.

Braus. Morph. Jahrb., xxvii, 1899.

Budgett. Trans. Zool. Soc. Loml., xvi, 1902.

Dahlgren and Kepner. Animal Histology. New York, 1908.

Ewart. Phil. Trans. Roy. Soc., 179 B, 1888.

Ewart. Phil. ’l‘ran.~x. Roy. Soc., 179 B, 1889.

Ewart. Phil. Trans. Roy. Soc., 183 B, 1892.

Felix. Anat. Ileftv (Arb.), viii, 1897.

Felix. Hertwigs Haudbnch der Entwicklun,<,-slehre, iii, 1904.

Field. Bull. Mus. Comp. Zool. Harvard, xxi, 1891.

Piirbringer. Entwick. der Amphibienniere. Heidelberg, 1877.

(See also Morph. Ja.hrb., iv, 1878.) .

Goodrich. Quart. Journ. Micr. Sci., xxxvii, 1895.

Guitel. Bull. Soc. Sci. et Méd. de l’Ouest, x, 1901, and xi, 1902.

Gregory. Semons Forscliiingsreison. i, 1906.

Hochstetter. Morph. .|abrb., xxix, 1900.

Jungersen. Arb. zoo].-zoot. Inst. Wurzburg, ix, 1889.

Jungersen. Zool. Anzc-iger, xvi and xvii, 1893, 1894.

Kerr, Graham. Proe. Zool. Soc. Lond., 1901.

Kerr, Graham. The Work of J olm. Samuel Bridgett. Cambridge, 1907.

Kerr, Graham. Quart. Journ. Micr. Se.-i., liv, 1910.

Koltzofi‘. Bull. Soc. Imp. Nat. Moscow, xv, 1901.

Lankester. Quart. Journ. Micr. Sci., xvii, 1877.

Lebedinaky. Arch. mikr. Anat., xliv, 1895.

Marshall. Vertebrate Embryology. London, 1893.

Marshall and Bles. Studies Biol. Lab. Owens College, ii, 1890.

Mollier. Anat. llefte (Arb.), iii, 1893.

Mliller, E. Anat. Hefte (Arb.), xliii, 1911.

Nuasbaum. Arch. mikr. Ana1.., xxvii, 1886.

Poll. Hertwigs Handbnch der Entwicklungslehre, iii, 1905.

Poole. Proe. Zool. Soc. Lond., 1909.

Rabl, G. Morph. Jahrb., xxiv, 1896.

Rabl, H. Arch. mikr. Anat., lxiv, 1904.

Rosa. Al‘ClllVl0 zoologico, iii, 1906.

Riickert. Arch. f. Anat. u. Entwick., 1888.

Schapitz. Arch. mikr. Ana.t., lxxix, 1912.

Schmalhausen. Zeitsclrr. wiss. Zoo1., C, 1912.

Schreiner. Zeitschr. wiss. Zool., lxxi, 1902.

Sedgwick. Quart. Journ. Micr. Sci., xx, 1880.

Sedgwick. (,),uart. J ourn. Micr. Sci., xxi, 1881.

Semon. Keibels Normentafeln. iii, 1901.

(Also in Zoo]. Anzeiger, xxiv, 1901.)

Semper. Arb. zool.-zoot. Inst. Wnrzburg, ii, 1876.

Vincent, Swale. Trans. Zool. Soc. Lond., xiv, 1897.

Wijhe, van. Ilber die Mesodormsegmente und die Entwicklnng der Nerven des Selachierkopfes. Verhaml. Akad. Wet. Amsterdam, xxii, 1883. (Reprinted, Gren- ingen, 1915).

Wijhe, van. Arch. mikr. Anat., xxxiii, 1889.

Zarnik. Zool. Jahrb. (Ana.t.), xxi, 1904.

CHAPTER V THE SKELETON

TH E skeletal tissues of the animal body show a variety which is at first sight quite bewildering. (Jloscr scrutiny however reveals certain general principles which are at Work. In a very restricted set of cases We see that the supporting structure consists of a row or red of cells which is rendered stiff through the individual cells being blown out or distended with fluid. Such turgor of cells is a far less con- spicuous feature in the animal kingdom than it is in the vegetable. lt is well seen in the axial row of endoderm cells which supports the tentacles of the Hydrozoa. In the Vertebrate it is seen in the notochord.

Far more usually the support is given by a definite supporting substance with such physical qualities as rigidity, tensile strength, elasticity, as may be required in the particular case.

These supporting substances of the animal body again show the greatest variety in their morphological nature but they may all be classed between two extremes——in one of which the supporting substance consists clearly of modified cells or portions of cells and in the other of dead intercellular substance. Examples of the former are seen in the rema.rkable phagocytic organs of nematode worms where an enormous cell becomes developed i11to an immensely com- plicated branched structure of stiff horny consistency upon the terminal twigs of which are perched innumerable minute blobs of phagocytic protoplasm. A good example of the second type is seen in the skeleton of ordinary coral——a mass of hard calcareous material lying clearly outside the limits of the living cells.

It is necessary to emphasize the fact, which is frequently lost sight of, that the ditl'erenees between these two types are superficial rather than fundamental. They are merely the extremes of a series and are connected up by innumerable intermediate conditions. The

skeleton of an Arthropod such as a Lobster is in the early stages of its-

development simply the stiffened and hardened outer layer of the cytoplasm of the ectoderm cells, while in its latest stage, immediately before it is shed, it has become a thick layer of dense chitinous and calcified non-living substance lying outside the limits of the living protoplasm. 288 CH. v THE SKELETON 289

Non - living material “ secreted” by cells consists no less of modified cytoplasm, although here the cytoplasm so modified does not form a continuous mass and retain its original position in regard to the rest of the cell body. It arises commonly as isolated droplets or particles which may secondarily run together within the body of the cell or, without this happening to any obvious extent, are extruded from it, passing on to a free cell surface or into the intercellular matrix. The process can be followed by observation, naturally, only in cases where the secretion runs together into discrete droplets or particles sufficiently large to be visible under high powers of the microscope, as commonly happens in gland—cells. Far more frequently the extruded particles are so small———possibly molecular- as completely to elude observation. Such is the case where the inter- cellular substance undergoes skeletal modification: all that can be observed is simply a gradual transformation in the physical and chemical characters of the matrix, due in some cases to a gradual change in the metabolic activities of the_ cells which inhabit it, in others to the immigration into it of cell-colonists of a new type.

The supporting skeleton of the Vertebrate is an endoskeleton ;1 it is developed not on the outer‘ surface of the body but within its substance. In this it contrasts with the skeleton of the Arthropod or Mollusc which is exoskeletal—-con sisting of thickened and stiffened cuticle. In the case of the most ancient skeletal structure in the Vertebrate body———the notochord—-—the. stiff’ supporting character is due to the individual cells being distended by fluid secreted in their interior but as a rule in other skeletal tissues the stiffriess is given not by the cells but by the intercellular matrix.

THE NOTOCHOBD AND ITS SHEATHS.——A comparative study of the Vertebrate skeleton shows that it illustrates three phases of evolu- tionary progress (1) the notochordal phase, (2) the cartilaginous or chondral phase and (3) the bony or osseous phase. Of these the primitive is indisputably the first. It is a phase which is passed through during ontogeny in all Vertebrates and it remains permanent throughout life in Amphiowus.

The notochord is in its origin a rod of cells split oil‘ from the endoderin along its mid—dorsa1 line. This is seen in all the lower Vertebrates. In some of the more primitive members of the group the notochordal rudiment is for a time deeply grooved on its lower side, so as to form an inverted gutter along the middle of the enteric roof, and it may well be that this is to be regarded as the primitive mode of formation of the organ.

The notochord becomes constricted off as a cylindrical rod extending along the dorsal side of the alimentary canal from a point just behind the tip of the infundibulum to the tip of the tail. The

‘ It is regrettable that the term exoskeleton has crept into use by writers on Vertebrate anatomy for structures such as fish-scales. As will be seen later these are really endoskeletal, even the enamel being-developed on the inner surface of the

epidermis. VOL. II U 290 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

individual cells develop in their cytoplasm fluid vacuoles which increase in size and become confluent until at last the cell takes the form of a comparatively thin layer of protoplasm surrounding an enormous vacuole and containing embedded in its substance at one point the nucleus. The turgescent condition of the cells inflated With fluid gives them the firmness which enables the notochord to carry out its function as a supporting structure.

The inflation of the cells with fluid carries with it another result namely a great increase in size of the individual cells. This in turn causes a great increase in size of the notochord as a whole, showing itself particularly by increase in diameter but also by increase in length. '|‘he latter is not able to take place with perfect freedom and the result is that the individual cells tend to be compressed into the forin of transverse discs.

The notoehordal rudiment at an early stage becomes covered by a thin, elastic, highly refracting, cutieular formation known as the primary sheath of the notochord (“ elastica ea:terna”). After the formation of this the superficial layer of the notochord soon resumes its cuticle-forming activity but now in a somewhat modified form--a secondary or “ fibrous ” sheath, thicker and more jelly-like in appearance, being produced internal to the primary sheath. In Cyclostomes and Sturgeons this secondary sheath remains through- out life without conspicuous change beyond increase of thickness a11d the assumption of a tough fibrous character and is physiologically the most important part of the axial skeleton of the trunk region.

The superficial layer of notochordal cells, lying in immediate contact with the inner surface of the secondary sheath, do not as a rule undergo the process of vacuolation which affects the inner cells. They remain as a layer of compact protoplasmic cells known as the notochordal epithelium.

In some Vertebrates (Dipnoi, Agar, 1906) a short stretch of noto- chord, from the tip backwards, degenerates within its primary sheath at an early stage, breaking up into loose mesenchyme. As the notochord behind the degenerated portion grows in length its front end is pushed forwards so as to re-occupy the vacated portion of primary sheath. The extent to which this process takes place throughout the Vertebrata in general, and also its meaning, are deserving of further enquiry.

HYPOCHORD (Subnotochordal Rod).—In the anamniotic Verte- brates there is formed what is apparently an accessory notochord lying ventral to the true notochord and hence known as the Hypo- chord or Subnotochordal rod. This organ (see Gibson, 1910) arises after the notochord and in an entirely similar manner, '£.e. as a longitudinal rod of cells split off’ from the endoderm in the mid- dorsal line and sometimes possessing a distinct groove along its lower surface facing the enteric cavity. On its surface it normally develops a primary sheath precisely like that of the notochord.

We may he sure, from its wide distribution amongst the more v THE SKELETON 291

primitive Vertebrates (Lampreys, Elasmohranehs, Teleostomes, l)ipnoi, Amphibia) and the early stage of development at which it appears, that the hypochord is an organ of great antiquity in the Vertebrate stem, but we have no definite knowledge 01' its ancestral significance. The fact that it does not occur in A'rnp/z/ioams has rendered possible the suggestion that it represents the longi- tudinal groove which in this animal runs along the mid-dorsal line of the pharyngeal wall. But this idea is negatived by the fact that the hypochord extends right back to the tail and is not merely a pharyngeal organ, a11d the probability seems to be that it has come down from a period in evolution long before the appearance of Avnplmloams. It is perhaps simplest to regard it merely’ as an accessory notochord.

Whereas the true notochord plays an important physiological role as the main part of the axial skeleton during early stages, and as the foundation for the vertebral column of later stages—the hypochord has no such justification for its persistence. It lasts only for a short time and eventually breaks up and completely disappears.

In the Aumiota there is no typical hypochord developed but it is possible that a thickening of the mid-dorsal endoderin which is frequently found in the pharyngeal region ((5.9. in the Second day Fowl embryo) may represent a last vestige of it.

SKELETAL DEVELOPMENTS OF THE CONNECTIVE TISSUE

Whereas the notochord is derived directly from the endoderm, the cartilaginous and bony components of the skeleton on the other hand are modifications of the mescnchyme or connective tissue, which forms a considerable proportion of the entire bulk of a typical vertebrate.

Connective tissue in its least specialized form may be seen in practically any late vertebrate embryo as a reticulum or spongework--—a syncytial framework—- of mucl1-branched cells, the processes of which are continuous from one cell to another, while the meshes are occupied by a clear fluid or jelly-like matrix. Masses of this tissue form a kind of packing between and around the various epithelia of the body, while it also, in the form of discrete wandering cells, actually invades the epithelial tissues and colonizes them. Such immigrant elements are found for example between the muscle-fibres, in the substance of the central nervous system, and even frequently between the epithelial cells of the epidermis.

The primitive or embryonic connective tissue undergoes gradual differentiation in accordance with the physiological role which it.has to play in difi"erent localities. This differentiation finds expression in such superficial features as shape and arrangement of the individual cells and more fundamentally in the peculiarities in metabolism which lead to its storing up particular substances in its protoplasm 292 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

--—pigment of chromatophores, fat of the cells of adipose tissue~—or again in the influence exerted by the metabolic activity of the cell upon the character of the matrix. This matrix is commonly described as intercellular, which is quite correct, b11t the important point is not the question whether it is inter- or intra-cellular but the fact that it is in iinmediate relation to, and under the influence of, the living protoplasm of the cell. The portion of matrix in con- tiguity with one of the irregularly shaped connective-tissue cells {is comparable with an intracellular vacuole the outer wall of which has thinned out and disappeared. The matrix has been l'or1ned by the breaking down of living substance and it seems merely a matter of phraseology whether we speak of it as modified protoplasm or as dead “ formed ” Inaterial.

The most familiar difierentiation of the matrix of connective tissue consists in the development within it of thin tough fibres, characterized by the physical property that they soften and dissolve, yielding gelatin, under the action of boiling water, and that they become further toughened by the action of tanning agents. These fibres run indiscriminately in all directions or, in the more specialized conditions, are definitely orientated, as in the case of tendon where they are parallel and arranged in longitudinal strands, or of aponeuroses where they are arranged in thin layers, those of one layer perpendicular to those of the next. Other portions of the matrix take the form of elastic flbres———characterizcd by their elasticity, by their connexion together to form a network, by their being much less easily affected by boiling water, and by their not yielding gelatin.

The amount of matrix present ditfers greatly in different localities. It may be reduced to a very small amount-——to a mere demarcating line——between closely fitting plate-like cells, as in the case of the endothelium covering the surface of a tendon, or it may be large in amount and comparatively rigid as in the case of the two great skeletal tissues cartilage and bone.

CARTILAUINOUS on CHONDRAL SKELETON.—-—Tl1e cartilage is char- acterized by its cells taking on a rounded form and becoming separated by an abundant semitransparent,elastic, chondrin-containing matrix. The process of chondrification becomes apparent first of all in the somewhat dense packing tissue (“ skeletogenous layer ”)‘ immediately surrounding the notochord. This connective tissue becomes_locally modified to form little blocks of cartilage known as the arch-elements (arcualia—Gadow, 1895), lying just outside the primary sheath and arranged in four longitudinal rows, two dorsal composed of the rudiments of the neural arches, two ventral»--the rudiments of the haemal arches. These arch-elements are apparently in the primitive condition duplicated in each segment, "Le. within the limits of a

myotome or sclerotome there are situated two pairs of neural and two pairs of haemal arch-elemen ts.

1 The term prochondral is applied to the young cartilage in its early stages before the characteristic intercellular matrix makes its appearance. V THE SKELETON 293

There now takes place in two of the more primitive groups of V ertebrates——the Elasmobranchii (including the Holocephali) and the Dipnoi—--a remarkable process whereby the secondary sheath of the notoehord becomes converted into a sheath of cartilage. Certain of the cartilage cells in the arch rudiment take on an amoeboid character and burrowing their way through the primary sheath, apparently by the help of a digestive ferment, invade the secondary sheath (Fig. 145, me). Continuing their migration they become distributed equally throughout the whole substance of the secondary sheath, including those portions in the head re- gion which will later on form part of the cranium. The immigrant cells fin- ally settle down in the substance of the second- ary sheath and the latter becomes a cylinder of cartilage. ‘

‘It is important, with an eye to the evolution of the vertebral column in Vertebrates higher in the scale, to bear in mind , that this invasion of the Secondary Sheath by fin‘ Fm. 145. -- Part of a transverse section through a migrant cartilage cells Lepidosiren of stage 38, traversing one of the neural takes place at four points 91°“ "‘“““‘°“*'“~ in the transverse plane, rm, _notu(:1un:«lul epithelium; m.c, migrating ('m'l.il:1;.;'c celg; corresponding to the bases ii-1:Jli.'i§iiiiilgfigatiiihU’ "Mal mh’ ‘i’ ’""""“y ”'"”"'h’ 5’ of the four arch rudi- ments, and that this arrangement is repeated twice within the limits of one segment owing to the arch rudiments being so repeated. Consequently if we suppose the colonization of the secondary sheath to be restricted to the neighbourhood of the trans- verse plane in which the arch rudiments are situated the result would be the formation of two rings of cartilage within the limits of a single segment. ‘

In the ease of Lung-fishes and Holoeephali the chondrified secondary sheath undergoes no further modification but in typical Elasmobranchs it becomes divided up into segments, which form the centra or bodies of the vertebrae, in the manner to be described later on. In this process the originally uniformly flexible notoehord with its sheaths becomes replaced physiologically by a series of rigid masses, flexibility being given to the whole by the presence of the intervening joints. As this jointed condition of the vertebral column originated in evolution at a time when the longitudinal muscles of the body were already divided into myotomes, we may 294 EMBRYOLOG-Y OF THE LOWER VERTEBRATES "CH.

take it as probable, for obvious mechanical reasons, that the rigid skeletal masses arose in a position alternating with the muscle segments. The individual vertebral centra were in other words from the beginning iiitersegmcntal in position in relation to the general body iiietamerism. i In sketching out in somewhat greater detail the further develop- ment of the vertebral column the assumption will be again made use

of, as it was in dealing with the mesodcrm segments, that the trunk‘

region has in all probability departed least froin the primitive con-

dition, and the facts quoted will in the main be taken from this regioii of the body.

The student who goes on to peruse original memoirs will notice

' ' that this rule is by no

A. 3- _ A. 1.3- means always accepted. Seine

I


,1 l .——”' writers will he found to assume that the caudal re- gion is more nearly primitive, and, in accordance with this assumption, to interpret the phenomena observed in the trunk vertebrae by those observed in the caudal, in- Fie. l46.—"'[\rr:-ingernent of dorsal arch-elements in Stead mice v6TSa"_ _ hinder trunk region of a 1’6C’I‘07n]/£.Il)')'l. larva 95 In l3l1lS COIIIIBXIOH 1l21I1115l'I mm. in length. (After Schauinsland, 1906.) [)3 borne jn f,ha,[',' the A, anterior, B, posterior neuralarcli-clenieiits;(hr, dorsal Vertebrate is above all I.‘.T..’3it‘f.l ?fEI$i§‘..5i.'.‘;‘i: ??..T‘3.f.lZ.'iIf‘r33E‘I§’3‘;’,§ll.:.1'$.13?-§T;i”"“' ‘Miss essentially a coelomate animal. No one doubts that whatever the common ancestor of the Vertebrates was like it was at least coeloinate. And most morphologists would admit further that the weight of evidence indicates that in this ancestor the splanchnocoele extended throughout the greater part of its length and that the existence of a considerable stretch of body towards the hind end devoid of sp1aii.chnoeoelc (13.6. a tail region) is secondary. But if the caudal region has in this way undergone profound secondary modification of its structure it is clear that it is not in this region of the body that we should expect to find persisting primitive modes of development of the axial skeleton. It is now necessary to follow out the fate of the arcli-elements. As already mentioned the primitive arrangement of these appears to have been two pairs to each segment, above and below, so that corre- sponding with each myotome there were, on each side, two neural elements an anterior (A) and a posterior (B), and two liaemal elements an anterior (a) and a posterior (b). NEURAL A1iCHEs.1-—Apparently the most nearly primitive arrange-



‘ In writing thes_e sections on the vertebral column much use has been made of Sclia.u1ns1and’s descriptions (1906) to which the student is referred for a more detailed account than is here possible. V NEURAL ARCH ES 295

mcnt of the arches is that which occurs in the hinder trunk region of the Lamprey (Fig. 146). In this animal, as is well known, the dorsal (sensory) and ventral (motor) nerve-roots are still separate and are spaced out alternating with one another at approximately equal distances along the sides of the spinal cord. The dorsal arch elements alternate, in their turn, with the nerve-roots, so that there are, on each side, an anterior (A) and a posterior (B) neural arch- element within the limits of a single myotome.‘ It should be noticed particularly that of these the anterior is situated between the sensory and the motor nerve-root belonging to the segment. This suggests a possible explanation of the later evolutionary history of these eartilages (A) which in the typical Fishes tend very usually



3A. A. 1.

,m.,. 3.1/.r



Fro. 147.———A, arrangement of arch—elements in mid-trunk region of a Carcharias embryo

85 mm. in length; B, do. in anterior region of a Sturgeon (.~lm',oenser lmso) 36 mm. in length. (After Schauinsland, 1906.)

A, ante-rior neural arch-clement; B, posterior do. ; a, anterior haemal arch-eh-nu-nt; h, post:-xior do. ; d.o', sensory nu-rvo-root; v, blood-vessel ; wr, motor nerve-root.

to become reduced in size, even to the point of disappearance. It may be that this reduction in size is connected with the fact that in the Fishes, as indeed in all gnathostomatous vertebrates, the two nerve-roots have become approximated together to form a common sensori-motor spinal nerve. On the other hand this explanation would leave untouched the fact that a similar reduction in size may occur in the corresponding ventral or haemal arches.

The reduction in size of the “A” elements, which is of so frequent occurrence amongst the typical fishes, is well shown in Figs. 147, A and B, which are based upon Schauinsland’s recon- structions. This marked reduction is by no means of universal occurrence. The two common Dog-fish-303/llmm and Acanthzas-— are familiar examples of fishes in which the “A ” elements

1 In the anterior trunk region the arrangement is apt to be morlified—the inter- segmental vessel, which forms the anterior limit of the segment, coming to lie on the tailward side of the A cartilage of that segment (Schauinsland). 296 EMBRYOLOGY OF THE LOWER VERTEBRATES en.

(“ intercalary pieces,” “ interdorsals ”) remain nearly as Well developed in the adult as the “B” elements} .

In Lung-fishes (here and there) and in Urodele aiiiphibiaiis the “A” pieces can still be recognized (cf. Fig. 148); they haye also been observed in the embryos of various Reptiles. _In this case they usually lose their individuality at an early period, becoming completely merged in the definitive neural arch formed by the “ B ” elements lying next to them on their headward side, but in some cases, ag. in the tail region of Lacesrta, they have been found 130 persist as discrete structures even in the adult, forming a vestigial second neural arch behind the main arch.

The neural elements become prolonged dorsally and meet so as to form a complete neural arch and the apex of this becomes pro- longed as an unpaired piece in the mesial plane to form the neural spine. The complete neural arch formed in this way frequently becomes segmented up into separ- ate pieces of cartilage. The arcualia in such cases become each divided into a larger basal (basi- d0rsal—B, interdorsa.l—A,Ga dew) and a smaller apical (supradorsal) portion. The spine may segment into three superimposed rod-like portions.

HAEMAL ARCHES. —— In the

FIG. 148.—Arrangeineiit of arch-eleineiits ‘ 1 ' - 3 in anterior caudal region of a Siredoii 50 Cyc Ostonles typlcal haenlal {Hobbs

mm. in length. (After Schauinsland, are , absent’ although possibly 1906.) vestiges of them are represented

Reference letters as in Fig. 147. _ by a continuous ridge of cartilage

occurring in the tail region of

Petromyzon where the neural arches have also been reduced to a similar continuous ridge (Schneider).

Of haemal arch elements there were apparently primitively two pairs to a segment just as in the case of the neural arches. This seems to be clearly indicated by Callorhynchas (Fig. 149). It is also Well shown in the young Sturgeon (Fig. 147, B) where the anterior element (a) in each segment has undergone reduction in size exactly as was the case with the corresponding neural element (A). A similar condition is found in many Elasmobranchs, though not in all, the “ a ” elements being in some cases apparently completely absent.

. 1 The examination of one of these Dog-fishes brings out another point of general importance namely that the arch-element as it increases in size is apt to spread round a nerve-root in its neighbourhood. The result is that in the adult the nerve- roots may pass out, not between the arch—elements, but through them. The lesson to be learnt from this is that the topographical relatiop of skeletal elements to nerve- l3lI‘ll!1l{s is not to be taken as infallible evidence as to the primitive situation of such e ements. . v HAEM AL AROHES ' 297

In various Fishes, for example Laemargus and Amvla. ‘(also in some Amphibia, see Fig. 153, B), the haemal arch-element in the trunk region segments into two pieces—one of which carries the rib and may become shifted dorsally, while the other becomes displaced in a ventral direction. The ventral pieces come to form projections downwards from the centrum of the vertebra on each side of the aorta and have been termed “aortic supports." They may be termed haemal processes as they appear to be homologous with the knob-like structures bearing this name which are to be

seen in the caudal region of ag. Laemaw-gas, projecting inwards from.

the haemal arch into the tendinous septum which underlies the caudal aorta.

In the caudal region the haemal arch—elements are commonly much longer than in the trunk. They bend round to meet one another and are prolonged into a haemal spine. These features are associated with the extension of the body in a dorsal and ventral direction correlated with the use of this region of the body for the purposes of movement.

Towards the head end of the series it not uncommonly happens in Cartilaginous fishes that the haemal arch - element becomes ,1 .= -.w - -. broadened out over the surface aéad "M zidb. ab £1. 5- of the notochord indicating the beginnlngs of the evolutmn of F110 cm. embryo of Uallorhynchusa few seg- Perlchordal centra (593 below) ments posterior to the hind end of the This is well shown by Callorhym skull. (After Schauinsland, 1906.) chi”/3 Where iI10iPi9Ilt Reference letters as in Fig. 147. centra are distinctly seen, formed by the much-enlarged and fused haemal arch-elements (a and b).

In the air-breathing vertebrates there are no longer double sets of complete haemal arch -elements but a distinct trace of this condition is seen in such a Urodele Amphibian as Siredon (see Fig. 148) where a large perforation through the haemal arch element, traversed by the intersegmental blood-vessel, betokens its double origin.

A characteristic feature of the Amniota is that the haemal arch (the cartilaginous forerunner of the “chevron -bone”) tends to become displaced forwards so as to assume an intervertebral position or even [to become fused with the vertebra lying in front (Anguis- Goette). ' '

VERTEBRAL CEN'1‘RA.—Except in the case of Cyclostomes, Holo- cephali and ‘Lung-fishes, the elastic notochord becomes replaced physiologically during development by the series of vertebral centra. In the various subdivisions of the Vertebrata we find two distinct 298 l4}MBRY('_)L(,)ClV OF THE LOWER VERTEBRATES on.

methods by which vertebral centra are produced (1) by the segmentation of the cartilaginous secondary sheath (sheath centra; chorda centra——Gradow) and (2) by the enlargement of the bases of the arch—elements which grow round the notochord and give rise to centre outside the primary sheath——(perichordal centra; arch eentra———G-adow). ’

S1-IEATH CENTRA are seen in Elasmobranchs. In the region which. will develop into a eentrum the chondrified secondary sheath becomes thickened so as to bulge inwards and constrict the noto- chord (Fig. 150). A more deeply staining “ middle zone” soon becomes distinguishable in this thickened part of the secondary sheath (Fig. 150, me) having a shape something like that of a dice—box, its central part lying much nearer to the axis of’ the


Flu. 150.-——Part of sagittal section through the secondary sheath of a Scyllium of 61 mm. total length showing an early stage in the development of a centrum. (After 0. Rabl, 1893.),

'i.2, inner zone: m..z, middle zone; o.z, outer zone; N, notochord; sl, pi-imary sheath.

notochord than do its two extremities. This middle zone becomes the main part of the Wall of the amphicoelous centrum, its substance becoming usually strengthened by the calcification of its intercellular matrix. _

The inner zone (Fig. 150, 73.2.’) may grow in thickness so as to cause greater and greater constriction of the notochord. This process attains to its maximum in the Skates where it extends inwards to the axis and causes the formation of thick septa which divide the notochord into isolated intervertebral fragments. More usually however the inner zone does not undergo this increase, it tends to become absorbed at each end and forms simply a ring of cartilage in the centre of the vertebra.

The outer zone in many Elasmobranchs becomes calcified in parts: the calcified regions often showing a regular arrangement ag. concentric cylindrical shells or radiating septa.

PERICHORDAL CENTRA.-———AS a matter of fact the purely chorda- centrous condition is merely a temporary one in the Elasmobranch, as in later stages of development the sheath centra become sur- V VERTEBRAL CENTRA 299

rounded by a layer of cartilage provided by the bases of the arch- elements, which spread over the surface of the sheath centrum (Goette, 1878).‘ This outer layer of cartilage may undergo calcifica- tion later and become continuous anteriorly and posteriorly with the edge of the calcified middle zone. Meanwhile the primary sheath of the notochord is liable to disappear so that there is no obvious clue left to the independent origin of the portions of the vertebral body derived from the sheath and the arch-elements respectively.

In the course of the further evolution of the vertebral body this outer layer of perichordal origin, which in an ordinary Elasmobranch like S’cyll'iwm or Acwntlmlas serves merely to reinforce the sheath- ccntrum, is destined to become all-important while the sheath portion is destined to disappear. A step in this direction has already been made by the Rays where (Torperlo-——Schauinsland) the secondary sheath remains thin and where the primary sheath soon disappears so as to bring about complete fusion between the thin sheath layer of cartilage and the much thicker external mass derived from the arches.

In those ’1‘eleostomatous fishes which possess centra the spreading of the bases of the carrtt'la.gt'ncus arch-elements round the notochord is commonly not marked: possibly this is correlated with the pre- cocious development of the bony centrum.

In the Urodele Amphibians the vertebral bodies develop in the manner illustrated by Fig. 151. A series of ring-shaped cartilages (“intervertebral cartilage,” Gegenbaur: Fig. 151, A, 0) make their appearance round the notochord in mid-segmental positions. These rings gradually extend for a considerable distance in a headward and tailward direction, immediately superficial to the notochordal sheath, and between it and a thin, tubular, segmented sheath of bone (non-cellular) which has already made its appearance (Fig. 151, A and B, b). '1‘he intervertebral cartilage also increases considerably in thickness, bulging out between the adjacent somewhat expanded ends of the bony tubes already mentioned. In various of the more primitive Urodeles the vertebral bodies practically remain in this condition, flexibility being given to the vertebral column as a whole by the intervertebral cartilages interposed between the rigid bony segments. In the more highly developed Urodeles on the other hand there is a tendency to form an opisthocoelous joint, ale. a joint concave on its tailward and convex on its headward side (Fig. 151, C). This may find expression merely in a softening of the cartilage along what would be the surface of the joint, or a layer of the cartilage may be liquified so that the intervertebral cartilage is completely divided across into a smaller concave anterior part and a larger convex posterior part fitting together by regular articular surfaces, forming in other words a completely developed joint.

1 In Petromyzon a few of the most anterior neural arches (e. g. 4th and 5th in an old specimen of 1’. _/luxwiateilis) have expanded bases which spread ventrally almost com- pletely round the notochord so as to form a kind of arch centrum which carries rib-like projections laterally (schauinsland, 1906). 300 EMBRYOLOGY OF THE LOWER V]+]RTEBRATES en.

Fm. 151.-—Illustrating the development of the vertebral centre in Urodeles as seen in horizontal su-ctions. (Based on figures by Schauinsland, 1906.)


A, .S'u.lwmu-ml-ru, :32 mm. ; B, '.rc_:d.un, :30 mm. ; C, T: -itu-n, 16 mm.; In, home; r, c:u-t.il:1,«,:e; «-I, connective ti.~4s1u- ; -m, myotume; N, notochord; mr, nutoclun-«Iul L-:u't..iI.-:«_-e-. ; r. 1»1«;o«l-wssel. V VERTEBRAL CENTRA 301

The notochord becomes more or less constricted by the ingrowth of the intervertebral or joint cartilage which pushes the sheath in front of it. Besides this constriction of the whole notochord with its sheath the substance of the notochord becomes eventually, sometimes at a relatively late stage of development, interrupted by the development of 'int'ravertel)ral cartilage which may form a com- plete cartilaginous partition across the notochord at about the middle of each vertebra (Fig. 151, B, no). The origin of this cartilage is disputed. Some (Lwoff, Zykoff, Gadow) derive it from immigrant cartilage cells which have penetrated through the notochordal sheath from outside, while others (Gegenbaur, Field, Elmer, Klaatsch, Schauinsland) believe it to originate by the metamorphosis of actual notochordal cells, probably cells of the notoehordal epithelium. In spite of a possibly greater volume of evidence supporting the latter View it is difficult to avoid the impression that the former has in its favour the balance of a priori probability.

The lleptiles are commonly regarded as the least specialized of the three subdivisions of the Amniota and it may therefore be con- venient to let them form the basis of our description. Schauinsland’s work may be referred to for more minute detail.

The sclerotome tissue grows actively and comes to be specially concentrated immediately round the notochord to form the peri- chordal layer. .This layer is at first———in accordance with its origin from the sclerotomes——segmented (Fig. 152, A, sol) but the original segmentation soon disappears so that it forms a perfectly continuous investment to the notochord. A secondary segmentation now becomes visible in as much as the perichordal layer is decidedly thicker in a position corresponding to the middle of each original segment. These thickenings mark it off into a series of reel-shaped pieces each of which is a primary vertebral body (Fig. 152, B and C, pub). lt will be understood that the hinder half of each primary vertebral body is derived from the front half of a sclerotome while the front half of the same primary vertebral body is derived from the posterior half of the next sclerotome in a headward direction.

In other words each primary vertebral body is formed from the adjoining halves of two original segments, and as a result of this the primary vertebral bodies necessarily alternate in position with the myotomes, each myotome running from about the level of the middle of one primary vertebral body to a level about the middle of the next in the series (Fig. 152, B).

The portions of the sclerotomes lying outside the perichordal layer undergo fusion also. This outer part of the sclerotome bulges out between the myotomes while it extends dorsalwards so as to arch over the spinal cord. It is in the wall of the tunnel so formed that the neural arch-elements make their appearance while the sclerotome tissue ventral to them takes part in the -formation of the body of the definitive vertebra. The superficial part of the vertebral body arising in this way from sclerotome tissue outside the perichordal layer 302 l4‘.MBRYOL()GY OF THE LOWER VERTEBRATES CII.

(Fig. 152, O, S) is best developed laterally (Sphenoalon) though it extends as a thinner layer over both the dorsal and ventral sides of the pcricliordal layer. Eventually chcndrifical.i0n takes place and the vertebral body, derived partly from periclumlal and partly from sclerotome tissue lying outside and continuous with the neural arch portion, becomes converted into a mass ol' cartilage in which the only clue to its compound origin is the scmiewhat llattened shape of the cartilage cells in the inner part derived from the perichordal layer (Fig. 152, C, p.'v.b).

During the development of the vertebral centre the notochord becomes constricted. across much as in Urodeles. A complete septum of notochordal cartilage is Formed across the middle of each

FIG. 152.-—--Diagram illustrating the mode of development of the vertebral centra in a Reptile as seen in horizontal sections. (Based mainly on Schauinsland’s figures of

' S1‘;/I-emu/U-H, 1906.)

my, myotomo-; ,.\', iml()l'lH))'d; -}:.r.h, pI'im:u'_\' \'o-rlr-In-al |md_\'; N, Hllpl-‘1"ll(!l:ll pm-1:ir,m of Ct-‘l)l.)‘llln arising outsirlv ]"‘l‘l('liHl‘4l.‘ll lay:-r: S-.'/, -“[5119-1is"il11!«f‘1i0l1; -‘v'/, .~‘vla-ro1..unn-; r, blood-vessel. In cmnpm-in‘: the segim-nt.-ii re-I.-ition.s' of A and 13 the ii1i:.«-rse;.-nu-ntal l)lOU(l-\'u..s's|el8 (4?) form iiseful ];mdm;u-k,~.;,

vertebra in Sp/aenodon and in the Lacertilia. In the ordinary Lizards this appears to arise as a ring-shaped ingrowth of carti.lagc which constricts the notochord, pushing the primary sheath in front of it (Gadow, 1897) while in Spltenodon and also in the Geckos the cartilage makes its appearance i.nterna1 to the notochordal sheath (Howes and Swinnerton, 1901). It may be suspected that in the latter case immigrant cartilage cells have made their way through the notochordal sheath though this has not so far been demonstrated.

RIBS. — The ribs are long cartilaginous projections from the vertebrae which run outwards and ventrally in the substance of the myosepta and serve to support and strengthen the wall of the splanchnocoele. As Goette (1878, 1879). first showed, there are included under the name “ribs ” two morphologically different v RIBS 303

structures, which may be distinguished by the names dorsal or upper ribs and ventral or lower ribs. '

In Poly/pterus both sets of ribs are well developcd—-—the dorsal ones larger towards the head, the ventral larger towards the tail. In other vertebrates the rule is that only one set is developed, though the other may be represented by more or less distinct rudi- ments or vestiges. Thus in Actinopterygian ganoids, Teleosts and Dipnoans the ribs are ventral ribs while in Elasmobranchs, Amphibians and Amniotes they are dorsal ribs.‘

Both types are associated with the niyosepta but whereas the dorsal ribs lie at the level of the horizontal septum which divides the lateral musculature into a dorsal and a ventral half, the ventral ribs, on the other hand, lie along the peritoneal edge of the myo- septum where it abuts on the lining of the splanehnocoele.

Probably both sets of ribs are to be interpreted morphologically as outgrowths from the vertebrae and the balance of evidence appears to favour the view that both are fundamentally outgrowths from the series of haemal arch-elements.

Ventral Ribs.——-This is clearly the case with the ventral ribs which are simply the ventral prolongations of the haemal arch- elements, frequently jointed off from the basal stump of the arch (transverse process) by the conversion of a thin layer of the cartilage into fibrillar material. In the skeleton of a Lung-fish, a Crosso- pterygian, or an Actinopterygian the ribs are seen to form a perfectly continuous series with the haemal arches of the tail region.

Dorsal Ribs.——The nature of the dorsal ribs tends to be obscured by the fact that their point of attachment to the vertebra shows much variation rag. they may appear to arise not from the haemal but from the neural arch. That we have to do here with a secondary shifting in a dorsal direction is indicated by various considerations. Amongst the Rays it can sometimes be seen that the ribs towards the head end of the series become more and more displaced dorsally, so that they come to project from the neural arch. Then it will be remembered that in various fishes the haemal arch-element becomes divided into a ventral part (haemal process) and a dorsal part which latter carries the rib and may undergo a considerable displacement in a dorsal direction.

In Urodele Amphibians Goeppert has shown that the apparent attachment of the rib to the neural arch has come about in a some- what complicated fashion as illustrated by Fig. 153. The most nearly primitive condition is that shown in the larva of such a perennibranchiate form as Necturus (Fig. 153, A). Here the haemal arch-element (h.a) sends oil‘ a strong outgrowth (r.b), the “rib- bearer,” which passes in a dorsal direction closely applied to the

1 Distinct traces of dorsal ribs occur in various Teleosts, e.g. Salmonids and Clupeids. The numerous little bones found in the myosepta. of various Teleosts in addition to the true ribs are probably to be looked on as independent and secondarily developed “ tendon bones. ” 304 '-EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

surface of the neural arch, from which however it is marked off by a thin fenestrated layer of bone (b). It will be seen from the diagram that the bases of the neural and haemal arches and the base of the rib-bearer enclose a space through which runs the vertebral artery (am). The haemal arch-element passes horizontally outwards beyond the base of the rib-bearer and the rib itself forms merely a prolongation of the haemal arch-element, becoming segmented ell’ from its proximal portion (“ transverse process”) by the dc - ve1opm.eut of an intcrealary zone of fibrillar joint tissue. A little way out from its base the rib grows out “into a projection which is directed dorsally and fqtowards the median plane. This dorsal process is prolonged into a liga- ment which is attached at its end to a mass of bony tissue developed on the outer side of the rib~bearer and indicated in the diagram by the dia- gonal shading.

In the larva of Salama/n.d'ra, maca- losa the condition is found which is illustrated by Fig. 153, B. The most important difference from the condition seen in Nectaras is that the basal part of the haem.al arch-element has become greatly reduced, and is new attached to the notoehordal sheath FIG 153 ——Illustrating the attachment merely- by a thin thread of bone.

cl‘ rib. to vertebra in the Urodela The nb'bearer grows out from the according to Goeppert (1896). haemal arch-element as before but it A, trunk vertebra of Ncvtnrus larva; B, iS.Sh0rter.a'nd is lnore completely fused trunk vertebra of Su.Imu.¢.m.d1'a. maculosa. Wllill lille neural arch. The dorsal larva; C, trunk vertebra of ’1'm'ton.alpesm'.s process of the has increased in larva. b,:bone; h..a, haomal arch-element; - ,,_,,.., hnema, pmc,.___,,,; N, ,,0mc,,o,d; M,’ strengtlrand now extends right to the neural arch: 9', rib: r.b. rib-bearer; t, dorsal end of the rib-bearer and is

dorsal rocess ofrib: 'v.a., vertebralarter . . ' ' ,..EI2.°.:.°“£§£2.‘;“’ °*‘ A =39 §§JJ‘i§s3$Z§“§§ J£v3§;.E°d§3%lJl‘§a§é3

character with a practically equally strong dorsal and ventral attachment through the substance of the rib-bearer to the neural arch.

Finally in the larva of Triton alpestmls the condition is found which is illustrated by Fig. 153, C. The original basal part of the haemal arch-element which lay ventral to the vertebral artery has disappeared, so far as cartilage is concerned, its place being taken by a thin thread of bone. The rib-bearer is shorter and stouter

than in Salcmnandra and its fusion with the neural arch still more V RIBS AND STERN UM 305

complete» The double-headed rib has all the appearance now of simply articulating with a massive projection of the outer side of the neural arch: its original connexion with the haemal arch would never be suspected.

The question naturally arises whether in other Amphibians in which the transverse process and rib projects from the neural arch, the dorsalward shifting has come about in the same manner as is apparently the case in Urodeles. The probabilities appear to be against this. In the remaining two groups of Amphibia—the Anura and the Gymnophiona-——the transverse process, though it springs from the neural arch, lies still ventral to the vertebral artery, which suggests that there has taken place here a simple shifting dorsalward of the whole of the haemal arch carrying the rib, including its basal portion.

In the case of the Amniota, Schone (1902) has carefully investi- gated the development of Reptiles’ and has failed to find anything corresponding to the rib-bearer of Urodeles. In all probability here as in Anura there has taken place a simple dorsal movement of the rib and transverse process.

The Amniote rib appears to arise generally in continuity with the anterior half vertebra. (a) 73.6. from material derived from the posterior half of the sclerotome. In the case of Sp/wnoclon (Schauinsland) the sacral and usually the caudal ribs, on the other hand, appear to contain material derived from both halves of the vertebra, the ribs being in these regions much broader than they are elsewhere and marked by a longitudinal groove indicating their double origin. In the last vertebrae of the tail these may give rise to two separate transverse processes attached to each side of

the vertebra tipped each one by a small rib-rudiment. The uncinate processes on the ribs of certain Reptiles and Birds

arise as independent centres of chondrification. They may later on ossify and fuse completely with the rib (most Birds) or they may never show complete fusion (Apteryw, Sphenodon). In Sp/Lenodon they become simply calcified without undergoing true ossification (Schauinsland).

STERNUM.——'.l‘he sternum of the Amniota arises typically by the fusion together of the ventral ends of a number of the anterior rib- rudiments into a continuous plate on each side. The two lateral plates so formed undergo fusion across the mesial plane to form the definitive unpaired sternum, a plate of cartilage still continuous with the ribs. Eventually the sternum becomes segmented off from the ribs and may become calcified by the deposition of limy particles in the intcrcellular matrix (Reptiles) or replaced by bone (Birds).

In Amphibia also the sternum arises by the fusion together of two longitudinal bands of cartilage but no connexion can be traced between these and the ribs. This peculiarity, as compared with the Amniota, is apparently to be correlated with the comparatively short extension of the ribs in a ventral direction which is characteristic

VOL. II X 306 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

of this group of Vertebrates. In the Fishes the sternum has not yet made its appearance.

SKULL.—-—’.l‘he skull is a mass of condensed and strengthened mesenchyme serving essentially to support and protect the organs of the head. It protects the brain and sense organs: and it forms a support and framework for the masticatory and other apparatus connected with the mouth and pharynx. ln correlation with this its characteristics in detail are secondary to characters of the brain and other organs. '

The skelctonization of the mesenchyme does not take place continuously but commences in irregular patches which gradually spread and eventually join together. Though there is frequently considerable agreement between different Vertebrates in the position of the centres of skeleton formation in the head there are in other

.cases equally well-marked variations between forms known to be

phylogenctically closely related. It is as a rule impossible to say definitely whether or not the first appearance of skeleton at particular points is of phylogenetic significance or is on the other hand related merely to existing arrangements of the adult.

Under the circumstances all that will be attempted here is a short sketch of the general features of cranial development without entering at all into minute detail. For a full and detailed description reference should be made to the admirable work of Gaupp (1906).

As has already been indicated there is a marked tendency for the arch - elements to undergo fusion towards the head end, the axial skeleton being necessarily rigid instead of flexible in the brain region. Eventually towards the front end of the series both neural arches and vertebral centra become completely fused together to form part of the skull.

The skull consists in its simplest form primarily of a chondro- cranium———a trough of cartilage, the cavity of which is occupied by the brain and more or less open on -its dorsal side. Somewhere about the middle of the floor of the chondrocranium there exists a recess in which rests the infundibulum of the brain, and the portion of cranial floor lying behind this is distinguished by having the notochord embedded in it——this organ having its anterior limit just behind the tip of the infundibulum. We are thus brought into touch with a deep-seated distinction between the posterior or epichordal (chorda1——K6lliker) region of the cranium and the anterior or prechordal (Kolliker). We are probably justified in regarding the epichordal region of the cranium as being morphologically a metamorphosed portion of vertebral column in which the processes of ,fusion, already indicated as frequently occurring in the anterior region, have attained to their maximum. As will be explained later the process of incorporation of a few vertebrae (the number varying in different groups) into the binder end-"of the cranium can still be observed in ontogeny and it is probable that during the contemporary v CHONDROCRANIUM 30’?

evolution of some of the lower Vertebrates (Elasmobranchs, Sturgeons) there is still going on a process of spreading backwards of the hinder limits of the skull with the incorporation into it of additional vertebrae.

As regards the evolutionary origin of the prechordal part of the cranium we have so far no clue.

The primary cartilaginous cranium does not remain by itself in any Vertebrate. There become inseparably fused with it the cartilagin- ous capsules Which surround and protect the nose (olfactory capsule) and the ear (otic or auditory capsule). Cartilage may also develop in the sclerotic of the eyeball but owing to mobility of the eyeball being necessitated by its having to be turned towards the direction from which impressions are received, the cartilage in this case does not undergo fusion with the chondrocranium.

’l‘herc are also associated with the cranium, and more or less closely connected with it, the series of hoop-like cartilages of the visceral arches‘ and finally in many of the subdivisions of the Vertebrata. important bony elements become added on to the chondro- cranium. The description of the development of the skull will therefore fall naturally into three sections: (1) The Chondrocranium including the sense capsules, (2) The Visceral Arches, and (3) The Bony Skull.

THE OHONDROCRANIUM. —— The chondrocranium shows many differences in detail in the various groups of Vertebrates. The epichordal portion commonly makes its appearance as a pair of rods of cartilage——the parachordal cartilages—~—lying one on each side of the front part of the notochord. The prechordal portion similarly takes its origin in a pair of trabeculae which lie dorsal to the buccal cavity on each side of the infundibulum. Important differences are seen in‘the relations of these primary cranial cartilages in different members of the Vertebrata. Thus in Elasmobranchs, Ganoids, Teleosts, Reptiles and Birds the trabeculae are at first quite isolated from the parachordals while in‘ Lampreys, Amphibians and Lung- fishes they are continued at their hind ends into the parachordals (Sewertzoff).

Again in many Vertebrates, apparently in correlation with the great development of the eyes in early developmental stages, the cranial cavity no longer extends forwards between the eyes. Its Walls have come together to form an interorbital septum and fore- shadowing this, the trabeculae are closely approximated or even fused in the median line. Gaupp applies the term tropibasic to such a type of cranium and contrasts it with the platybasic type in which

1 In the neighbourhood of the margin of the mouth there frequently develop in the lower Vertebrates (Fishes and Amphibians) isolated pieces of cartilage (labial cartilages). These are sometimes termed the precranial skeleton, and various specu- lations have been made as to their possible evolutionary significance. Up to the present there appears to be no convincing evidence that they are other than mere secondary developments and consequently they will not be referred to further in this book. 308 EMBRYOLOGY OF“ THE LOWER VERTEBRATES CHI

the cranial cavity still extends forwards between the eyes and the trabcculae retain their primitive parallel position some distance apart. The skull in aetinopterygian Ganoids, Teleosts, Anmiotes and certain Elasmobranchs develops after the tropibasic type while in Amphibians, Lung-fishes, Crossopterygians and some Elasmobranchs it retains the platybasic condition.

As the platybasic type of cranium is admittedly the more primitive we shall deal with it first and will take as our example the cranium_ of the Lung-fishes Lep'icl0si7'en and Protupterus as described by Agar

1906 .

( l.)llJVl1‘.|.(‘)PMl1lN'I‘01l' OHommocRAN1uM IN Lzrpmoszmav AND PROTO- P'rImUs.——'1‘lie first rudiments of cranium become apparent about stage 31 in the form cl‘ a longitudinally situated condensation of mesenchyme——-the rudiment of the trabecula «lying upon each side beneath the thalamencephalon and mesencephalon. At its front end the trabeeula rapidly extends dorsalwards to form a vertical plate of prochondral tissue lying against the side wall of the thalam- encephalon, and terminating in front against the optic nerve. The dorsal portion of this plate, just internal to the deep ophthalmic nerve, is the orbito-temporal process (Fig. 155, A, o.t). From the outer surface of the trabecula, just in front of the main portion of the trigeminal nerve, there projects outwards a horizontal shelf of cartilage (Fig. 155, A, gxr). This is the rudiment of the portion of cranium which contains the ganglia belonging to the Trigeminal and Facial nerves (Gasserian recess, Bridge). The cranial rudiment becomes prolonged backwards, the backward prolongation represent- ing the parachordal cartilage of meroblastic Vertebrates.

This parachordal rudiment lies on each side of the front portion of the notoehord but, unlike what is more usual in other Vertebrates, it is separated from the notoehord by a considerable space (Fig. 154, A). The cranial rudiment so far described gradually becomes chondrified. About this time there appears a condensation of mesenchyme round the outer side of the otocyst: this is the outer wall of the auditory capsule (Fig. 154, A, a.c). A little later than the stage mentioned a knob of cartilage begins to develop on each side of the notoehord at the level of the septum between metotic myotomes III and IV. This is an enlarged and precociously developed neural arch which, becoming, as will be seen presently, incorporated in the skull, is known as the occipital arch. The base of this spreads forwards along the dorsolateral surface of the notoehord to form the occipital plate (Figs. 154 and 155, B, o.p). .

By stage 34 the chondrocranium has reached the condition shown in Figs. 154 and 155, B. The trabeculo-parachordal cartilage has spread outwards and has become continuous with the rudiment of the auditory capsule so that the greater part of the lateral portion of the definitive chondrocranium is now laid down in cartilage. The two trabeculae have extended forwards, converging towards one another and passing in front into an unpaired mass of cartilage The floor of the cranium has made little pro for the most part, occupi

grese, its position being, ed by a. large bas1cran1a,l fontanelle.

FIG. 154.——Il1ustrating the development of the chondrocranium in Lung-fishes. (After Agar, 1906.)

A, Protopte-ms, stage 31; B, Le1_aido.-£rcn., stage 3-}; C, L-ep-idosiren, stage 36 +. a.c, auditory capsule; (10.1), antorbital process; u-rt, foramen for artery; bc. , basicranial fontanelle; b.;;__ ‘nasilar plate; Hy, hyoid arch; £n.s, internasal septum;; M, mandibular arch; N, notochoni; oc.u, occipital arch; oc.r, occipital rib; o.p, occipital plate; pn..c, prenasal cartilag ; \'2.\'3.VII lat, foramen for exit of maxilla.r_\' and mandibular divisions of trigeminal nerve, together with buccal and superficial ophthalmic branches of Facial and communicating branch between Facial and Vagus ; VII hm,

foramen for hyomandibular branch of Facial; Vllsp, foramen for superior palatine branch of Facial.

lying between the olfactory organs—-the internasal septum (ring).

V

CHONDROCRANIUM 309 310 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Posteriorly the occipital plates have spread forwards but are still separated by a distinct space l'rom the true chondrocranium. The precoeiously developed occipital arches (om) have reached a large size and the pair of corresponding ribs———the occipital ribs (oc.'r)—-— which are so characteristic a feature of the Dipnoan skull have also made their appearance.

In the next stage figured (Fig. 154, O) the occipital plates have become continuous with the parachordal cartilages forming a broad basilar plate (lap) in which is embedded the notochord, except its tip which is still to be seen projecting freely into the basieranial fontanelle but which later on disappears. The side wall is extending dorsalwards and has enclosed the roots of the trigeminal and facial nerves, forming the outer wall of the Gasserian recess. Further forwards the front part of the basicranial fontanelle has become in great part obliterated by cartilage continuous laterally with the anterior extensions of the trabeculae. The lloor of the cranium is still deficient except anteriorly and posteriorly. Laterally a long antorbital process (cap) has grown out from the dorsal edge of the trabecu1a,.passing forwards into the upper lip.

The olfactory organ has by this stage become enclosed in a char- acteristic olfactory capsule. From the anterior end of the internasal septum a horn-like outgrowth spreads outwards and backwards to form the ventral edge of the capsule, meeting posteriorly an independ- ently developed subnasal cartilage (Fig. 155, C, s.n.c). This horn-like cartilage is met by four cartilaginous outgrowths from the dorsal side of the internasal septum which arch forwards and outwards over the olfactory organ. The roof of the olfactory capsule owing to this mode of origin has a characteristic fencstrated appearance. Between and in front of the olfactory capsules the internasal septum comes to project forwards slightly as the prenasal cartilage (Fig. 154, C, pn.c).1

In the last stage figured (Fig. 155, C) the chondrocranium has reached practically the condition of the adult. The occipital arches have extended dorsally so as to fuse, on the one hand, with one another to form the median supraoccipital ridge, and, on the other, with the auditory capsule. A horizontal shelf . of cartilage has grown outwards from the side wall of the cranium and quadrate cartilage (see below) enclosing a space (P.0) in which lies Pinkus’s organ (see p. 133). The dorsal portion of the internasal septum extends backwards slightly as the mesethmoid cartilage (me).

1 When investigating the development of Lcpidosiren in South America in 1896 I was struck b the fact that badly macerated skeletons of about this stage, with the lower jaw an other cartilaginous arches detached, as is commonly the case, and with the olfactory capsule frayed out at its edge, presented a remarkable resemblance to the remains of the curious little “ lamprey ” Palaeospondylus described by Traquair (1898). The resemblance was such as to» eave little doubt in my mind that Palaeo- spondylus is really a Dipnoan—--either larval or an adult form of small size and primitive structure. This conclusion is supported independently by the investiga- tions of W. and I. Sollas (1903) who conclude that Palaeospoadylus is an Amphibian. Any one without practical knowledge of young Lung-fishes would quite naturally suppose their imperfect remains to be those of young Amphibians. V CHON DROCRAN I UM 31 1

FIG. 155.—I11ustra.ting thé development of the chond1‘0cra.nium in Lung-fishes. (After Agar, 1906.)

A, 1"rotoptems, stage 31; B, Lepi.dn.sn1rcn, stage 34; C, Lepidosivre-n, stage 38. um, :mdit.m-_v u:xpsul.e; gar, floor of Gusseritm rm.-.\..~“.s; Hy, hyoid at-ch; Lo, interopercular; me, mesethmoid; M, uumdibular m-ch; N, notochord; o, ope-.rcula.r; oc.a., occipital arch; ocxr, occipital rib; o.p, occipital plate; o.t, or-bito-temllporal; _p._q, pectoral girdle; P1). pm-ziticm of I’inkus's or-gnu ; p.q, palat()q11:ulr-ate; (J, quad- rate; s.n.c, subnasul c1u'til:l-ge; l, flrst. hraun,-lniul arch ; III, fu1':ll!ll‘l1 for Oculomotor; V, \'l l. l'uz-.-unina for T1-igeminal and Facial. 312 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

In the case of the young Protopterus the cartilage goes on extending considerably with growth. In the region of the auditory capsule it spreads dorsalwards and reaches the middle line so as’ completely to roof in the cranial cavity at this level. Further forwards in the region of the Gasserian recess the side wall of the cranium also extends dorsalwards, though in this region there remains a wide deficiency in the cartilaginous roof.

In 1;epz'do.s~e"ren the increase in cartilage is less marked, in fact the ehondrouranium of the adult remains in many respects in the same condition as that of the larval Protopterus.

The Dipnoan ehondroeranium obviously belongs to that type in which the primary basal eartilages of the skull are continuous on each side—almost or quite from the beginning, the separation into distinct trabecular and paraehordal portions, if visible at all, being confined to a very brief period, and in which the cranial cavity extends forwards between the orbits (platybasic type). Seeing that a similar type of chondrocranium occurs in the majority of the holoblastic lower Vertebrates the probability is that it represents a more neaiily primitive condition than the tropibasic type, with paraehordal separate from trabecula, which is more usual in the meroblastie vertebrates.

DEVELOPMENT or TIIE CHONDROCRANIUM IN ELASMOBRANCIIS.-— As the chondrocranium has for its main function the support and protection of the brain, and develops in close relation with it, it will be of interest to compare with the development of the Dipnoan cranium that of one of the meroblastie Vertebrates in which the brain is modified in early stages by a greatly developed cerebral flexure. The Elasmo- branchs are admittedly the most nearly primitive of such forms and may therefore most suitably be taken as the example.

Here (Pristiurzos and Aca/nth'£as——Sewertzoff, 1899) the first indica- tions of skull development make their appearance about stage 25 (see Chap. XI.), as a concentration of mesenchyme on each side of the notochord about the level of the otocyst. This spreads, headwards and tailwards, as a paraehordal strand of prochondral tissue, extend- ing anteriorly as far as the Facial nerve and continuous posteriorly with the rudiment of the vertebral column. As the paraehordal plate takes definite form it develops in its occipital portion segmentally arranged rounded swellings which project dorsally between the nerve- roots and correspond in position with the intermuscular septa.

The prochondral paraehordal plates gradually become chondrified and at the same time they extend outwards and dorsalwards to form the basilar plate. As they do so the metameric projections flatten out and disappear in the anterior portion while posteriorly they become more pronounced, growing up dorsally between the nerve-roots and finally meeting over the roots so as to enclose them in distinct foramina. In the region behind the definitive skull the swellin gs in question do not fuse but develop into discrete arch-elements. The last of the swellings to be included in the skull would appear.to be, as a rule, in Sharks and Dog-fish that between metotic myotomes 7 and 8 (Braus, 1899) v CHONDROCRANIUM 313

though in all probability variation occurs as between different genera. and species and possibly even between individuals of the same species. The trabeciilae are strikingly different in their relations during early stages from those of the Dipnoan. Instead of being continuous with the parachordals they are at first separated from them by a wide gap in which appears on each side a small nodule of cartilage the Polar cartilage (van Wijhe, 1905). Further the long axes of trabeculae and parachordals instead of being in line are practically at right angles to one another. It is probable that both of these peculiarities are to be associated with the greatly developed cerebral flexure. The fore-brain has as already described been bent down- wards into a kind of retort shape," the floor of the thalamencephalon coming to face in a tailward direction. As the thalainencephalic floor has undergone this displacement the tra- beculae have been carried with it, so as to assume a practi- cally dorsiventral direction, and this same movement has probably brought about the severance of their original con- tinuity with the para- chordals. It will be noticed from Fig. 156

FIG. 156.——Clioiidroc1'aiiium and visceral arch skeleton of

that the d1SPla(?ement an embryo of Acam-thins. (After Sewertzolf, 1899.) has gone even (1.19, auditory capsule; 131, B5, bram_.-liial arches; II, hynid; M, than has been 1nd1" mandibular arch;n.t,urbito-teinporal;1».¢-,paraclim-clal; Ir, l.l'H.l)I'(f.lll:l.

cated so far, for the _ _ _ _ trabecula has been translated bodily in a tailward . direction so that

it lies in a plane considerably posterior to the level of ‘the anterior end of the parachordals. _

At the front end of the parachordal a plate of cartilage (Fig. 156, 0.15) develops in the side wall of the cranial cavity correspond- ing generally with the orbito-temporal plate of the Lung-fish (_Ali- sphenoid plate, Sewertzoif ; Sphenolateral, Gaupp). This is described by van Wijhe as being an outgrowth from the parachordal, while Sewertzofl" states that it is at first distinct. .

The auditory capsule originates according to Sewertzoff as a simple outgrowth from the parachordal which gradually spreads outwards and dorsalwards round the otocyst, while according to van Wijhe the first trace of cartilage is on the outer side of the otocyst and is independent. in

The cartilage belonging to the various elements which have been mentioned spreads outwards from each till they form a continuous trough-like chondrocranium. The trabeculae become continuous 314 EMBRYOLOGY or THE LOWER vsnrssaarns on.

with one another first towards their morphologically anterior ends- a vacuity persisting for a time beneath the infundibulum. As might be inferred from the study of Fig. 54 (p. 93), which shows how the floor of the thalamencephalon gradually assumes its definitive horizontal position, the trabecular portion of the cranial floor pawl passu. swings forwards and comes to be more nearly in line with the parachordal portion. The displacement of the trabeculae which we have associated with the exaggerated cerebral fiexure is then a temporary phenomenon which tends to become corrected during subsequent development. The cartilage formed by the fusion of the anterior ends of the trabeculae becomes prolonged forwards between


'. ".’:'i..‘ - I’ " ‘-._-.‘E-"<2.-.-i‘ ‘\'?i.“'-§

er-2:244--czar."


FIG. "157.--Dia.grams illustrating the early development of the chondrocranium of Birds. (Based on figures by Sonics, 1907.)

A, Chick, 11 mm.; B, Duck, 13 mm.; 0, Duck, 15 mm; D, Duck, 14_mm.; E, Chick, 12 mm. a.c, auditory capsule; ucr, acrochordal cartilage; i.o.s, interorbital septum; mo, mesotic cartilage; ma, neural arch; p, polar cartilage; p. f, pituitary foramen; p.b.f, posterior basicranial fontanelle; p.c, pa:-achorclal; tr, trabecula.

the olfactory organs as a rod of cartilage, the rostral cartilage, which represents the ventral edge of the internasal septum.

The outer wall of the olfactory capsule appears as an, at first independent, -piece of cartilage on the anterolateral side of the olfactory organ which gradually spreads round the organ in question and becomes continuous with the rest of the cranium.

It is unnecessary to— follow out in detail the modelling of the definitive cranium but it should be noticed that the cranial cavity gradually becomes‘ roofed in by the upgrowth of its side walls, and that, in some cases at least (Pvastim-us), this roofing-in process becomes completed first in the region between the auditory capsules. This fact is of interest when correlated with the persistence of this portion of the chondrocranial roof in I’rotopterus——suggesting that v OHONDROCRANIUM “ 315

this is probably the most archaic portion of the cranial roof of the Vertebrate. At the same time the possibility must not be lost sight of that instead of being of ancestral significance this feature may be associated merely with particular activity of cartilage- formation in the region of the otocyst, connected with the need of protecting that superficially placed organ of sense.

DEVELOPMENT or THE CIIONDROCRANIUM IN BIRDS.-—AccOI‘ding to Sonics (1907) the first cartilage to make its appearance is anunpaired plate arranged in a frontal plane and surrounding the notochord at its highest point in the cerebral or mesencephalic flexure. Sonics terms this (Fig. 157, acr) the acrochordal cartilage and states that it makes its appearance in the 5-day embryo of the chick. What appears to correspond to it in Apterya: is described by 'J‘. J. Parker as the prochordal cartilage, though in this case it lies quite anterior to the notochord. Very soon after the acrochordal cartilage, the parachordal makes its appearance——-ensheathing the notochord. As this is thickest laterally and very thin ventrally and especially dorsally (where indeed it may be absent) it presents when viewed as a transparency from the dorsal or ventral side a misleading paired appearance. In Apteryw however the parachordals have apparently retained the actual paired condition. ’

For a time the parachordal and acrochordal cartilages are separated by a wide gap but later (11-12 mm.) this becomes filled in by the development of the paired elongated mesotic (basiotic) cartilages (Fig. 157, B, mo). In the Duck these are at first independent, but in the Chick they appear to be, even at the time of their first appearance as cartilage, continuous with the parachordals. Extending forwards they become continuous with the acrochordal, bounding upon their mesial side a space in which no cartilage is present~——the posterior basicranial fontanelle (Fig. 157, E, pbj). Postero—externally the mesotic cartilage fits round the lagena, form- ing the rudiment of the cochlear part of the auditory capsule.

The parachordal cartilage spreads out on each side forming the basilar plate of cartilage and in embryos of about 7 days (13-14 mm.) two pairs of neural arch-elements make their appearance as lateral projections near its posterior end (Fig. 157, E, n.a)—-the posterior, situated between the Hypoglossal and the First Cervical nerve, developing first. In the Kestrel (Tvlmmnculus alaualarius) Suschkin (1899) found three such occipital arches (Fig. 158, ma) and Gaupp looks upon this as probably the typical number for Birds.

The acrochordal spreads out and forms a transversely situated plate of cartilage.

The trabeculae appear in the chick embryo of about 11 mm. as paired parallel rods of cartilage underlying the fore-brain. Posteriorly each passes into a swelling lying lateral to the pituitary body and as in the Duck and Starling (Stm-nus) this forms at first an inde- pendent piece Sonics terms it the polar cartilage. Even in the Duck embryo this polar cartilage (Fig. 157, C, 10) becomes very soon 316 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

continuous with the trabecula in front and with the acroehordal cartilage behind. The connective tissue between the anterior ends of the trabeculae gradually chondrifies in continuity with them in both Chick and Duck (Sonics). In the Kestrel Suschkin found a, for atime independent, intertrabecular plate of cartilage in this position (Fig. 158, vltr). This intertrabecular tract of cartilage serves to bound anteriorly the fontanelle (Fig. 157, D, pf) in which the pituitary body lies and through which pass the two internal .‘.tl.1‘Ol3l.(l arteries. Posteriorly this fontanelle is demarcated from the posterior basieranial fontanelle by the acrochordal cartilage later the posterior boundary of the sella twcica. It appears to be character-

FIG. 158. ———-Early smge in the development of the chmulrocraniiun of the Kestrel (Tinmmculus alcmdw-n‘.-us). A, side view; B, dorsal view. (/\l“r.-er Suisttliliin, 1899.)

Mr, auditory capsule; itr, intertrabecular c:zi-tilug-u; w.u., neural ‘arch:-s: tr, trabecula; III, foramen for oeulumotor nerve. . ‘

istie of Birds that this dorsum sellae undergoes a considerable amount of reduction during later development.

In Chick embryos of about 12 mm. a patch of cartilage has made its appearance external to the otocyst between the lateral and the superior (anterior) semicircular canals which gradually spreads forming the external wall of the auditory capsule and closely moulded to the surface of the canals. This periotic cartilage (Fig. 157, E, (ac) is for a time separated by a wide gap from the

basilar plate but this gap gradually becomes more and more encroaehed ‘

upon until reduced to a. narrow fissure through which cranial nerves IX, X, and XI find their exit. A.part from this fissure the basal and periotic cartilages become continuous. As the wall of the auditory capsule extends dorsally it remains incomplete at two points where perforated by the Facial and ‘Auditory nerves.

The roof of the chondrocranium is represented by a quite v CHONDROCRANIUM I 317

inconsidcrable tectum synoticum, which originates as a pair of at first separate cartilaginous rods (Chick 21 mm.). 'l‘hese very soon become continuous with one another and with the auditory capsule.

DEVELOPMENT or CHONDROCRANIUM IN GENERAL.——Tl1e three examples of chondrocranial development which have been dealt with

. will suffice to give a general idea of the process with its variations.

A survey of the known facts in Vertebrates generally shows that the first rudiments of the chondrocranium consist of paired elongated pieces of cartilage (preceded by prochondral tissue) lying on each side of the mesial plane and on the morphologically ventral side of the brain. These rudiments are divisible into a (para)chordal portion lying at the side of the notochord, and a prechordal portion lying anterior to this. A break in the continuity of the cartilage frequently occurs somewhere about the limit between these regions and this had led to the regarding of the portions so separated- trabecula in front and parachordal behind - as being fundamentally distinct morphological elements. As a matter of fact the break, when it does occur, appears to vary in position: thus in I’etrom,g/zen the “trabeculae” extend back for some distance beyond the tip of the notochord, so that their hinder parts are parachordal in position. In many cases the break is visible only for a very short period, while in others (Lepidosiren) there is complete continuity between trabecula and parachordal. On the whole it appears justifiable in the present state of our knowledge to regard the break in continuity between trabecula and parachordal not as marking a demarcation between two originally distinct morphological elements but rather as a secondary solution of continuity correlated with exaggerated cerebral (mescncephalic) flexure.

The parachordal cartilage in the case of the Elasmobranchs passed backwards by perfectly insensible gradations into the cartilage of the vertebral column. In that portion (occipital region) which lies between the hinder limit of the definitive cranium and the vagus nerve there appear for a time evidences of segmentation, corresponding with that of the vertebral column, and it is therefore justifiable to regard this portion of the parachordal cartilage as representing a region of fused vertebrae. In -the anterior or mesotic portion there are no visible metameric swellings but, as the relations to the notochord are otherwise identical, it is difficult to refuse a homology in this case which is granted in the case of the hinder portion. Here again, then, we should be inclined to regard the distinction between the mesotic and the occipital portions of the parachordal as merely a secondary differentiation in what was once a continuous structure or series of structures: in other words we should regard the whole of the parachordal region of the cranium as representing a modified portion of vertebral column which has been absorbed into the cranium.

The foundations then of the vertebrate chondrocranium are laid 313 EMBRYOLOGY or THE LOWER VERTEBRATES en.

in the form of paired basal eartilagcs which are eventually continuous throughout parachordal and trabccular regions but which may for a time consist of separate portions lying one in front of the other. As chondrification spreads from each of these primary elements, they become united together in a continuous plate of cartilage, forming the floor of the chondrocranium. From this in turn chondrification spreads upwards to form the side walls and roof, and forwards into

the ethmoid and nasal regions. To the brain-case so formed there become added the protective

capsules of the olfactory organ and otocyst. As each of these organs is a development of the external skin, we may assume with a con- siderable degree of probability that their cartilaginous capsules were originally independent of the cranium.

Any repetition however of this completely independent stage of the sense-capsules in question has apparently become obliterated from ontogenetie development. Portions of the sense capsule may arise from separate centres of chondrification 8.5;. in the case of the auditory capsule the first rudiment may be in the form of an independent patch of cartilage in the region of the lateral semicircular canal. Even in such cases however the inner portion of the capsule develops in continuity with the chondrocranium. Again the Dipnoan arrange- ment, where the otic capsule is without any wall upon its mesial side so that it takes the form merely of a bulging of the lateral cranial wall, is to be looked upon as secondary.

SKELETON or THE VISCERAL ARCHES.—-—The anterior portion of the alimentary canal forms a tube leading from the mouth back underneath the cranium, its lateral walls perforated, and therefore weakened, by the visceral clefts. The coelomic space being no longer present in this region somatopleure and splanchnopleure are in continuity, a continuous mass of mesenchyme ex tending from ectoderm to endoderm. This mass of tissue is divided by the clefts into the series of visceral arches and each of these is characteristically strengthened by a tract of tissue in‘its interior undergoing con- densation and chondrification to form half-hoop shaped cartilaginous arches. These arches are named according to the mesenchymatous arch in which they lie-—Ma.ndibular (I), Hyoid (II) and First

branchial (III), Second branchial (IV) and so on. The skeletal branchial arches differ in number in different verte-

brates, just as do the corresponding mesenchymatous arches (see p. 153). In the Lamprey,‘ which probably in this respect shows the most

nearly primitive arrangement, the two half-hoops of a pair become con- tinuous with one another ventrally. In the gill-breathing fishes the

«hoop typically becomes divided by joints into four segments on each

side, with a median ventral copula——no doubt an adaptive arrange- ment to facilitate the movements of respiration. Where branchial respiration is reduced the arch has reverted to its primitive un-

1 There is in the writer's opinion no sufficient evidence to doubt that the visceral

-skeleton of Cyclostomes is homologous with that of Gnathostomes. v VISCERAL ARCHES 319

segmented condition (Lepidosiren, Amniota) and no trace of segmenta- tion appears during ontogeny.

In Elasmobranchs (Dohrn, 1884) chondrification begins on each side and then spreads dorsally and ventrally. Segmentation takes place first into a dorsal and ventral half and later each of these segments again. The gill rays develop independently of the hoop and only come into contact with it later. .

The hyoid arch corresponds closely with the branchial arches in its mode of development.

The arches so far dealt with——branchial and hyoid———having to do primarily with the function of branchial respiration show their typical development in Fishes. With the disappearance of this f'unc- tion they become degenerate. This degeneration makes itself manifest in (1) reduction of segmentation, (2) tendency to fusion between successive arches and (3) reduction in number from behind forwards.

Thus in a Newt four cartilaginous branchial arches make their appearance but they are for a considerable period continuous dorsally and ventrally with their neighbours in the series, and they develop only one joint upon each side c'.e. the half-hoop consists of two segments instead of four. In a Lizard only two cartilaginous branchial arches make their appearance, and in a Bird only one.

The hyoid and the anterior branchial arches have probably been saved from complete disappearance in the higher Vertebrate by the fact that they have taken on important functions in connexion with the tongue and have become specialized in accordance therewith. Thus in the case of the frog tadpole there is found, when the branchial apparatus is at the height of its development, a continuous cartilaginous hyobranchial skeleton, in which may be recognized parts corresponding to hyoid arches, copula between these, and 4 pairs of branchial arches continuous ventrally. At the time of metamorphosis this becomes greatly modified to give the adult condition (Gaupp, 1894): the mid-ventral portions become greatly expanded to form a flattened plate—the so-called “ body of the hyoid ”: the hyoid arch becomes an elongated slender rod which serves to suspend the apparatus from the skull: the branchial arches disappear except the ventral end of the second which persists as a stump (“ Postero-median process ”).

MANDIBULAR ARCH.-—-The usually accepted idea of the mandibular arch is to regard it as a half-hoop shaped cartilage resembling the other arches, to which is added a forwardly projecting outgrowth— the palato-pterygoid bar-——which forms the primitive upper jaw

.skeleton. In actual ontogeny there is always a less or greater

amount of departure from this general scheme.

In the Amphibians and Lung-fishes the hoop-like character of the main portion of the arch has been most completely retained. Here (Fig. 155, A) the arch develops on each side as a curved bar of cartilage—-a mid-ventral copula having been detected in certain cases. The cartilage soon becomes completely continuous at its 320 EMBRYOLOGY OF. THE LOWER VERTEBRATES CII.

upper end with the chondrocranium and its dorsal end becomes segmented oli", as the palate-quadrate cartilage, from the larger ventral portion —-Meckel’s cartilage-—which forms the primitive skeleton of the lower jaw.

In the animals mentioned the lower jaw remains throughout life connected with the cranium through the dorsal portion of the original arch. This must be looked on as in all probability the primitive mode of attachment of lower jaw to skull and such skulls may there- fore bc termed protostylicfl

Both in Lung-fishes and Urodele amphibians the palate-pterygoid process is much reduced. In Urodeles it makes its appearance only at a late stage of development and is of comparatively small size. In Lepidusirem. and P7-otopterrus it has become eliminated almost entirely from development, being represented for a short time by a slight condensation of tissue which never becomes chondrified. This is probably to be interpreted as a modification of development induced by the precocious development of the bony skeleton of the upper jaw which in the forms mentioned replaces functionally the originally cartilaginous skeleton.

The Elasmobranch fishes do not exhibit this reduction of the palato-pterygoid bar for this becomes the functional upper jaw. On the other hand an important modification of development has taken place in correlation with the fact that in these fishes the original dorsal end of the mandibular arch has lost its primitive function of suspending the jaw, this function having been taken over by the enlarged dorsal end of the hyoid arch (Hyostylic type of skull). In correlation with this the portion of the mandibular arch lying above the pterygoid outgrowth is, all through development, greatly reduced. It is apparently represented by the prespiracular cartilage, which develops comparatively late.

The mandibular arch makes its appearance in Acanthias (Sewertzoff, 1899) as a G—shaped rod of cartilage lying in the rim of the buccal opening on each side (Figs. 156 and 159). The lower half of this segments off as Meckel’s cartilage, while the upper half, which develops from behind forwards, clearly represents the pterygo- quadrate bar. This becomes continuous with and later articulated towards its anterior end with the trabecula--a doubtless secondary connexion with the cranium seeing (1) that it arises from the anterior and later developed portion of the palate-pterygoid outgrowth and (2) that in primitive sharks such as Notidanus, in Lung-fishes, and in Urodele amphibians, the attachment of mandibular arch to skull is further back in the auditory region~—-in fact in the region of the original dorsal end of the mandibular arch.

In the lower Vertebrates apart from those mentioned the development of the cartilaginous mandibular arch takes place on

1 Graham Kerr, 1908. Attention is drawn in this paper to the need of an additional term to designate the more primitive ty e of so-called autostylic skull. A similar suggestion had, however, already been m e by Gregory (1904). V . THE SKELETON 321

similar lines. In the Reptiles and also in Birds the palato-ptery- goid outgrowth is again reduced in size———in correlation with the fact that in the Tetrapod Vertebrates the tooth-bearing function of the original upper jaw or palato-pterygoid bar has been taken over by the secondary upper jaw composed of bones such as the maxilla and premaxilla. '

BONY OR OSSEOUS SKELETON

Bone, like cartilage, is a modified connective tissue. In its typical form it differs from cartilage in the facts, that its matrix yields on being boiled a larger proportion of gelatine, that the matrix is rendered rigid by being strongly calcified, and that the cytoplasm projects as slender branching processes which ramify


FIG. 159. —Skeleton of visceral arches and pectoral girdle of 20'5 mm. embryo of Spinax. (After Braus, 1906.) -

I:I_, [;5__ In-um-lnial :u'clw.'~':: Hy, ll_\'0l(l: I, labial cartilage; M, nuuulilmlar :1l"(‘.ll: p, palato-pterygoid lmr ; ,.._/; rudiment of ]-«'x't‘.a_n'ul lin ; p.g, pectoral girdle; (3, knob for attachment to trabecular region of skull. -

through the matrix and are commonly continued into those of other cells. Many dill'erent varieties of bony tissue exist. In ordinary bone the cell elements are completely surrounded by the calcified matrix. On the other hand some of the cells may have the main part of their cell-body outside the calcified mass, only a slender prolongation being surrounded by it (Bones of Lepidosteus _and Amie). Or this peculiarity may apply to all the cells (Dentine of higher Vertebrates) or finally no cells or parts of cells are enclosed within the ‘hard matrix—-as is often the case in early stages of development ‘and as occurs in the adult condition in many Teleostean fishes.

Probably the most archaic type of bony skeleton in existing Vertebrates is seen in the Placoid scales Of the Elasnlobra-nchs and consequently the mode of development of these will logically fall to be considered first.

The appearance of the‘ scale is foreshadowed by a localized eorldensation of the dermal connective tissue immediately beneath

VOL. 11 Y 322 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the epidermis. Presently this begins to bulge upwards like a dome into the epidermis. The epidermal cells imniediately liounding this little dermal elevation take on a columnar shape; they constitute the enamel epithelium (Fig. 160, e). The (dermal) cells‘ on the surface of the dome or papilla, immediately underlying the enamel epithelium, also become distinct and form a. definite layer of odonto- blasts. ‘

The hard substance of the scale makes its appearance as a cone of dentine fitting over the surface of the dermal papilla and in turn ensheathed by the enamel epithelium. The dcntine cone, which usually becomes directed tailwards, gradually thickens, encroaching upon the dermal papilla or pulp which it surrounds. It lies

FIr_:. 160.——-Longitudinal vertical section through the skin of an crI1l'n‘_\'nni«: Shark to show a developing placoid scale. (From Balfour’s _IvJm.l2r.qnlo_:/y: ligun-. li_\'(_iL'gl_‘lll_)(l.l1I‘ afti.-r Hertwig, 1874.)

l'.', epidermis; «J, enamel epithelium; 0, enamel ; p, dermal papilla.

immediately outside the odontoblasts and as it increases in thiclmess the outer portion of some of the odontoblasts persists as a fine thread of cytoplasm extending out through the substance of the dentine, so that when dried the dentine is seen to be traversed by innumerable fine slightly diverging canals each of which contained a protpplasnniic thread.

The hard material of the dentine is cominonly regarded as calcified matrix but there is evidence which points rather to its being formed of modified cell cytoplasm. This point will be returned to in connexion with the development of the teeth.

Towards the surface of the cone the calcified substance changes its character. It becomes extremely hard (consisting of very dense calcium carbonate), transparent and highly refracting, and the terminal branches of the tubules within it are reduced to an extreme degree of fineness. This outer layer is commonly known as enamel. It may be comparatively thick, or on “the other hand it may be extremely thin as for example in the scales of Acantlums except the V PLACOID SKELETON 323

enlarged spine-like scales in front of the dorsal fins, on the anterior face of which it is well developed. -The enamel is in turn covered on its surface by an extremely thin membrane-like layer———the enamel cuticle-—and Huxley (1859) made out the important point that this is continuous with the basement membrane of the epidermis outside the limit of the scale rudiment.

In the teeth of the higher animals, which as will be seen later are simply modified placoid scales, the enamel is sharply marked off from the dentine and it is usual to regard it as of totally different origin namely as a kind of cuticular formation by the inner ends of the enamel epithelial cells. The chief reasons for this view are the sharp differences in appearance and composition from the dentine in these higher Vertebrates, and the fact that the cells of the enamel epithelium undergo a shortening as the enamel layer thickens—as if the inner ends of the epithelial cells were undergoing conversion into enamel from within outwards.

It is however curiously difiicult to find evidence sufficiently con- vincing to justify the almost universal acceptance of this idea even as ‘regards the higher Vertebrates. And in the case of the Fishes the evidence—---such as the location below the basement membrane and the frequently quite gradual transition between the so-called enamel

and the den tine—stron gly supports the idea that the former is simply a modification of the outer layer of the dentine.

The basal edge of the cone of dentine comes to spread outwards all round parallel to the surface of the skin as irregular trabeculae forming a strong basal plate by which the scale is firmly fixed in the dermis. This basal plate is usually of homogeneous appear- ance but its substance shows a gradual transition to the typical dentine of the spine, and in the case of 0a.llo7'7z.3/'n.clu4.s (Schauinsland, 1903) the basal plate as a whole shows, just as it does in the ancient fossil Coelolepids, dentinal structure. There seems then no reason to doubt that the basal plate is in its nature closely allied to dentine or in other words that it is bone in the broad sense of the term.

TEE'1‘II.—A section across the jaw of an ordinary Dog-fish is sufiicient to demonstrate the important morphological fact of the homology of the teeth and the placoid elements of the skin. Teeth are simply placoid elements belonging to that portion of the outer skin which is carried inwards to form the stomodaeum. Or con- versely the spines of the placoid scales are simply teeth which have not been carried inwards into the ston1odaeu1n. In accordance with this the placoid scales were long ago (1849) named, by Williamson, dermal teeth. The demonstration of the homology in detail will be found in a classical paper by O. Hertwig (1874).

The lining of the buccal cavity being morphologically part of the outer skin the probability is that originally teeth or placoid elements were distributed equally all over it. But in the evolution of the Vertebrata there has clearly taken place a restriction of the teeth to particular parts of the lining where they can be most 324 EMBRYOLOGY OF THE LOWER VERTEBRATES on.

effective. In some of the lower fishes (many Elasmobranchs, e.g. Acanthvlas) teeth of a simple character, practically unmodified plaeoid elements, are still to be found scattered over the roof of the buccal cavity and even extending back into the pharynx. Un- fortunately the development of these has not been worked out in detail. In Teleostean fishes however a very simple type of tooth development has been described ag. in the Pike (Esoa: lucius). The teeth are here no longer scattered equally over the buecal lining; they are restricted to the dentary, maxilla, vomer, palatine, and the inner surface of the visceral arches. The teeth on the roof of the month arise as simple conical dermal papillae which project into the epidermis and develop enamel, dentine, and an irregular trabe—

FIG. 161.-~~—Early stage in the development of the tooth in (A) '_..'u/‘ulm/1/..s* and (B) Lr:.pz'¢_/n.s-irm. (A after Semen, 1899.)

«I, dent'.inc; (M1, 0cl0ntul')lasts.

cular bony base on the same general lines as described above for the typical placoid element.

Relatively primitive conditions are found again in the Dipnoi and Amphibia (Urodela and Gymnophiona) in the latter of which the teeth may be very widely distributed (3.9. on preniaxilla, maxilla, vomer, palatine, pterygoid, parasphenoid (Spelerpes), as well as on the dentary and occasionally on the splenial. In the simplest cases the tooth originates as a simple conical or rounded dermal papilla which projects upwards into the ectoderm (cf: Oemtodus, Fig. 161, A) but in other cases an onward step has been made and the “ ectoderm ” in the region where the papilla develops tends to grow down below the general level of the ectoderm into the underlying connective tissue of the dermis (of. Lepidosiren, Fin‘. 161, B . ' ..

In )the Amniota this tendency becomes more pronounced, the ectoderm covering the tooth-germ not merely projecting downwards into the underlying mesenchyme but becoming constricted off from v THE TEETH 325

the rest of the ectoderm so as to remain connected with it only by a narrow stalk or isthmus.

The tooth is built up of precisely the same elements as the placoid scale~—-dentine, enamel and basal plate. I ts modifications are such as to make it more efficient for its special purpose. The projecting spine becomes exaggerated to form the functional part of the tooth: it remains conical, or it becomes a flattened blade with plain or serrated edge, or it becomes a low flattened crushing plate.

To secure greater strength the pulp may become traversed by hard trabeculae (vaso—dentine). .

Regarding each of the three elements mentioned above there is a certain amount of controversy. As regards the dentine there is the question of its origin-——whether it is to be regarded as calcified matrix or as modified cytoplasm. The evidence ol' Le/7id0sz'7'c7b - which on account of the size of its cell elements is always of weight in such questions~—scems very clearly on the side of the latter view. As shown in Fig. 161, T3, the cytoplasm of the odontoblast passes uninterruptedly into the calcified dentine, the spaces between the odontoblasts on the other hand dying away as the dentine is approached. But if in a relatively archaic creature like Lepidosiren the main part of the dentine is undoubtedly modified cytoplasm this at once raises a strong presumption in favour of the same being the case in the higher Vertebrates even if it be not actually obvious.

Again as regards the enamel it is taught practically universally that it is formed after the manner of an internal cuticle by the cells of the enamel epithelium. This idea has come down to us from the days of the early investigators who devoted themselves especially to the investigation of Man and those Vertebrates most closely allied to him. In those days the structure of the lower animals was interpreted according to the data obtained from Man and his allies. The whole outlook was the opposite of that which holds in these evolutionary days when the accepted principle of all morphological work is to interpret the higher and more complex animals by data obtained from these lower in the evolutionary scale. Applying this principle to the case of the teeth of the most archaic Vertebrates we see in the Elasmobranch fishes that the outermost layers of the dentine develop the special modifications already

‘alluded to——extreme denseness and hardness, transparency and high

refraction, reduction of the proportion of organic material, reduction of the tubular cavities. Here the enamel is undoubtedly modified dentine.

But if this be so there are only two alternatives open to us in interpreting the enamel of the higher Vertebrates. It is either to be regarded as a further stage in the differentiation of the outer layer of dentine or it is to be regarded as something quite new, a new substance formed by the enamel epithelium. This latter is the generally accepted View and in accordance with it the hard layer on the teeth of fishes was given by Williamson the name 326 EMBRYOLOGY OF THE LOWER VEBTEBRATES on.

Ganoine to distinguish it from the true enamel of the higher Verte- brates.

As regards the basal plate the main question at issue is the evolutionary one whether or not the view of Gegenbaur should be accepted that these basal plates constitute the iirst phase in the

. evolution of the bony skeleton. This question will more suitably be

discussed in connexion with the bony skeleton in general.

Lastly questions of general interest are raised by these excep- tional cases where the developing tooth cannot be traced into immediate relationship with the ectoderm. In Lepidosiren and P'ro£opte'ru.<: as well as in the Urodele Amphibians portions of the lining of the bueeal cavity which give rise to the teeth have the appearance of being derived in the embryo from cndoderni. Again in Teleostean fishes teeth are developed far back in the pharyngeal region, in other words in a portion of the alimentary canal which is lined with endoderm.

Such cases obviously cause serious trouble to those who apply

the germ layer theory rigidly. They explain them by supposing

that there takes place in development an actual spreading inwards of ectoderm over the surfaces on which teeth will‘ develop. As indicated in Chapter III. in dealing with the buccal lining of Urodeles and Lung-fishes the writer of this volume believes that the evidence adduced so far of the ingrowth required by this explanation is not to be relied upon. He would rather explain such cases as due to the more or less broad debatable zone between the ectoderm and cndoderin, the influence of one layer being liable to spread into the other and there being no sharp line the position in regard to which decides definitely to which layer a particular organ belongs. EGG-TOOTH or REP'I‘ILIA.—-—In the embryos of Reptiles there appears a precociously developed “ egg-tooth” at the tip of the

upper jaw which has for its function the rupture of the egg-shells

In Geckos there are a pair of these present, attached to the pre- maxilla close to the mesial plane. In other Reptiles the left egg- tooth appears only as a transient rudiment and the functional (right) tooth takes up a practically median position so that it appears to be unpaired. It is of interest that this holds also for snakes in which there are no definitive teeth in the premaxillary region (Rose, 1894). POISON FANG OF VIPERIDAE.—The poison fang of the Viperidae is highly specialized for the injection of poison, its pulp being traversed by a longitudinal tube, composed of dentine and attached to the outer wall of the tooth along its anterior face. The inner tube---or poison canal——passes at each end into an open groove the openings so formed serving for entrance and exit of the poison respectively. The main features of the developmentare illustrated by the transverse sections shown in Fig. 163 (p. 329). The poison canal makes its appearance as a longitudinal infolding of the dentine, the ectoderm of the tooth-germ seeming to push the dentine in before it to form a groove (Fig. 163, 7). The groove deepens and its V v THE TEETH 327

lips meet (6) so as to convert it into a tube. From its mode of formation thi.s tube is at first filled with ectoderrn of the tooth- germ. Eventually however this ectoderm disintegrates and leaves an open tubular cavity‘ (2).

Up till this stage the tooth is still enclosed in the ectodermal germ which has increased much in size (2) but eventually this

‘ectodermal mass also disintegrates with the exception of its outer-

most‘ layer, so as to give rise to the cavity of the sheath in which the functional tooth is contained. As the tooth becomes functional this cavity comes to communicate with the duct of the poison-gland so that it receives the poisonous secretion, and owing to the poison- canal retaining the form of an open groove towards the basal end of the tooth it in turn receives the poison from the cavity of the sheath.

'F‘I(:. 162.———Diagram illustrating tooth .\'1lm'C.<sin1l in an El:Is1noln':u1ch (A. primitive, J3, existing condition) ; an Amphibian ((_') ; and n l'i('}IillL' (l.uc¢-Wu) (1)),

41.11, lh-ntal gmova-: «Ll, «la-nml hnnina ; ,u..~'. plavoid .»-.-_-.-ah‘-; I, tooth.

Enamel is present as a thin layer towards the point of the fang: traced towards the base it passes into a simple fine cuticle-like basal

membrane. SUCCESSION or TEE'1‘H.-—In cases where the teeth have become restricted to special areas, and more particularly in cases where from

their shape and from the habits of their owner they are liable to be.

broken off or damaged by wear or otherwise, it is usual to find a special arrangement for the replacement of the lost or injured teeth by new ones. Such an arrangement is seen in its simplest form in an ordinary ])og-fish or Shark (Fig. 162, A and The portion of skin——-ectoderm with its strong and fibrous underlying dermis———to which the tooth-bases are attached is gradually, by processes of differential growth, caused to shift its position in an outward direc- tion over the edge of the jaw which supports it. This is brought about by the skin UI1d€l.'g()lll:_[' :1. continual slow process of absorption or atrophy along the outer margin of the jaw (about the point 328 EMBRYOLOGY OF THE LOVVER VERTEBRATES CH.

marked by a slight indentation of the surface in Fig. 162, B), while there takes place a compensating process of growth, or formation of new skin, along the inner margin of the jaw, near the bottom of a deep groove (Fig. 162, A, d.g). The young skin arising in this region, like young skin elsewhere, produces placoid elements and it is these which, growing older as they gradually move forward over the jaw surface, become the functional teeth. Under normal circum- stances the rate of outward progress is such that by the time the tooth is becoming inefficient through wear it gets into the absorptive region and is shed.

It should he mentioned that, to make it more easily intelligible, the above account has been simplified i11 one important detail. The replacement groove is as a matter of fact in the Elasmobranchs mentioned no longer an open groove. lts walls have become fused together, so as to obliterate its cavity and form a solid lamina of eetoderm which dips down into the mesenehyme round the boundary of the mouth, just within the jaw, as shown in Fig. l62, P».

In Amphibia the general arrangements for replacement of the teeth are similar to those of Elasmobranchs, as may be gathered from Fig. 162, C, but an advance beyond the Elasmobranch condition is found here in that the functional tooth is firmly fused to the jaw. It remains stationary throughout its period of active functioning and it is only at the end of that period, when it is lost either by being broken off or by a process of natural shedding accompanied by resorption, that the next replacement tooth in the series moves up to take its place.

In Reptiles (Fig. 162, D) again the general arrangement is similar except that economizing of material has now taken place, the dental lamina being relatively reduced in bulk between the tooth- germs, so that the latter project prominently from the lamina instead of being embedded in its substance as was the casein the Amphibian.

Amongst the Lizards certain modifications occur which are of importance as foreshadowing arrangements which occur in Mammals. Thus it may happen (Iguana, Leche, 1893) that the first generation of teeth to be formed never become functional but disappear before hatching. Again the replacement mechanism may become reduced in the anterior part of the series (Agama-—~Carlsson, 1896; Ohamaeleo-— Rose, 1893). In the Chameleon the ordinary replacement mechanism is no longer functional except at the extreme hind end of the jaw, where alone new teeth are produced.

The large poison-fangs of poisonous snakes are peculiarly liable to injury and we find, as might be expected, that the replacement mechanism is in their case particularly well developed. The dental lamina (Fig. 163, (U), which is very extensive and thinned down to such an extent as to become perforated by numerous openings in its more superficial and no longer active portions, is curved scrollwise upon itself and upon its concave surface develops tooth-germs in V i THE TEETH 329

rapid succession, as many as ten being visible at one time in the ordinary Viper. As the tooth-germs develop and approach the surface they take up a position in two rows (3, 5, and 2, 4, in Fig. 163)

The maxilla, which carries the functional fang, has two bases of attachment for teeth, an inner and an outer, and these are made use of alternately——a functional tooth with external attach- ment being succeeded by one with internal and con- versely.

The replacement takes place approximately syn- chronously in the two maxillae a pair of teeth attached to the right-hand bases of attachment of the two maxillae (in other words attached to the outer base of attachment on the right maxilla and to the inner base of attachment on the left maxilla) being replaced by a pair attached to the left-hand bases of attachment (inner on right maxilla, outer on left max- illa), In consequence of this F1c..163. ——I’art of transverse section tlirongh upper arrangemtant the individual Jaw of a young Viper. (After Rose, 1894.) teeth of 3 functional pair "Fhe ('ll'.\-’6l(')1)lll_',f'_ to-eth arr nninlwrml in m-ch-rot.‘ so-qin-lice.

- IV). 8 is not shown. lmrk tunn.:¢-ctodvnn; pale him.- are the Same dlstance from mesenchymt-. |.h-I1t'im'- is shown in black. The.-. cnamvl 0118 another as their pI‘ede- which is present. as a thin sheath over the apical l‘ll‘l of the “"33” 3"?‘ th"h'.*3“°°?SS°1‘S-. E33333-SZTJIZTi'.‘.'.T‘l21i..iii-1.71"‘.?ff???fT'iiifii'i“UT-£'l'f.’.'i3Z}.’.'.7I.§’3’?II

In this Inodlhcamon of 0 this has bemiiiv (‘.(lll\‘(!l'LI‘(l intoa lulu‘. (pm) still Iillml with the p1‘in1itiVe linear 0(.'l'.-U1lt'I‘lll (_‘t‘-ll:':l.‘,.lll :2 the cells luu-v de;_.--ein-1-:itml. lea\'iii;_-; :1 arrangement of the reP1eee- I;l°;?iT',.'i"‘SII",,‘i.Lii.l.§f}.it°i.§iIf §’.‘.l'.3t°‘iI;i'Ii'lJ.’.".iL';ll5I3 if,’ i3'.?..‘."i’i'$ ment teeth We have doubt‘ (~.a.\'it.y of the tOOt-ll-Sllflltll (S). The functional tooth (I) in less a nlechanigln to secure l-l|1'—.\‘8('ll()ll he-re ilgm-ad lwlmigs to tlic unto!‘ series. in whit-h

rapid      

in this connexion it 18 of , interest to notice that the replacement of the functional tooth is not dependent upon its having already suffered injury or become worn out but takes place at regular intervals (about six weeks in the case of the European Viper, Kathariner) while the snake is leading an active life. .

In the Crocodiles the dental lamina becomes broken up into a network, and finally reduced to a strand of tissue running longitudin-

ally along the jaw, slightly to the inner side of the tooth-bases. A V TOOTH-PLATES 331

The originally separate denticles develop as already explained (p. 324) in typical placoid fashion, giving rise to little hollow cones of dentine. Trabeculae of bony tissue (“trabecular dentine,” or

FIG. 164.—~Illustra.ting the dental arrangements in young Lung-fishes. (A .'l.ll(l B :ll'lu1' Sc-.n1nn, l_{\‘!)9.)

A, ronl nfnlmlt-l1 (of :1. Ca-mtodus of stage -15, showing Hui .<ep:1r:ile rnliivnl lo'~.'l.h ; IL 1:.-will of roof of mnuth frnm :i h'll;_5lll.l}' ynll-I120!‘ specimen (stage 46) after the sull fi.~'s11r‘.s' h:i\'n been l_‘ll‘ll.l'l?l(l :1.\\':i_\' h_\' Hm notion of dilute :1lk:Ili : (‘., LepidU.s‘i’I'8'lL, niact-~ml:ed upper jzuv of 5mm_: .»'pm-inn--.n, .~zlmwin;_-; l.ln- pnintvzl L‘.usp.s' still ['m~.s'o-nl. ml lhv luUl.lI-plat:-s; nIf1._ :1nlvrim' nuris; n/_/'13, pUHII‘I'inI‘ n:u'i.~a.

“pulp dentine") spread inwards from the bases of these cones through the underlying mesenchyme, so as to join up the various dentioles by a loose calcified spongework. As development goes on the trabeculae of this thicken, the pulp-filled meshes become pro- V TOOTH-PLATES 33 1

The originally separate denticles develop as already explained (p_. 324)_i_n typical placoid fashion, giving rise to little hollow cones of dentme. Trabeculae of bony tissue (“trabecular dentine,” or

FIG. 164.-——ll1ustra.ting the dental axrmngeinents in young Imn;_.;-lislu-s. (A and B allot‘ Sunmn, .l8!l‘.l.)

A, roof of mouth ofa Ceratodus of stagt: -Is, slum-ing thu .~wp:n':L1uwnlivsll tvvth : ls, tw-*l.h of roof of mouth from El. sllglitly }’()x1hRer sP*‘cln1en(atM§e 46) :xI‘lm- the .-on tissllc-s haw law-H ('|I*:ll'Ptl :n\‘:n_\' l»_\' the action of dilute» nlk.-zli: C, Lepi¢lo.z:inrn, nmcvmted tumor‘ _i.‘l\\ or .\oIHI:." -‘lN‘t'iIHt‘II, -lNH\'iH_=.'_' ll!" l"'i"“"l ('.11Hp.~i still 1)l'('.\'¢'lll ml 1..l|«- Loot-ln-]Il:lLc-.-; u/fl. !llllI‘l‘ll|l‘ nnris: u/_/"~', post:-riu1'11:u'is.

“pulp dentine”) spread inwards from the bases of these cones through the underlying mesenchyme, so as to join up the various denticles by a loose calcified spon §_v'v.\\'<'11‘l<. :\:-: development goes on the trabeculae of this thicken, the pulp-lilled meshes become pro- 332 EMBRYOLOGY OF THE L()WER VERTEBRATES CII.

portionally reduced, and the trabecular mass becomes the compact substance of the adult tooth. In the functional tooth the tips of the original denticles have completely disappeared.

In Lep7§a?o.s'7§ren and Protopterus the separate denticle phase of development is not so distinct as in Ueratodus but a reminiscence of it is seen in the pointed cusps which are present on the teeth in early stages (Fig. 164, C).

THE BONES IN GENERAL.--.The view is now accepted by many morphologists, following Hertwig and Gregenbaur, that the true bony skeleton has come about in evolution by the spreading inwards of bone-forming activity from the skin, where it arose in association with the coating of placoid scales which occurs in the lowest Gnatho- stomata. The probability of this view being correct is rendered apparent by a Sl1I‘\'ey of the phenomena of development of some of the bones in the lower Vertebrates. Both in Lung-fishes and in Amphibians the bones of the skull which carry teeth are found to arise in development in the form of more or less trabecular bony

tissue which spreads outwards from
the tooth-bases in the same way as

i has already been described as occur- 2 ring in the development of the com- pound tooth in Lung-fishes (Fig. 164) i ‘O. Hertwig (1874*‘) found for . example that the vomer, palatine 3:1 and opercular of Urodele Amphibians are developed in this way, forming perforated bony plates studded with conical teeth (Fig. 165). In the case of dentary, maxilla, and premaxilla, part of the bone arises in exactly the same way, while part on the other hand spreads through the mesenchyme without having teeth on its surface. It is to be noted that these bones at first, as frequently happens in the development of bony tissue, have no cells actually enclosed in the calcified substance. Later on the teeth in some cases disappear, leaving behind merely the basal plate of bone which gradually increases in thickness. On turning to the Anura it is found that the bony trabeculae develop precociously and form the basal plate of bone while the teeth belonging to it are delayed in their appearance and may even be omitted.

The embryology of the Amphibia then teaches us (1) that typical bones may be developed from the basal trabeculae‘ connected with placoid elements and (2) that a secondary modification may arise in which the tooth formation is delayed or suppressed, the trabecular basal plate simply developing by itself and becoming converted into the definitive bone.

L The facts as narrated by Hertwig for Amphibia do not stand alone. On the contrary an exactly similar mode of development is seen in


FIG. 165.—--Vomer of a 2'5 cm. larva of Axolétl (X45). (After 0. Hertwig, 1874*.) s V ORIGIN OF THE BON Y SKELETON 333

the “membrane” bones of the roof of the mouth in Teleosts, and in the tooth-bearing bones of Lung-fishes. Again there are present minute enamel-tipped teeth scattered over the surface of the dermal bony plates of Urossopterygians and various Siluroid Teleosts such as Lorz'car'ia, Hg/postoma, Oallvlchthys.

Such facts as those just enumerated seem to justify the acceptance, as a Working hypothesis, of the view that at least the more superficially placed dermal bones of the Vertebrata have actually arisen in the course of evolution from the basal trabeeulae or plates connected with placoid scales.

Admitting this a further question presents itself. What was the evolutionary origin of the more deeply situated masses of bony tissue, for example those which replace cartilage? Has the tissue which gives rise to these gradually been infected with bone-forming activity which has spread inwards from the skin? Or has this bone-forming power in the deeper tissues arisen independently? It is in this connexion extremely instructive to study the gradual spreading of the irregular shreds of bony material from the tooth-base of a Lepidosiren. They gradually spread onwards through the connective- tissue matrix like crystals forming in a fluid, and there is no apparent reason why such spreading should not continue for relatively great distances, provided the necessary pathway of connective tissue is present. It appears in fact thoroughly reasonable to regard the deeper portions of the bony skeleton, like the more superficial, as having arisen in evolution by the spreading inwards of bone-forming activity from the skin.

In considering this important morpliological problem, the origin of the bony skeleton, it must be borne in mind that the all-important fact, which far outweighs all other evidence available, is that in the Elasmobranchii, the group of gnathostomatous Vertebrates which is admittedly the most archaic, the placoid scales are the only elements of the osseous skeleton which have as yet made their appearance. There is no suggestion that the ancestors of existing Elasmobranchs ever possessed a bony skeleton apart from the’ placoid scales. Con- sequently in the Vertebrate groups which have been evolved sub- sequently to the Elasmobranchs the bony tissue must either be a further development of the bony basal plates of the placoid scales, or else a new independent development. If the former View is shown to have in its favour a reasonable degree of probability we are bound to accept it as our working hypothesis until a better is suggested, for it alone of the two views mentioned is really constructive, the other offers no explanation but merely the negation of an explana- tion. In the opinion of the present writer the reasonable degree of probability has been amply demonstrated by the facts which have been quoted.

It is also necessary to avoid attaching too great importance to the differences in detail which have arisen in the. evolutionary history of bony tissue under different circumstances. Such differ- 334 EMBRYOLOGY OF THELOWER VERTEBRATES cu.

ences may become conspicuous and highly characteristic —— for example the difference in relation to the calcified material——--whether the cell elements are completely surrounded by it as in the ordinary bone of the higher Vertebrates, or have merely a prolongation of the cell—bo(1y embedded in it as in ordinary dentine.

Such dit'l'ercnces in detail may be of great interest in themselves. For example the bony tissue forming the scales of Lrpidosteus is characterized by the fact that some of the bone - cells show the dentinal characteristic that the main part of the cell-body lies on the surface of the calcified material and only a prolongation of it is enclosed within the hard substance. Now Goodrich (1913) has made out the important fact that this peculiarity is not confined to the scales but extends to the whole of the bony skeleton. Such a fact is obviously a strong additional evidence of intimate evolutionary relationship between the scales and the rest of the bony skeleton.

Again such detailed differences may raise interesting problems, for example Whether the “ordinary bone ” type or the dentinal type (as is perhaps probable) is the more primitive type of bony tissue.

Interest in such details must not be allowed to obscure the main conception of bony tissue as contrasted with cartilaginous, or the problem of its evolutionary origin. As regards that origin We seem Justified in believing that bone formation has during the evolution of the Vertebrata spread from the dermis—fro1n the neighbourhood of the placoid scale bases——into the deeper tissues and so given rise to the deeper portions of the bony skeleton.. On the other hand we do not appear to be justified in regarding the evolution of the deeper parts of the skeleton as being due to a sinking downwards of actual individual placoid elements. Nor, in the author’s opinion, is there reliable evidence, so far, hearing on the further problem whether or not the first scleroblasts or bone-formin g cells of the Vertebrata were immigrants from the ectoderm. This view, which was supported by Gegenbaur, has a considerable amount of a- priori probability in its favour in view of the facts of skeleton formation in the lower invertebrates.

It is no longer possible in the present state of knowledge to classify bones, as did the older workers, simply into two sharply defined sets membrane bones and cartilage bones. The most that we can do is to recognize various stages in the process of shifting inwards from the skin, from which as already indicated they probably arose in the early stages of their evolution.

Firstly We have the most primitive type which may be termed dental bones, which are superficial in position and which still are connected at one period or other with teeth. Typical examples are the bones already referred to in the roof of the mouth in Amphibians.

A second category consists of bony plates which have lost their tooth structures and have sunk down to a deeper level. These frequently become applied to the surface of the cartilaginous skeleton, remaining separated however from the cartilage by a layer of un~ V B\ON ES AND SCALES 335

modified connective tissue. Such may be termed investment bones (Allostoses, Gaupp).

Finally a third category of bones are the substitution bones (corresponding roughly to the old group of cartilage bones; Auto- stoses, Gaupp). ln these the formation of bone has spread into the connective tissue in immediate contact with the cartilage, and as the tissue is formed, room for it is made by the destruction of the pre- viously existing cartilage, which it therefore comes to replace.

While it is convenient to recognize these three types of bone development, and probably justifiable to interpret them as represent- ing successive steps in the evolution of bone, it must not be supposed that they are absolutely distinct : intermediate forms occur frequently and a single bone of the adult may arise during ontogeny in part according to one type and in part according to another.

Bony tissue being rigid and inextensihle, it is essential to the functions of movement and growth, that it should not be continuous throughout the body. It consequently. takes the form of separate bones, the junctions between which are specialized either for move- ment, or for addition of new bony tissue at their margins. Each bone arises by the spreading outwards of bony tissue from one or more centres of ossification. The study of the arrangement and homology of the various bones constitutes an important part of the science of Comparative Anatomy———particular1y important for the reason that it is the bony skeleton alone which is as a rule preserved in the fossil remains of Vertebrates belonging to past phases of Evolution.

lt should be borne in mind that a single bony plate in such a part of the skeleton as the skull may represent ossification which has spread out irregularly from the bases of a large number of the original placoid elements. In View of this it will be realized that great caution must be exercised in homologizing apparently similar bones in different groups of the lower Vertebrates. Thus the same name ——implying homology-——is commonly given to similar bones in the skull of a (lrossopterygian, an Actinopterygian, and a Lung-fish or Amphibian. There is no guarantee of any precise homology in such cases and the student should be on his guard against taking very seriously the nomenclature of such bones as expressing exact and Well-determined homology.

FISH SCALEs.————In the Fishes, that is in those Gnatliostomata in which the skin has not yet become specialized for Respiration (Amphibians), or for protection against desiccation (Reptiles), or for diminishing loss of heat (Birds and Mammals), there is commonly present a coating of dermal bones which most usually take the form of scales. Such scales are in the most general terms simply plates of bone in one or other of its varieties. The development of what is probably the most primitive type-—the placoid scale—--has already been dealt with. It need only be added that individual scales, inter- spersed regularly amongst the others, pause in their development, and 336 EMBRYOLOGY OF THE LOWER VERTEBRATES Cu.

only proceed with the process when room is provided by the already developed scales becoming spaced out during the growth of the bod .

Tlfhilc the placoid scale is simply an individual dermal tooth the ganoid scales as seen in the surviving .l’0lg/ptems or Lejmltlosteus are on the other hand tooth-plates, numerous minute dentieles being associated with each sca.le. In these fishes also there is a certain amount of independence between the dermal plate of bone and the actual dentieles which are at first quite separate from it (N ickerson, 1893 ; Goodrich, 1908). The reduction in size of the dental cones and the loss of their attachment to the bony plate are steps towards their complete disappearance which has been reached in the scales of ordinary Teleostean lishcs.

In the ganoid scale of I’o/;c/pter-as or lugnidosteus the protective power of the bony plate has been greatly increased by its superficial layers undergoing modilication of an analogous kind to that of the superficial layers of the dentine cone in the teeth of fishes. This portion of the scale is extremely dense, hard and enamel-like and is without cells embedded in it. Like the corresponding layer in the tooth of a fish it is commonly known by Williamson's name Ganoine. The advisability of using this name, rather than enamel, rests mainly upon the assumption that enamel is a substance fundamentally

dilferent, derived from a different eell—layer, from bone or dentine. If

it be the case however that enamel is merely the superficial layer of dentine which has undergone secondary modification then there seems no particular harm in adhering to the custom—-until- recent years quite general-—-—of using the word enamel for the superficial layer of the ganoid scale.

Ganoid scales are still comparatively thick and bulky struc- tures but in the typical Teleosts the scales have become very thin plates of bone so modified as to be very tough and flexible, and overlapping like slates on a roof so as to be able to slide over one another during flexure of the body. This overlapping has been rendered possible owing to the surface of the scale being no longer inseparably linked to the ectoderm by the development of teeth. The scale is developed in the thickness of the dermis and it is only at its posterior edge, if anywhere, that it is even in early stages connected with the epidermis.

Vestiges of dermal dentieles have been described in Teleostean fishes and these deserve fuller investigation. Marett Tims (1906) describes a stage in Gadus in which the scale consists of separate platelets each with a tooth-like spine projecting from it, while in such South American Siluroids as Uallvlc/Ltlz,y.s and Loricaria the plates of the bony cuirasse bear numerous small spines which appear

to be typically tooth-like in structure. The scale is a plate of bone immersed in the dermis and it

therefore naturally grows by the addition of new bone all over its surface. In the ganoid scale the quality of the bone differs on the V THE SKELETON 337

inner and outer surfaces, that formed on the outer surface being the enamel or ganoine already referred to. In the highly evolved Teleost, where the scale has increased in area at the expense of thickness, the addition of new bone on the Hat inner and outer surfaces of the scale is relatively small in amount as compared with that round the edges. \

In accordance with variations from time to time in the metabolic activity concerned in the production of the new bone the latter tends to show variations in rapidity of growth, density and other characters, and consequently to show a more or less distinct layered arrangement. Where there are periodic variations in the metabolism of the fish-—associated it may be with sexual‘ activity or with food supply or with changes in the physical environment (6.;/. seasonal changes of temperature)»-these variations may be duly chronicled in the contemporary layers of the scales. Such scale records are often particularly distinct and easily observed in the scales of the Teleostei owing to their thin flat character and the predominance of growth at their edges. "

The development of the Cycloid scales of Dipnoi has not been investigated in detail. So far as the main features of their develop- ment are concerned they apparently resemble the scales of Teleosts. Like them they are, except at their posterior edge, deeply embedded in the dermis. On their outer surface they are prolonged into numerous, often recurved, spines which in all probability represent true denticles although they have lost their primitive relation to the epidermis.

BONY VER'1‘l«:nRAI. COLUMN.——II1 all gnathostoniatous Vertebrates, except the Elasmobranchs (including Holocephali) and Sturgeons, the vertebral column becomes in great part bony. The process of ossification is found in its first beginnings in the Lung-fishes, where the arches become ensheathed in bone.

In the bony Ganoids and Teleosts ossification usually commences in the connective tissue bounding the surface of the areualia, the first shreds of bone being completely coll-less. From the arches the bone spreads over the surface of the chordal sheath (in Amia it develops here iirst -Schauinsland) to form the rudiment of the bony centrum. In 6'0'reg()'Iuts it is stated (Albrecht, 1902) that for a time two bony rings can be distinguished round each centrum (of. variation in Am/(Ia. mentioned below on p. 339). From the thin superficial sheath of bone an irrogl1la1' spongework of bony trabcculae spreads outwards and forms the bulky centrum of the definitive vertebra. As this process goes on the basal portions of the cartilaginous arches become surrounded by bone and may persist as four tracts of cartilage running outwards through the bony centrum (e.g. Esom—Pike). Most usually the arches become completely bony: the original bony sheath covering their surface becomes perforated on its median side by invading vascular connective tissue which destroys the cartilage and deposits bone in its place.

VOL. II Z 338 EMBRYOLOGY OF THE LOWER VERTEBRATES C11.

The neural spine even when segmented in the cartilaginous condition becomes ensheathed in a continuous layer of bone.

In the case of Teleosts the cartilaginous stage of the haemal arches is frequently completely eliminated from development, the arches being laid down as bone in the connective tissue.

ln the Urodele Amphihia bone makes its appearance as a cell- less sheath round the surface of the centrum, which gradually increases in thickness and becomes cellular, enclosing connective- tissuc cells, and also spreads over the surface of the areualia. The cartilage becomes gradually absorbed and replaced by the bone.

ln S'phenucl072., which may be taken as an example of the more primitive Reptiles, a bony sheath similarly develops round the eentruin, but according to Sehauinsland it consists at first of a distinct dorsal and ventral half. The bony tissue of the dorsal portion spreads upwards so as to enclose the bases of the neural arch- elements but the main portion of the arch-element on each side becomes enclosed in an independent bony sheath of its own. This latter appears first on the outer side of the cartilaginous arch and may persist as a separate bony element for a long period, even throughout life in the Crocodiles and various other Reptiles.

Bone formation also spreads inwards into the substance of the cartilaginous centrum along what possibly corresponds to the boundary between the primary centrum and the chondrified tissue external to it (Fig. 152, p. 302). Thus arises a deep-seated centre of active bone formation.

From these various centres ossification spreads, the cartilage being gradually supplanted by bone. Not the whole of the bone so deposited is permanent: a great part of that lying outside the primary centrum becomes again absorbed, leaving a superficial tract connected with the more central portion only by sparse bony trabe- culae, the meshes being occupied by intrusive connective tissue.

An interesting adaptive feature is found in the tail region of certain Reptiles (Lizards, Sphe’It0/.l07t) which enables the possessor to break off its tail suddenly when seized by an enemy. In these animals the halves of the centrum derived from successive sclero- tomes have reverted to a condition of incomplete fusion—the ossifica- tion being more or less interrupted in the plane of contact of the two successive sclerotomes by a transverse septum of cartilage. As at the same time the corresponding connective-tissue septum between consecutive myotomes remains weaker than usual a violent contrac- tion of the caudal muscles is able to tear across both cartilaginous and connective-tissue septum and break off the distal portion of the tail.

In the case of the Birds it would appear that the main centre of ossification of the centrum corresponds to the deep-seated one in Sphenodon, the superficial bone-forming activity being much reduced. A characteristic feature of the Birds, associated primarily with their peculiar respiratory movements, is the extensive fusion which takes place between the vertebrae of the trunk region. v THE DEFINITIVE VERTEBRA 339

THE OoMPo‘s1'r1oN or THE DEFINITIVE VERTEBRA.--—-A fascinating but difficult chapter in Vertebrate morphology is that which deals with the composition of the definitive vertebra. We have already, in describing the development of the cartilaginous vertebral column, mentioned the elements which go to build it up—-neural, haemal, and central. The difficulties of interpretation arise from the fact that great variety shows itself in the ultimate fate of these elements and in the manner in which they undergo l'usion with their neighbours. This can perhaps best be illustrated by the case of Amie as described


FIG. l66.——Variation in vertebral column of A inria, according to Schauinsland (1906).

A, tail region of a 7-5 nnn. larva; B, postc,-.rier trunk region of an 18 cm. specimen ; 0, mid trunk 1-egion(lS (:.m.); 1), anterior trunk region (18 cm.). Fig. A is more highly magnified than B, C, and D. The position of the boundaries between success- ive myot.-omes or segnients is indicated by the intersegmental blood-vessels (-19). A, Ii, neural arch-elements; a, h, haemal arch do. ; a, 3, central do.

by Schauinsland. Here in some cases two amphicoelous centre (a. and ,8) are developed corresponding to a single segment, each one carrying its pair of neural and pair of haemal elements, those attached to the anterior centrum (A and ca) being relatively small, those on the posterior centrum (B and b) on the other hand well developed. Variations from this diagrammatic arrangement are found in different parts of the body. In the tail region (Fig. 166, A) the original condition frequently persists although in aged individuals the arch-elements (A, a) of the , anterior vertebra of the segment are liable to become completely over- grown and hidden by bone. On the other hand there frequently 340 EMBRYOLOGY OF THE LOWER VERTEBBATES CH.

takes place fusion between adjacent centra so that compound centra are produced. Most usually in this case it is thetwo centre (a, B) of one segment which undergo fusion but in some cases the posterior centrum of one segment fuses with the anterior centrum of the next so that the resulting compound vertebral centrum (/3 a) belongs to two successive segments. Again in some cases as exemplified by the specimen figured (Fig. 166, A) three successive centra may undergo fusion.

Towards the front end of the tail and throughout the trunk region the two centra of one segment undergo fusion but apparently the hinder ccntrum has undergone reduction in size with the result that its neural arch-element (B) becomes displaced on to the top of the smaller anterior element (A) (Fig. 166, B, (3).

Towards the extreme front end of the trunk the neural element B becomes practically intervertebral in position (Fig. 166, 1)). It is to be noticed that in these cases, where the element 12’ has been dis- placed, the bony splints which develop on its surface never spread downwards from it, so that it remains throughout life without any continuity of structure with the rest of the vertebra.

Towards the tip of the tail there is, apparently no regularity, all kinds of fusions and modifications of the various elements taking

lace. P To sum up we see in Amvla two complete potential vertebrae corresponding to each segment,‘ each with its central, its neural and its haemal elements.

The two vertebrae of a segment may be represented by the

A B formulae and !l3'where a and /3 are the centra, A a11d;1>’_{tl1e neural a 5 '

arch-elements and a and b the haemal arch-elements. In the trunk region the ordinary compound vertebra may be represented thus

AB

in./3 but in occasional cases fusions take place so as to produce vertebrae ab

BA ABA ABAB of the type or of the type pnfiti or'a-[id/3 . Such a fusion as ba. aba _c-zlbalii

that shown in the last formula produces a vertebra with a very long body upon which may persist four sets of neural and four sets of haemaleleinents. -

The great range of variation seen in Amie from the presumedly original condition, even in different regions of the vertebral column of one individual, emphasizes the need of much caution in the laying down of general principles regarding the composition of the definitive vertebra. It seems justifiable to admit the two pairs of neural and two pairs of haemal arch - elements into the general

1 Diplospondylous condition-—von Jhering. v THE SKELETON 341

scheme of a vertebral segment in addition to the central element. But a difficulty is at once raised by the not infrequent appearance (as in Amid and many Elasmobranchs) of two centra within the limits of a segment. To get over this it has been suggested that primitively there were actually present two complete vertebrae, each with centrum, neural arch and haemal arch, within the limits of a single segment (dispondylous or diplospondylous condition). Physio- logical considerations however support the probability of there having been primitively a single vertebral centrum, extending from about the middle of one pair of myotmnes to about the middle of the next pair. If it is borne in mind that the material of the anterior and posterior halves of the centrum is derived from two independent sources—-successive pairs of arch-elenients or of sclerotomes—it will seem reasonable to explain the occasional duplicity as due primarily to absence of the complete fusion which normally comes about between the halves of the centrum derived from the two sources, the two halves proceeding with their development independently. Con- versely a more complete fusion, extending over a number of these potential half-vertebrae instead of merely two of them, would lead to cases of elongated definitive vertebrae carrying a number of arches.

BONY Sl(ULL.——Tl10 caution expressed on p. 335 is especially necessary in connexion with the bones of the skull. Here we have a department of morphology which took shape in the early days of that science. The efforts of the older anatomists were devoted to the working out of homologies between the bones of different groups of Vertebrates and individual bones were given the same name-—they were decided to be homologous——mainly on the basis of similarity of relations in the adult animal, with only the most slender basis of either palaeontological or embryological knowledge. The consequent uncertainty as to the precise homology of similarly named bones in different groups of Vertebrates makes it in the author’s opinion impossible to write a satisfactory account without treating each of the main groups in detail by itself. As to do this would require more space than is available he would refer readers who desire such information to Gaupp’s volume and the literature there cited and will confine himself here to a very brief sketch.

In the lowest Gnathostomata, as represented at the present day by the Elasmobranch fishes, the skull retains throughout life its cartilaginous character, the bony tissue being confined to the placoid elements of the skin. In the other groups of Gnathostomata the purely cartilaginous condition is temporary, the cartilaginous skull becoming strengthened and in places, though never entirely, replaced b bone.

y In the floor of the chondrocranium there make their appearance a mid-ventral row of replacement bones the basi-occipital, the basi- sphenoid, and the presphenoid. Laterally to each of these elements the cartilage becomes replaced by a pair of bones known respectively 342 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

. as the exoccipitals, the alisphenoids and the orbitosphenoids. In

the occipital region the cartilaginous roof gives way to a replacement bone——tl1e supraoccipital--—but in the corresponding region further forwards the cartilage either does not develop or if it does develop never becomes replaced by bone.‘

In addition to the three groups of replacement bones mentioned, the cartilage of the sensory capsules gives way to replacement bones. In the wall of the auditory capsule there develops anteriorly a prootic and in the remaining part of the capsule wall there develop other ossifications differing in number in dilferent groups——as many as four in the ease of Teleostean fishes ———~ epiotic, opisthotic, sphenotic and pterotic. The wall of the olfactory capsule similarly becomes replaced by ethmoid, b«_mos—varying much in the difl'erent groups. The wall of the eyeball in a few cases develops a ring of flattened replacement bones in the substance of the sclerotic.

In addition to the replacement bones already indicated there develop others in connexion with the visceral arches. Thus we find the upper portion of the mandibular arch becoming replaced by the quadrate, the upper portion of the lower jaw by the articular and the palato—pterygoid outgrowth by palatine and pterygoid elements which however exhibit great difi"erences in their mode of development. The segments of the hyoid and branchial arches become replaced by various hyal and branchial bones in the bony fishes.

The bony skull is completed by numerous more superficially placed bones—-some belonging to the ordinary investment type, others retaining their dental character. The cranial roof immediately in front of the occipital region is typically covered by a pair of parietal bones; in front of these are a pair of frontals. In the Amniota particularly are developed such additional elements as squamosal, postfrontal, jugal. The Ethmoidal region develops such bones as nasal, prefrontal, lachrymal, septomaxillary.

In the region of the buccal cavity we find a particularly rich development of investment and dental bones. The function of upper jaw, originally exercised by the palate-pterygoid bar, becomes taken over by new bones--the maxilla and premaxilla~—-lying external to it, the palate-pterygoid bar becoming shunted inwards except its hinder quadrate portion and no longer forming the margin of the buccal cavity. Behind the maxilla or jugal there may develop a quadrato-jugal: in the region of the palate-pterygoid parts of the palatine and pterygoid are of this origin and so also are the vomer and parasphenoid.

Just as the primitive upper jaw becomes replaced function- ally by bones of superficial origin, so also with the lower jaw-- the original Meckel’s cartilage becoming ensheathed by splenial and dentary (the latter taking on the tooth-bearing function) with such other bones as angular, supra - angular, and coronoid. In

1 The pleuroccipital bone of the Dipnoi arises as afelose investment of the occipital arch (Agar, 1906). V BONES OF SKULL ' 343 '

the region of the Hyoid arch the Teleostomatous fishes with their greatly developed operculum develop a series of opercular bones.

Of these various bones mentioned in relation with the buccal cavity and pharynx the majority show more or less distinct evidence of their dental origin [l’remaxilla, Maxilla, Dentary, Palatine, Pterygoid, Vomer, Parasphenoid, ()pereular——cf. Hertwig,18'74*]. In some cases this maybe apparent only in part of the bone, the rest developing as an ordinary investment bone, while in a few cases bone which is in one part of the investing type may in another part present all the features of a replacement bone.

The student should recognize that the ossification of the skull, though tlillering greatly in degree in diflerent Vertebrates, is never complete. In the adult Elasmobranch the cranium is entirely cartilaginous, bone being confined to the placoid scales : in a Sturgeon there is still a well-developed chondroeranium but the surface of the head is covered with large bony plates: in such a T eleost as the Salmon the chondroeranium also persists to a great extent but extensive tracts of the cartilage are replaced by bone while the superficial plates of bone are now in much more intimate relations to the surface of the cartilage: in such a Teleost as a God again the cartilage is reduced in the adult to such an extent as to be quite inconspicuous. It never however completely disappears and the macerated skull of a Vertebrate as seen in an osteological or palaeontological collection is imperfect, being without parts which maybe of great morphological significance.

AUDITORY SKELETON.-——ln those Tetrapoda in which a tympanic membrane is present the vibrations of this membrane are trans- mitted through the tympanic cavity to a movable portion of the wall of the auditory capsule by a special arrangement of skeletal structures. These reach their highest development in the auditory ossicles of the Manimalia which have attracted much attention from students of mammalian anatomy and have been the centre of much controversy as to their phylogenetic origin. In the non-mammalian Vertebrates the two outer members of the chain of ossieles—the malleus and incus—have not yet made their appearance so that we are only concerned in this volume with the inner or stapedial portion which is represented in the Sauropsida and most of the Anura by the columella. auris. It will be convenient to study the develop- ment of this in the case of the Lacertilia in which it has been recently investigated by Versluys (1903), Cords (1909) and Good- rich (1915).

It will be recalled that the tympanic cavity is the dilated outer end of the spiracular or hyomandibular gill pouch, and the Eustachian tube is the inner or pharyngeal portion of this pouch. The pouch is for a time open to the exterior, forming an ordinary spiracular cleft, bounded in front by the mandibular and behind by the hyoid arch. In the hyoid arch is situated the main branch (hyomandibular) of the Facial nerve and from this, near its dorsal end, there comes off a branch—the chorda. tympani——-which runs in a ventral direction ‘344 EMBRYOLOGY OF THE LOWER VERTEBRATES on.

behind the cleft to its lower limit and then curves forwards beneath the cleft towards the region of the lower jaw and floor of the buccal cavity.

The external opening of the spiracular cleft gradually closes, from below upwards, as is usual with this cleft, a stage being passed through in which only the dorsal end of the cleft is open - precisely as in the adult of an ordinary Elasmobranch. As the lower limit of the opening gradually shifts dorsalwards the ehorda tympani remains in close relation with it so that the portion of the nerve on the ventral side of the opening assumes a more and more dorsal position. Eventually even the dorsal vestige of the cleft closes so that the spiracle has now reverted to the condition of a pouch. Owing to the shifting in position of the ehorda tympani as it followed the

retreating lower edge of the spiracular opening this nerve now .

passes forwards dorsal to the main portion of the pouch, instead of entirely ventral to it as it did originally.

The dilatation of the outer end of the pouch to form the tympanic cavity is brought about mainly by active growth of the lower portion of its posterior wall. This bulges outwards and spreads forwards and dorsalwards beneath the epidermis, from which how- ever it remains for a time separated by a considerable thickness of mcsenchyme. Later on this thins out relatively so that the three layers bounding the tympanic cavity on its outer side—endoderm, mesenchyme, ectoderm——form a thin 1nembrane——-the tympanic membrane. As the tympanic dilatation goes on expanding in a dorsal and anterior direction the ehorda tympani becomes displaced in front of it still further from its original position.

In the mesenehyme of the hyoid arch there takes place a gradual condensation to form the rudiment of the skeletal arch. The lower and main portion of this condensation becomes the cartilage of the definitive main cornu of the hyoid. lts dorsal portion also becomes converted into cartilage, taking the form of a stout red the inner (“stapedial”) end of which fits into the fenestra ova.lis——a vacuity in the wall of the auditory capsule——while its outer portion (“extra-columella,” Gadow) extends outwards towards the skin, embedded in the mesenchyme of the posterior wall of the spiracular pouch or tympanic cavity. Finally the lining of this cavity grows actively dorsally and ventrally to the columella so that it bulges backwards both above and below the columella. The pockets of tympanic lining so formed meet round the columella and fuse together so that the columella, instead of being embedded in the hind wall of the cavity, new passes right through it, enclosed in a delicate sheath of mesenchyme covered with endoderm. The exten- sion of the tympanic cavity backwards past the columella causes an extension of the tympanic membrane in the same direction so that the point at which the tip of the extra-columella reaches the skin, instead of being situated behind the tympanic membrane as it was originally, comes to be about the centre of that membrane. V AUDITO RY SKELETON 345

The general mode of development of the columella and the cavities associated with it as seen in Lcu:e"t(t appears to be typical of the Sauropsida in general. It is now necessary to refer to a few additional details. _

The inner end of the eolumella (stapes) fits into the fenestra ovalis. It is for a time, during prochondral or cartilaginous stages or both, continuous with the Wall of the auditory capsule and is probably to be interpreted as a portion of this wall which has become separate and movable.

C‘»hondrification of the columella commences in the Lacertilia from three centres according to Versluys and it is to be noted‘ that the separation between extra-columella and stapes arises secondarily within the region of cartilage which develops from the innermost centre.

In Birds an interesting variation has been discovered (Goodrich, 1915) in the relations of the chorda tympani. In the Duck these are normal, agreeing with what has been described for Lacerta. In the ordinary Fowl and the Turkey however the stage in which the chorda tympani is posterior to the hyomandibular cleft is omitted from development. Even in early stages it is found to pass in front of the pouch or cleft.

This is one of those cases which emphasizes the need of caution in regarding the course of a nerve as a necessarily deciding factor in discussions as to the morphological nature of particular organs. Position in regard to a particular nerve-trunk often affords us most valuable evidence regarding the primitive position of an organ. Here, however, we have it impressed upon us that we must never rely absolutely upon such a piece of evidence taken by itself. Were we to do so in this case we should be led into the absurdity of con- cluding that the tympanic cavity of the Turkey is not homologous with that of the Duck.

As a matter of fact nerve-trunks do not always form impassable barriers to the evolutionary change in position of organs. A skeletal structure may spread round a nerve-trunk (e.g. neural arches of Dog-fish) and becoming absorbed behind it may come to be transposed entirely past the nerve. In the case of the chorda tympani and the tympanic cavity it is clear that the nerve lay primitively behind and below the cavity and we may probably take it that, in accordance with the general principle that nerve-trunks tend to shorten and so economize material, in the course of evolution it became shifted dorsalwards through the mesenchymatous middle layer of the outer wall of the tympanic cavity before it became thin and membranous, so as eventually to lie completely dorsal and anterior to the tympanic membrane.

Incidentally the variation from normal development occurring in the Turkey and Fowl is one of those cases apparently impossible to explain on the outgrowth theory of nerve-development, but readily understandable on the view of nerve - development supported in 346 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Chapter II., according to which new nerve-paths may arise in response to the short circuiting of nerve-impulses.

A tympanic cavity with membrane and columella occurs in many of the Anura——as for example the ordinary Frogs and Toads (Rana, Bufo)--while in others such as the genera 1>’ombz'na.to7' and Atnlopus (“ P/W3/n?I.9cus") it is absent. In the Urodele Amphibians the tympanic cavity and membrane have not yet made their appearance. The inner end of the columella is however represented by a movable plate of cartilage fitted into the fenestra and in various genera the extra-columellar portion is represented by a rod-like outgrowth from this. The former apparently develops from the auditory capsule while regarding the latter there is much diflerence of opinion as to the extent of its relation to the cartilage of the hyoid arch. The disagreement between different observers probably means that there are actual differences between diiferent genera of Amphibia. This is quite what is to be expected, for whenever we find a single organ of which part is derived from one embryonic source and part from another the proportion contributed by the two sources is liable to vary, so that in one case it may be the portion derived from the one source which is conspicuous and in another case that derived from the other.

The tympanic ring‘ within which the tympanic membrane is stretched arises in the form of an outgrowth from the rudiment of the quadrate, 73.5. from the upper portion of the skeleton of the mandibular arch. This outgrowth separates off and grows round the outer end of the hyomandibular pouch in the form of a crescent the two horns of which eventually meet to form a complete ring.

SKELETON or THE MEI)IAN on UNPAIRE1) FlNS.———The median fin, thin and membranous as it is in its most highly evolved condi- tion, is supported by characteristic skeletal arrangements. Into these two distinct elements enter, one mesial represented by rays of cartilage or bone (radials), the other superficial and of dermal origin.

The mesial fin-rays are frequently i11 close ' relation to the neural and haemal arches and it is reasonable to suppose that in the process of evolution, as the hind end of the body became extended in a dorsal and ventral direction, so as to attain to the flattened form conducive to efficiency in propelling the body, the neural and haemal spines underwent a corresponding extension for the purposes of support. This view is corroborated by the existing Dipnoi in which the mesial fin remains a comparatively slightly difi"erentiated extension of the body dorsally and ventrally and in which the mesial supporting elements are simply the prolonged neural and haemal spines, each secondarily subdivided into three segments. The same is the case in Fishes generally so far as the ventral portion of the caudal fin is concerned in which the mesial supports develop also for the most part as typical haemal spines.

The mesial supports of the dorsal portion of the median fin on the contrary do not in Fishes generally show this relation to the i oped at particular points

V MEDIAN FIN SKELl£'I.‘ON 347

vertebral arches. They arise, any. in Elasmobranchs, in ontogeny as independent rods of cartilage without definite relation to the metamerism of the body and later on become segmented into three pieces. In those cases, so far as they have been investigated, in which the radial elements are connected with a continuous basal plate of cartilage, this latter appears to arise in ontogeny as a continuous plate, though there is no reason to doubt that it arose in phylogeny by the fusion together of the basal portions of originally separate rays.

This want of correspondence of themesial elements of the dorsal fin skeleton with the vertebrae is probably sufficiently explained as a secondary result of the prolonged working of the general principles which have governed the evolu- tion of the median fin and which find their ex- pression in the tendencies (1) of the continuous fin to become specially devel-

and to die away in the intervening spaces, (2) of the resulting separate fins to have their base of attachment to the body shortened and (3) of these fins to be situated on the body at the points

where they are mechanic- s

most effective. FIG. 167.——Two sticeessive stages in the development

DERMAL SUPPORTS OF of the lepi(lotl'iclnia of Srrlmu.

MEDIAN FINS. __ The A, Sahnon (afte-r Harrison, 1893); B, 'l‘rout.(aft:-r(l0odricl1, . fi b . . 1904). h.m, lmsa.-nu-nl. llll‘llll_|l':|nl‘: m'I_, m-hulv1'n1 : I, lvp1«'lo- nledlan ns 9138 PH’ triuhial rudiment: ; ma__'.~', lIln'.-‘o*.lIt'll_\'lIlt? of dc-r1ni.~'.

marily mere extensions of

the body in the vertical plane it would only be reasonable to expect that they would show traces of skeletal elements comparable with the placoid elements or their derivatives characteristic of the rest of the surface. And in fact the dermal skeletal supports of the median fins can, some of them, he clearly recognized as homologous with scales, while in others although this may no longer be recog- nizable their origin is found to be closely associated with the basement membrane as was the case with the dermal teeth.

_It will be convenient to consider first of all the dermal skeletal elements in which the direct relation to scales is most clear. Such are the bony fin-rays of Crossopterygian and Actinopterygian fishes. In an ordinary Telecst (Fig. 167) the fin-rays of this type (lepido- trichia, Goodrich) appear in their earliest stage, as shown by 348 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Harrison (1893), in the form of a localized thickening of the basement membrane underlying the ectoderm. This thickening becomes more and more marked and eventually separates oil’ round its edges in the manner shown in Fig. 167, ll, mesenchyme insinuating itself all round between the ray and the basement membrane, so that the former eventually lies free from the basement membrane (or in some cases still connected with it by narrow bridges) deep dow11 in the mesenchyme. The ray soon,beoo1ncs calcified. New layers are deposited on its inner and outer surfaces, mesenchyme cells become included within its substance and it becomes a plate of ordinary bone. The rays are elongated structures which develop from the [in base towards the tip. They often become jointed, either by calcification being interrupted at intervals (Goodrich) or by a secondary solution of continuity (Harrison).

ltays are formed in the manner described on each surface of the thin membranous fin. The rays of opposite sides correspond exactly in position and become later on [used across the mesial plane so as to form a single unpaired ray whose paired origin is indicated only by its forking at its inner end to embrace the tip of the median radial, the process of fusion between the two elements not taking place at this proximal end. In the dorsal and anal fins and in the ventral part of the caudal fin the lepidotriehial fin-rays correspond segment-ally with the true median skeletal elements the tips of which they embrace as indicated above.

In the more primitive Toleostoines the identity in nature of these fin-rays with the scales which cover the rest of the body is still more obvious. ln Pol;/pterus and Lepidosteus they develop a coating of ganoine and even hear distinct small dentieles on their surface. It is also interesting to notice that in the anal fin and ventral part of the caudal fin of Poly/pterus the fin-rays at their proximal ends merely pass in beneath the edges of the body scales and do not take .on any relation to the true median skeletal elements. The palaeontological fact may be recalled in passing that in some of the extinct fishes a perfect gradation can be traced between the fin-rays and typical body scales (see Goodrich, 1904).

In addition to the fin-rays just described, the homology of which with scales may be taken as well established, it is very usual to find another type of fin-ray in which this home- logy is not so obvious. This is exemplified by the horny fin-rays (see Goodrich, 1904) which occur in the fins of Elasmobranehs (including Holocephali), in the “adipose” fin of Salmonids and Siluroids, and towards the margins of the fins generally in adult Ganoids and Teleosts. These horny fin—rays develop either as thickenings of the basement membrane or at least in immediate contact with it. Mesenchyme cells insinuate themselves between the ray and the basement membrane as the ray separates ofi". The ray gradually becomes farther removed from the basement membrane and mesenchyme cells collecting round it deposit fresh layers on its V FIN-RAYS 349

surface so that its diameter becomes increased. In the fins of Elasmobranchs and in the adipose fins of '_l‘eleosts these horny fin- rays become much elongated, but ordinarily inlthe Teleostoinc they become relatively shortened during development, apparently being absorbed at their proximal ends while they grow at their distal ends, so that in the fully developed lin they form merely a marginal fringe-—the individual horny rays being concentrated about the ends of the lepidotriehia.

The fin-rays in question have‘the main feature in common with the lepidotriehia that they arise in very close relation to the base- ment membrane and later on separate from it and sink into the underlying mesenchyme. It is probably allowable to look upon them as being of the same nature morphologically as the lepidotriehia but as having evolved still farther from the primitive scale-like condition. In Lung-fishes horny fin-rays occur of somewhat inter- mediate character in the form of slender parallel rods irregularly jointed and branched distally. In the later stages of their develop- ment these rays are apt to assume a bony character, becoming strongly calcified and enclosing branched bone corpuscles in their substance. The early development of these rays has not, so far, been worked out

‘ in Oaratodus in which they are best developed. In Protopte'rus

Goodrich found them in early stages in at least very close proximity to the basement membrane.

We may probably look upon the dermal fin-rays of fishes as belonging to one morphological category, representing structures of originally placoid nature which have sunk down into the mesen- chyme and degenerated, or-——to use a preferable expression--become specialized, into more or less horny structures. The earliest stage in ‘this process we should see in the lepidotriehia of Teleostomatous fishes where the scaly nature is still quite clear. The fin-rays of Lung- fishes would represent a farther stage, in which the scale homology is no longer clear, and the horny rays of Elasmobranchs and Teleostomes would represent the final stage, in which all trace of the original nature had disappeared except the origin in close association with the basement membrane.

The lepidotriehia of the Teleost and its horny fin-rays would represent different generations, the lepidotriehia being a later genera- tion which have, as it were, had less time for modification. In this connexion it should be mentioned that the horny fin-rays of the Elasmobranch do not all belong to one generation. Their production from the basement membrane may go on for a long period so that instead of a single layer a thick mass of rays may be developed.

SKELETON OF THE PAIRED LIMBS

I. FINs.—-To be consistent with the plan adopted in this book we should commence with the development of the paired fin in its most

nearly primitive existing form. Before this can be done it is necessary 350 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

to decide which is the most nearly primitive of the various forms of paired fin met with in surviving fishes. The present writer takes the view that undoubtedly the most primitive type of paired fin known to occur in existing Vertebrates is the paddle-like limb of Ceratodus.

Physiologically this type of fin is as clumsy and archaic an organ in‘

comparison with the paired fin of a Shark or Actinopterygian fish as is the most primitive type of savage’s paddle compared with a racing car. Further we know from the data of palaeontology that the Oeratodus type of limb is of great antiquity and that it was a common type of fin amongst the more ancient Sharks and Ganoids as well as amongst the Lung-fishes. There are only two possible explanations of its occurrence in the three groups mentioned. Either (I) it is an archaic type of 1111 inherited from the common ancestors of those groups or (2) it has been evolved independently in the three groups. The latter explanation seems very improbable-—~for such a type of organ would become evolved independently in different groups, only if it were physiologically very efiicient. But it seems quite impossible with knowledge of the structure of the limb of Cemtoalus and its use in the living animal to regard it as an organ of great locomotor efficiency. Apart from this consideration we have the historical fact that this type of fin has vanished away entirely in the two successful types of fish, the Sharks and the Teleostomes, and has persisted unchanged only in one of the surviving Lung-fishes. There seems then no escaping the first of the two possible con- clusions mentioned above, that the paddle-like fin of Oemtodus and the ancient Lung—fishes, Ganoids and Sharks is a common heritage from the ancestral group out of which these fishes evolved and is therefore the most archaic of the known types of paired fin.

DEVELOPMENT or THE PECTORAL LIMB SKELETON IN CERATODUS. The skeleton of the limb makes its first appearance (Semen, 1898) about stage 45 (Fig. 201) as a rod-like condensation of connective tissue along the axis of the limb which tapers gradually towards the apex. Histological difierentiation, by which this rod-like structure passes through a prochondral into a completely chondrified condition, proceeds from the base towards the apex. While in the prochondral condition the rod is a continuous structure, chondrification takes place from separate centres, with the result that the rod becomes converted into a series of blocks of cartilage, each separated from its neighbours by a thin layer of unchondrified tissue. The basal block, lying within the body wall and spreading in a ventral direction, is the rudiment of the pectoral girdle; the rest of the series forms the axis of the limb.

The lateral rays make their appearance later, the development again proceeding from base towards apex, and those on the preaxial or definitively dorsal side preceding those on the postaxial or definitively ventral. Each ray spreads out from the prochondral tissue between two segments of the axis and it is noteworthy that rays develop from the first (proximal) of these intersegmental joints although in V PECTORAL LIMB SKELETON 351

the course of further development these, doubtless to give greater mobility to the fin, disappear. In the details of its development each ray repeats that of the main axis.

In addition to the normal rays, which are attached to the axis at the level of the intersegmental spaces, occasional rays make their appearance opposite the segments themselves. According to Semen these sprout out from the thin superficial layer of the axial tissue which like that between the segments persists for long in an unchondrified condition. These extra rays are most frequent on the postaxial side of the limb which in the pectoral limb becomes ventral, in the ease of the pelvic limb dorsal (see Chap. VII.).

Growth of the fin and of its enclosed skeleton continues for a long period-—-even after the adult condition is attained. As regards the skeleton this continued process of growth takes place by‘ two methods (1) by a simple continuation of the extension at the apex, and (2) by the already formed elements of the cartilaginous skeleton, axial orradial, continuing their individual growth in size.

As the definitive condition of the ‘skeleton is reached, the inter- scgmental tissue chondrifies, towards the apex forming soft hyaline cartilage with a sparse matrix and towards the body taking the form of fibro-cartilage. Towards the base of the limb this fibre-cartilage develops many fluid-filled cavities so as to assume an almost spongy character and in this way give greater mobility. This is specially marked at the junction with the shoulder girdle and between the first and second segments of the limb—axis and in these -two cases the apposed surfaces of hyaline cartilage are curved and concentric so as to afford a distinct development in the direction of a true ball and socket joint.

The details of development of the pelvic limb skeleton apparently agree with those of the pectoral. In this case also the girdle arises in the form of two originally separate halves.

ELASMOBRANCIIII.——The earliest stage in the development of the pectoral limb and its girdle in Elasmobranehs is that described by Ruge and by Braus (1904) in Spinaw where there exists a condensa- tion of connective tissue in the form of a curved rod on each side of the body close under the skin, in the position shown in Fig. 159, pg (p. 321). This forms the rudiment of the pectoral girdle. From it there grows outwards a short projection into the limb-rudi- ment which as it is clearly homologous with the axial cartilage of Oemtoolus we may call by the same name. The girdle rudiment increases in length both dorsally and ventrally, and ventrally the two rudiments come to be in apposition. On each side a tract of cartilage now develops in the prochondral rudiment: the two cartilaginous rudiments show a similar dorsal and ventral extension and presently they also come into apposition ventrally and form a continuous structure across the mid-ventral line.

In other Elasmobranchs (see Mollier, 1894) the conditions appear to be similar on the whole to those described in Spmaw. 352 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The prochondral rudiment of the axial cartilage extends out into the limb-rudiment, forming a broad plate which tapers oil’ posteriorly (Fig. 168). The condition is in its essentials the same as that in 0e-ratodas except that here the axial rudiment is laid back along the side of the body. The prochondral fin-rays arise, as in Ceratodus, in the form of outgrowths from the axial element. These are restricted to the outer (preaxial) side of the limb. They develop in Spi'na.:z3 (Braus) in series from before backwards except that anteriorly, in the region which will give rise to mesopterygium and propterygium, a few rays develop in the opposite sequence from behind forwards.

The chendrifieation of the limb skeleton appears to take place in 1l['u.stelus and ’1'o'rpedo continuously but in 5Ep'i'ru(.33 it sets in first in the axial portion and then in the rays in the same succession as they flrst appear. 'l‘he separate segments of the rays in Spirnaaz also Fm163___SMi0-nthrong}, develop in succession as separate centres of

pectoral lin of Torpedo chondrification.


<;:1::r5':;'I*af;‘::l161 To understand the morphological relations

.” - Q ' of these earl sta es it is advisable to refer

Moll1er,l«.94.) y - . _ _ _ back to the paddle type of limb as it exists

Tl‘ -h 1 Ir l'me1t . . . omif, _,“,’f:,‘.’,,;:,’,‘,1‘i,‘,“_..,,,,:,‘:,,',, ' 111 the ancient sharks oi the genus Pleura-

cant/ms. Here (Fig. 169, B) we find a limb resembling generally that of C'emto(lus but difl'ering from it in two conspicuous details. (1) The skeletal axis has become relatively larger and clumsier, its original elements having probably under- gone extensive processes of fusion both with one another, as shown by the fact that the eartilages of the axis are in places less numerous than the lateral rays, and also with the basal portions of these lateral rays. (2) The rays on the pestaxial side of the limb are much reduced in number, only a few persisting towards the limb apex. -

The tendency of the postaxial rays to disappear in these archaic sharks (and the same tendency is seen in Lung-fishes) justifies us in believing that the external side of the pectoral limb-axis in the young Elasmobranch is morphologically p’I’6a.’13ial. This conclusion raises the interesting question Are there any vestiges of postaxial rays to be found in existing Elasniobranchs ‘? This question has to be answered in the affirmative. In Oentrophorus (Fig. 169, C) Braus finds a number of postaxial rays near the tip of the fin in a late stage of development; in Spiaaa: at least one similar piece of cartilage occurs; and even in the adults of various Sharks Gegenbaur and Bunge found similar vestiges. As vestigial organs are notori- ously variable more extended investigations into the occurrence of such vestigial postaxial rays are very desirable. They should be carried out on as many different species of Shark as possible and V SKELETON OF THE PAIRED FIN S 353

on as large as possible a number of individual specimens of each 8pe(31(3S.

The skeleton of the pelvic girdle arises in a manner similar in its main features to that of the pectoral girdle. It is however character- istic of Elasmobranehs (except Holocephali) that the portion of the girdle dorsal to the attachment of the limb undergoes atrophy in later stages of development. As in the pectoral fin an axial cartilage appears with fin rays sprouting from its external side. Here also a separate cartilage develops anteriorly with a few rays attached to it but it is doubtful whether it is justifiable to homo- logize this in detail either with the propterygium or the meso-

pterygium of the pectoral fin. The cartilaginous skeleton ol' the clasper arises in continuity

FIG. ‘I69.-——Pectoral fin skeletons of : A, (}eratmlu.s- (Semen) ; B, l’lmI.1'rI,cantlm.9 (Fritsch) ; C, (}’0nt'rophrn'u.s' mnliryo (Braus); D, Actmt/u'u.s (Gegenhaur); E, Olatloselache (Bash- ford Dean) ; F, I’ul_z/pterus larva. (Budgett) ; G, I’olypteru.s' (Wiedersheim).

The outer or preaxial side of the limb is to the left, except in A.

with the rest of the fin skeleton and appears to consist of the tip of the limb axis with possibly a few modified rays. The claw-like structures are simply modified placoid scales.

TELEOS'1‘0MI.—-—As regards 1’olg/ptcms, commonly regarded as the most archaic of existing Teleostomes, our knowledge of the develop- ment of the limb skeleton is fragmentary. In the larva of stage 36 (Fig. 197) the skeleton of the pectoral limb is in the form of a thin lamina of cartilage with small irregularly scattered perforations. This is connected with a shoulder girdle rudiment consisting of a simple curved rod of cartilage. In the 30 mm. larva described by Budgett (1902) the girdle has become shortened into a compact block of cartilage and the cartilaginous plate lying within the limb itself has become thickened along its anterior and mesial edges. These thickened portions are separating off to form the rod-like “ proptery-

voL. II 2 A 354 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

gium ” and “ metapterygium ” (Fig. 169, F). The main portion of the plate is becoming split up towards its margin by a series of slits into a number of radiating pieces which represent the separate radii of the fully developed fin (Fig. 169, G-). The close correspondence between the fin skeleton at this stage of its development and the fin skeleton of a shark is obvious from Fig. 169.

In Actinopterygians the pectoral limb skeleton is in the pro- chondral stage a continuous mass, of which the anterior and mesial part separates oil’ to form the pectoral girdle, while the distal portion spreads outwards to form the rays.

The pelvic limb skeleton shows a fundamentally similar origin from a continuous proehondral rudiment, but here the skeletal rudi- ment appears first in the projecting limb and only secondarily spreads within the body wall into the region of the pelvic girdle. It appears to the present writer that no special weight need be attached to such cases where the skeleton develops earlier in the limb than in the body wall: they are probably to be regarded simply as special cases of the frequent tendency for highly specialized organs to be laid down precociously in development.

II. LIMBS or TIIE ’]‘Er1:AroDA.-—-In the Amphibia also the pectoral girdle and the skeleton of the limb itself are foreshadowed by a single condensation of mesenchyme which extends in a dorsal and ventral direction to form the girdle and out into the limb to form its skeleton. N 0 general rule can be given as to the relative time of development of the various parts. In Bombinator according to Goette the girdle rudiment appears first and the limb skeleton sprouts from it: in Proteus according to Wiedersheim the limb skeleton appears first and the girdle later. In the girdle rudiment the dorsal or scapular portion becomes apparent first. Chondriiication takes place separately in the girdle and the limb, the joint remaining unchondrified.

The cartilaginous pectoral girdle of the Amphibian, as of other quadrupeds, takes on the form of a A upon each side of the body- the three branches of the A being known as scapular, coracoid and precoracoid portions respectively and the glenoid articulation for the limb being situated at the meeting point of the three portions. As the two ventral branches of the A are in some cases continuous with one another at their tips through a strong membrane, it seems not improbable that they had originally the form of a continuous flattened plate of cartilage, of which the central portion has now disappeared, leaving the thickened marginal parts as precoracoid (anterior) and coracoid (posterior) respectively. On this view the epicoracoid when present would represent the persisting thickened ventral margin of the primitive girdle.

In actual ontogeny the three branches spread gradually outwards from the original rudiment, while the epicoracoid when present is formed by the coracoid spreading forwards at its ventral end and fusing with the end of the precoracoid. The two lateral halves of v SKELETON OF THE‘ LIMBS 355

the girdle come to overlap one another in the mid-ventral line and in the case of the higher Anura complete fusion takes place.

Amongst the Reptilia the first rudiment of the pectoral limb skeleton has been investigated by Mollicr (1895) and found to consist of a condensation of mesenchyme in the glenoid region corresponding partly to the glenoid portion of the girdle and partly to the basal portion of the limb skeleton——the two being thus again continuous at lirst. The girdle portion of the rudiment spreads ventrally to form the coracoid region, then dorsally to form the scapular. The chondrification of the various parts takes place in the order of their appearance.

In (Jhelonians the girdle takes on the typical A-shaped form with a more or less pronounced projection from the lower end of the coracoid forwards towards the lower end of the precoracoid which apparently represents the epicoracoid of Amphibians. In Sflmzodon and in Lizards on the other hand the ventral portion of the cartilaginous girdle consists of a flattened plate which may become perforated by several foramina. Whether this flattened ventral portion corresponds to coracoid and precoracoid is doubtful. It seems on the whole more probable (Goette) that the precoracoid has disappeared in these forms owing to its functional replacement by the clavicle, a process seen in its incipient form in Anura. This view is supported by the occurrence of a distinct strand of condensed connective tissue in the position where the precoracoid should be though in this case it does not become chondrified but becomes replaced by bone (clavicle) at a later stage.

In Birds the girdle forms a simple curved rod without any bifurcation into coracoid and precoracoid portions ventrally.

Each lateral half of the. pelvic girdle of quadrupeds is, like the pectoral girdle, typically of a A-shape, the three limbs being known here as ilium (dorsal, more correctly iliac bone or iliac cartilage), pubis (anterior) and ischium (posterior). The frequency with which the pubis and ischiuni are continuous at their ventral ends suggests that here also they represent the persisting thickened marginal parts of a once flattened plate-like ventral portion of the girdle.

As in the case of the pectoral girdle the three processes are formed by simple spreading outwards from the original rudiment. In Amphibia chondrification takes place apparently from a single centre on each side (’1'rv}ton, Bunge, 1880) giving rise to a pair of longitudinal plates of cartilage which meet ventrally.

In Reptiles each half of the pelvic girdle passes through the typical A-shape. The ventral end of the pubis, like that of the ischium, meets its fellow across the mid-ventral plane forming a symphysis. In some cases, e.g. Sphenodon and certain Chelonia, the pubic sy‘1nphysis becomes connected up with that of the ischia by a longitudinal bar of cartilage. ln the_Crocodiles the pubic portion of the girdle becomes eventually segmented off at its dorsal end from the

rest of the girdle. 356 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

In Birds the pelvic girdle during the prochondral stage passes through the }\-shape, the right and left halves distinct from one another and each at first continuous with the skeleton of the limb.

Pubis, iliuin and isehiuni usually chondrify separately but in many ‘

cases (e.g. in the Coniinon Fowl usually) ilium and ischiuni may become chondrified in continuity, and less frequently all three elements ehondrify in continuity. A highly characteristic feature of the avian pelvis is that the pubis swings in a tailward direction about its attached dorsal end until it assumes a position parallel with that of the pubis. ln the pelvis shown in Fig. 170, B, this rotation is just commencing.

The Gheiropterygium (Huxley), or skeleton of the limb in Amphibia and Aniniota, consists of three distinct portions corre- sponding respectively to the Upper Arm or Thigh, the Forcarin or Leg, and the Hand or Foot. As these por- tions are looked upon as homologous in the fore and hind limbs it is con- venient to have a niorphological name

 for the corresponding parts of the two

[2 B "L-=.s_ sets of limbs, and such names have

_ . _ been proposed by Emery and Haeckel

F”:' :70‘ i"‘Si‘1“;tT£l"“’}ffhP°l:‘“1g’g33l"‘-l --- Stylopodium, Zeugopodiuni or

“"',”’_"’_‘l" ‘(B “_ "_ Zygopodiuni, and Autopodium. In

 ;.;.:.;:'“  the autopocuum there may rurmer

' be recognized Basipodium (carpus or

tarsus), Metapodiuni (nietacarpus or inetatarsus) and Acropodium (phalanges). _ _ _ _ p

The limb skeleton is typically at first quite continuous. A rod- shaped condensation‘ of inesenchyme appears first in the limb stump -——the rudiment of the stylopodiuin (femur or humerus)———and as the limb grows this spreads outwards, bifurcating as it does so to form the rudiinents of the zygopodial skeleton: wi.tli further growth the two limbs of this unite distally to form the rudiment of the auto- podial skeleton. Chondrification takes place from the base of the limb outwards, each separate element of the adult making its appear- ance as a separate chondrification centre.

The skeleton of the autopodium originates in a flattened plate- like extension of the prochondral zygopodial skeleton. In this the various carpal or tarsal elements make their appearance as separate centres of chondrification. It seems unnecessary in a general text-book like the present to go into the great variations in detail which are found amongst the various tetrapods in regard to the skeleton of carpus and tarsus. It need only be said that the striking variations found in different groups from the schematic arrangement, such as is illustrated by Fig. 171, seem to have been brought about by enlargement or reduction 0'1’ individual elements, or the fusion together of originally separate elements. V SKELETON 0}" THE LIMBS 357

From the plate-like rudiment of carpus or tarsus there spread out radiating extensions normally five in number to form the skeleton of the digits. In the Amniota these appear practically synchronously although in Amphibians there is a tendency for them to develop in regular sequence according to the number of the digit (Rabl, 1901). In the substance of these the phalanges make their appearance as discrete chondrifications.

In the Birds the loss of individuality of the digits involved in the conversion of the tip of the pectoral limb into a rigid support for the flight feathers has been accompanied by processes of reduction and fusion of the orginal elements. In the proehondral stage five digits are laid down but only 11, Ill and IV proceed with their development. Of these metacarpal ll

becomes reduced to a small stump project— IL Ill 15/f ing from III: metacarpals III and IV I o 0 0 V become fused with one another at both 0 0 0 0 0 ends: and the three distal carpals liecdlile O 0 0 0 0 fused with the metacarpals to form the O 0 O 0 0 C5 carpo-metacarpus characteristic of the Bird. CL 0.0000‘? _o Ce

In the hind limb of Birds there are "6’-,_.-0 Q .0 -.- u" also laid down proehondral rudiments of O. i‘ the five digits and again I and V become R Q U. reduced although not so completely as in ' the fore limb. V reaches the stage of a H

small metacarpal nodule of cartilage which FIG. 171. —— Cartilaginous 916-

however soon disappears. Metacarpals II, Ilwnt-*4 whivh «M9101» in “W

I’ - ‘ ‘ , forelimb of I'.'m_?/szthe tigureis Ill and I\ fuse w1th- one another and cnmbincdfrom se\,em1stag,.s_

with a cartilage which represents the distal (After Mehnort, 1897.) pow of ttiagsals to 1l\}1‘p1ttl1elcpagacteristic 6,.’ ,,,.,,t,.,,,,.; P1,,’ ,,:_.,m, ,.,,.,,,,1;; arso—me a arsus. e a arsa isappears i, i_nt.ernu-dium'; H_. mnu-rush: . except in its distal portion. And finally T’ "“““““’ U’ “"“" "’ the two proximal carpals which are visible for a time fuse with the end of the tibia to form the tibio-tarsus. BONY SKELETON on THE LIMBS. PECTORAL GIRDLE.-—In the Sturgeons the original cartilaginous pectoral girdle persists, lying close under the skin of the posterior branchial recrion. Plates of bone corresponding exactly with those on the rest of the skin develop superficial to the girdle and serve to reinforce it. Of these bony plates there are two principal ones on each side, one in the region of the glenoid surface the cleithrum (Gegenbaur) and one extending ventrally to meet its fe11ow—-—the clavicle. In existing Crosso- pterygians where the evolution of the bony skeleton has reached a higher level than in the Sturgeons the same two bony elements develop but here the original shoulder girdle-its function being to a great‘ extent talienl over byil the cleithgum ——f_bte1c1:omies tlgelativeelylr rec 11CB( in size. t ies on t e inner sur ace 0 e c ei rum an its cartilage gives place in part to two replacement bones--the scapula dorsal, and the coracoid ventral. It is to be noted also 358 EMBRYOLOGY or THE LOWER VERTEBRATES on.

that the cleithrum sinks more deeply into the tissues while the clavicle remains superficial.

In the higher bony fishes Ganoids and Teleosts—-the conditions are very similar to those ol']’oly;ute'mzs-—tl1e primitive shoulder girdle being small and usually becoming replaced in great part by bone (scapula and coracoid) and the main supporting function being exercised by the independently developed cleithrum.

In the Dipnoi more nearly primitive conditions are retained as the original cartilaginous girdle remains well developed throughout life and retains its continuity with its fellow ventrally. (lleithrum and clavicle however are also developed and they show a higher condition in that they are developed in intimate contact with the surface of the cartilaginous girdle, the clavicle ensheathing the anterior face of the coracoid portion.

In Amphibians the scapula becomes replaced incompletely or completely by hone which spreads dorsalwards from the region of the glenoid articulation. The coracoid may remain cartilaginous (most Urodeles) or become replaced by bone. The precoracoid also tends to be strengthened by the formation of bone. ln the common frog (Rana) and Toad (Bu;/b) the bone (“clavicle”) is in the form of a splint lying along the anterior side of the precoracoid and originat- ing in the connective tissue some little distance from the cartilage. In other cases the bony tissue completely surrounds and to a great extent invades and replaces the cartilage. We may infer with con- siderable probability that the bone in question was originally in phylogeny a “niembrane” bone and that becoming more and more intimately related to the precoracoid cartilage it has in the latter form become more or less completely a “cartilage ” bone——a good example of the type of evidence which has led morphologists to minimize the importance of the distinction between these two types of bone.

In the Amniota scapula and coracoid are replaced nearly or quite completely by bone. A clavicle like that of Amphibians develops in relation to the precoracoid in Reptiles except Crocodiles. ln Birds What appears to be the same element (furcula) is widely separated from the eoracoid, probably for mechanical reasons con- nected with flight, while a separate centre of ossification appears at the apposed ventral ends of the two bones. In Reptiles a somewhat similar ele1nent——the episternum—n1akes its appearance and is con- tinued tailwards along the mid-ventral surface of the sternum and it is possible that in Birds the ossification lying between the ventral ends of the clavicles represents the anterior segmented off portion of this and the keel of the sternum the rest.

PELVIC GIR1)LE.——The cartilaginous pelvic girdle becomes replaced by bone less or more completely without receiving any reinforcement from investing bones. The iliac, pubic and ischial portions ossify each from its own centre except in Amphibia where the pubic region remains cartilaginous.

In bony Teleostomatous fishes eaclf half of the pelvic girdle V THE SKELETON 359

hecomos replaced by a plate of bone the morphological nature of which has been much discussed. Detailed studies of its development in a variety of different teleosts and in the more primitive ganoids are much needed.

LITERATURE

Agar. Trans. Roy. Soc. Edin., xlv, 1906. Albrecht, A. Entwickelung des Achsenskele-ties dcr Telcostier. (Diss.) Strassburg, 1902.

Braus. Morph. Jahrb., .\xvii, 1899.

Braus. Haeckelfestschrift (J cnaer Denkschriften, xi), 1904. Braus. l[cl‘twigs Handbuch der Entwicklungslehrc, iii, 1906. Budgett. Trans. Zool. Soc. Lond., xvi, 1902.

Bunge. Entwickcluingsgcscliiclite des licckeilglirtc-ls der Amphihic-n, Rt-ptilien

nnd V ogel. (Dis:-3.) Dorpat, 1880.

Garlsson. Anat. Anzciger, xi, 1896.

Carlsson. Anat. Anzeigr-r, xii, 1896*‘.

Cords. Anat. Hefte (A1-h.), xxxviii, 1909.

Dohrn. Mitt. Z001. Stat. Ncapel, v, 1884.

Gadow. Phil. Trans. Roy. Soc., B, clxxxvi, 1895.

Gadow. Phil. Trans. Roy. Soc., B, clxxxvii, 1897.

Gaupp. Schwalbcs Morph. Arbeiten, iii, 1894.

Gaupp. Hertwigs IIandb11ch der Entwicklungslchre, iii, 1906. Gibson. Anat. Anzeiger, xxxv, 1910.

Goeppert. Gegcnbaurs Fcstschrift. Leipzig, 1896.

Goette. Arch. mikr. Anat., xv, 1878.

Goette. Arch. mikr. Anat., xvi, 1879.

Goodrich. Quart. Journ. Micr. Sci., xlvii, 1904.

Goodrich. Proc. Z001. Soc. Lond., 1908.

Goodrich. Proc. Zool. Soc. Lond., 1913.

Goodrich. Quart. Journ. Micr. Sci., lxi, 1915.

Gregory. Biol. Bull., vii, 1904.

Harrison. Arch. mikr. Anat., xlii, 1893.

Hertwig, O. J enaische Zeitschr., viii, 1874.

Hertwig, 0. Arch. mikr. Anat., xi, Suppl., 1874*.

Howes and Swinnerton. Trans. Z001. Soc. Lond., xvi, 1901. Huxley. Todd's Cyclopaedia of Anatomy and Physiology, v, 1859. Kerr, Graham. Proc. Roy. Phys. Soc. Edin., xvii, 1908.

Leche. Anat. Anzciger, viii, 1893.

Marsh. Odontornithes. Washington, 1880.

Mehnert. Morph. Jahrh., xiii, 1888.

Mehnert. Schwalhes Morph. Arheiten, vii, 1897.

Mollier. Anat. Hefte (Arb.), iii, 1894.

Mollier. Anat. Heftc (Arb.), v, 1895.

Nickerson. Bull. Mus. Comp. '/.001. Harvard, xxiv, 1893.

Rabl, C. Morph. J ahrb., xix, 1893.

Rabl, C. Zeitschr. vsiss. 7.001., lxx, 1901.

Rose. Anat. Anzeiger, vii, 1892.

Rose. Anat. Anzeigcr, viii, 1893.

Rose. Anat. Anzeiger, ix, 1894.

Schauinsland. Zoologica, xxxix, 1903.

Schauinsland. Hertwigs Handbuch dcr Entwickelungslehre, iii, 1906. Schéne. Morph. Jahrh., xxxi, 1902.

Semon. Forschungsreisen in Australien, i. Jena, 1893—1913. Sewertzoff. Kuplfers F estschrift. Jena, 1899.

Sollas, W. and I. Phil. Trans. Roy. Soc., B, cxcvi, 1903.

Sonics. Petrus Camper, iv, 4, 1907.

Suschkin. Nouv. Mém. Soc. des Naturalistes dc Moscou, xvi, 1899. Time, Marett. Quart. Journ. Micr. Sci., xlix, 1906.

Traquair. Proc. Roy. Phys. Soc. Edin., xii, 1893.

Versluys. Z001. Jahrb. (Anat.), xix, 1903.

Wijhe, van. Comptes rendus 6"” Congrbs intcrnat. Zool. 1904, Berne, 1905. Williamson. Phil Trans. Roy. Soc., ex], 1849.

CHAPTER VI VASCULAR SYSTEM

As has already been indicated the vascular system of the animal body consists of strands of highly specialized niesenchyme——-the cells (corpuscles) along the axes of the strands being detached from one another and Heating freely in a fluid intercellular substance (plasma), while the superficial cells are united together to form the walls of. tubular channels-—~—the vessels. The vessel walls are provided with a coating of muscle-fibres and this muscular coat becomes greatly thickened and specialized at one or more points to form hearts which serve as pumps to force the blood through the system of vessels. '

The fundamental plan of the Vertebrate vascular system appears to have been like that of an Annelid worm, with two main longitudinal blood-vessels, situated respectively one on the neural side of the alimentary canal and one on the side opposite to this, connected together by a series of half-hoop shaped vessels encircling the alimentary canal laterally. In the Vertebrate the longitudinal vessel on the neural side of the alimentary canal is the dorsal aorta and in it the blood runs in a tailward direction. The longitudinal vessel on the other (ventral) side of the alimentary canal develops the heart on its course: its precardiac portion is the ventral aorta, its postcardiac the subintestinal vein. In this ventral vessel the blood passes in a headward direction. Half-hoop shaped vessels lying in front of the heart and connecting ventral aorta and dorsal aorta are the aortic arches.

0 c YER'1‘EBRA'rEs.— mongst holob as 10 er e rates the first steps in the deve opment of the vessels have been investigated in the Newt (Triton) by Mollier (1906) and his account will here be followed.

In an embryo with six mesoderm segments the lateral sheetsof mesoderm have met ventrally except in the region of the liver where they terminate in a free edge. This free edge is thickened and the thickening extends back along the mid-ventral line towards the cloaca as the rudiment of the subintestinal vein—the entire thickening having thus a Y-shape (Fig. 173, A).

360 (:11. VI ORIGIN OF THE HEARTANI) VESSIELS 361

At a stzigu with ‘twelve segments this Y-shaped vascular rudiment is continued forwards as a couple of strands of cells, lying on each side on the inner surface of the splanchnic mesoderm and apparently derived from it. These are destined to give rise in their hinder portions to the two vitelline veins and in their anterior region to the first rudiments of the heart (_ Fig. 17 2, A, (me).

At a stage with fifteen segments the paired strands of cells have assumed a disposition like that shown in Fig. 173, B. They approach one another as the mesoderm extends downwards and presently fuse across the mesial plane (Fig. 172, B and O), the fused portion being the rudiment of the heart while the two :1.nteri.or limbs represent the first (mandibular) pair of aortic arches and the two posterior

FIG. 172.——Ventral portions of transverse sections of young Amphibians to illustrate the tlcvclopment of the he:u't:.. (R.-is-a_sal on ligln-o_-s by M olli¢_.:r. 1906.)

A, B, I), E, ’1'*r-Him (A twelve segments, B sixt.oa_-n ¢ln., l) 1.\u-nty «ln., I41 twenty-six do.); (.‘, R--nu. a.l.mc, dorsal mesocardium; eat, ectoclernn; rm-, «-mlm-.:n-«lium; mm‘. t'lI(l(Nlt‘l'lll; me, myouardium; mes, mesoderm; pa, pm-icardiac cavity; -v.mn, ventral lIlcs()(‘u.l'Ilillll1.

limbs the vitelline veins. The heart rudiment is at first extremely short in an antero—posterior direction being much broader than it is long. This is correlated with the shortness of the foregut. As the latter lengthens the heart rudiment keeps pace with it, and becomes elongated (Fig. 173, D). As it does so the tissue within the rudiment becomes loosened and takes the form of a syncytial network with wide meshes. .

In the meantime the mesoderni on each side, now containing a wide coelomic (pericardiac) space, has grown down to the mesial plane ventral to the heart, so as to give rise to a ventral mesocardium which however only persists for a short time (Fig. 172,

C and D, mnc). About this same period fluid begins to collect in.

the interstices between the cells of the subintestinal strand, with the result that some of the cells in its interior assume a spherical 362 EMBRYOLOGY or THE LOWER VERTEBRATES on.

form and are recognizable as embryonic blood eorpnscles. Mollier notes that about this stage the subintestinal strand comes into extremely close relation to the yolk—cells, there being in places apparently complete continuity between the two-——hence the conclusion on the part of observers who did not study earlier stages that the vascular strand was actually derived directly from the endoderm.

About the stage with sixteen to eighteen segments the rudiments of the Duct of Guvier and dorsal aorta become apparent, in the form of cells at iirst scattered and later joined into strands. The aorta cells anteriorly often show eonnexions with the sclerotomes and

Fm. 173.-——Rongh diagram to illllstl.':1tc the form of the early rudiments of heart, vitel1ine_

veins, and -‘x-ul'»intestinnl vein in Triton as seen in plan. A, six mesoderm segment stage; B, fifteen segments; (2, eighteen segments ; l), twenty segments.

u..e.l, mandibular aortic arch ; cl, position of clones; H, heart; L, position of liver; .~:.-a'..v, subintestzinal vein; -v.-r, vitelline vein.

Mollier admits that some of them may actually be derived from the sclerotomes (see p. 364) though he considers that the main source of origin is the upper angle of the lateral mesoderm.

At the stage with twenty segments a network of fine channels has appeared over the surface of the yolk, between it and the mesoderm, foreshadowing the vitelline network of blood-vessels. The subintestinal strand has become still looser in texture and prolongations may be found passing from it inwards amongst the

~yolk—cells.' ” The heart has now attained the form of a. straight tube

the protoplasmic strands in its interior disappearing while its superficial cells take on an endothelial character, and are recognizable as the endocardium. The splanchnic mesoderm has become closely moulded round it ventrally and laterally (Fig. 172, D) forming the rudiment of the myocardium and the latter begins to show contractions causing slight movement of the fluid contents of the heart. VI ORIGIN OF THE HEART AND VESSELS 363

By the twenty-seven segment stage the anterior limbs of the subintestinal strand have become detinite (vitelline) veins with well-defined lumen filled with fluid in which spherical young corpuscles float freely. The large flat cells forming the wall are probably simply the modified superficial cells of the strand though Mollier thinks these may be reinforced by additional mesoderm cells from without. The vitelline veins are continued in front into the posterior venous limbs of the hea.rt and the heart itself is seen in transverse sections (Fig. 172, E) to be now completely enclosed in myocardium, the inner wall of the pericardiac space having become moulded right over its dorsal side. Where the two sheets of mcsoderm, one from each side, have met dorsal to the heart there still persists a septum——the dorsal mesocardium (Fig. 17 2, E, cl.m.<:) which serves to sling up the heart to the ventral side of the foregut.

The dorsal aorta is at this stage particularly instructive. Posteriorly it is represented by scattered cells, lateral in position, thus betraying their lateral origin. .Further forwards these have approached the mesial plane and form a pair of cellular strands. Further forwards still in the region of the first eight segments—— these have become still more nearly mcsial in position and ever part of their extent have undergone actual fusion to form the unpaired aortic rudiment.

About this stage the dorsal aortic rudiment is connected up to the vitelline network by a series of segmentally arranged vessels (segments 5-17) which had made their appearance about the twentysegment stage as segmentally arranged strands of cells.

The rudiments of the ducts of Ouvier make their appearance even earlier than that of the dorsal aorta, in the form of cells derived according to Mollier from the somatic mesoderm at the cranial side of the pronephros. These rudiments develop extensions iii a headward and in a tailward direction to form the cardinal veins. ’l‘he vessels of the head region develop in situ from the mesenchyme and the same may probably be said of the smaller vessels generally.

The Crossopterygian fish 1’ol_2/pterus (Graham Kerr, 1907) is, apart from its generally archaic character, particularly suitable for the study of the first beginnings of the vascular system owing to the fact that the long axis of the embryonic body is straight, so that horizontal as well as sagittal sections may be made passing through practically the whole length of the dorsal aorta during its early stages, when in its hinder portion it has not yet taken definite form.

The first conspicuous stage in the development of the dorsal aorta consists in the collecting together of irregular multinucleate masses of yolky protoplasm in a row beneath the hypochord (Fig. 174, A). Vacuolar spaces develop in these masses and foreshadow the aortic cavity. The masses of protoplasm become more closely aggregated into a cylindrical shape while the vacuolar spaces increase enormously in size and eventually flow together to form the continuous aortic cavity. In the specimen figured in 364 EMBRYOLOGY or THE LOWER VERTEBRATES on.

Fig. 174, B, the cavity was perfectly continuous towards the head end, while posteriorly it was still in the form of isolated vacuoles. The cells which form the rudiment of the dorsal aorta are from their coarsely-yolked character clearly derived ultimately from the primitive endoderm, but the question remains whether they are derived from the definitive endoderm directly or through the inter FIG. 174.-—l’m‘fiunH of horizontal sections through I‘ul‘:_/[1/wrI1.s' larv:u+ 0l'.~t1i.ge.s' ‘Z4 + (A) zunl 25 (B) showing the rudiment of the aorta in longitudinal section.

A, aortic rndinn-nt; m..r..-, vacuole-.~;.

mediary of the mesoderm. Such a section as that shown in Fig. 175 indicates that the latter is the case. The definitive endoderm shows a perfectly sharply defined surface, clearly marked off from the aortic rudiment, while the mesoderm of the sclerotome on the other hand is continuous with the aortic rudiment. We may say therefore with high probability that the aortic cells are derived from the sclerotome.

A remarkable feature has been noticed'in the development of the VI ORIGIN OF DORSAL AORTA IN POLYPTERUS 365

dorsal aorta of Polypterus which requires further investigation both in that genus and in any other Vertebrates in which it may be found to occur. In Polypterus in the stage immediately preceding that in which the aortic cells collect together the position of the future aorta is distinctly marked out by the arrangement of the delicate reticulum that is visible connecting up the various organ-rudiments of the larva. This reticulum is usually regarded as an artifact caused by the action of the fixing and preserving solutions upon the albuminous substances contained in the fluids of the embryonic body but the fact that it becomes arranged in this peculiar fashion to._ foreshadow the future aorta at once raises the question whether it is not really a reticulum of living substance.

The aortic cavity in Polypterru-s has been seen to originate by the fusion of intracellular vacuoles. The cavity is filled with clear fluid and T1 this condition persists even after the main channels of the vascular system are ].aid down. The blood is at first simply fluid or plasma without corpuscles. This plasmatic condition may persist even after circulation has commenced and the heart propels through the vessels simply the clear cell—less fluid. Here we find repeated in ontogeny an extremely archaic condition of the circulation. The Plasma’ beconles peopled with Ifie. ]75.—Portion of transverse section corpuscles comparatively suddenly. through Polyptcms of stage 25

The portions of vessel wall lying “1“;?"i“gt“:f :°1‘;;i°““l°_ft‘1‘° “°'l'°i° external to the lining endothelium rm ""9" (_ ) “C m °me('9°)' appear to arise from mesenchyme W’ °"‘°"“l’,,‘,"“n::Zc‘1,:;;,”d’_""'°t°""" cells.

SOURCE on THE CORPUSCLES.—'-The blood corpuscles are to be looked on, broadly speaking, as mesenchyme cells which have lost their connexion with their neighbours and float free

_in the plasma. Their precise sources in ontogeny ‘appear to be

various :—

(1) They can frequently be seen in process of being budded off by the wall of the embryonic blood-vessel into its cavity.

(2) In other cases the vessel with its contents is seen to arise as a solid mass of cells, those at the periphery becoming the wall (endothelium) of the vessel rudiment while those more deeply placed round themselves off, becoming separated by chinks containing fluid, and develop into corpuscles. This may be regarded as a 366 EMBRYOLOGY OF THE LOWER VERTEBRATES t CH.

modification of (1) brought about by a hurrying on of the develop ment of the corpuscles.

(3) In still other cases the cells of the mesenchyme reticulum in certain localities e._q. in the spaces between the tubules of the pronephros draw in their processes, round themselves ofi' and are carried away in the blood stream as primitive corpuscles.

This last mode of origin may account for the fact that the

Flu, 176.——-"Portions of transverse sections through Ela.~nnobrancl1 enibryos illustrating the origin of the heart.

A, ’1’m'pedo, stage with one gill cleft; B, 7'()1'pe(lo, stage with two gill clefts; (3, .1’r-istiu:-us, stag.-,-e with twenty-five st-.,«_»;ments; D, I-’ri..-I2'.u-rw.«:, stage with forty segments. (After figures by Riiekert ([388) and Mollier (l906).) cl.-nue, dorsal mesocardinm; mo, endoeardimn; and, endoderm of foregut: me, myoeardiunl; mes, mesodenn; N, notochord ; pt‘, pericardiuc cavity; 3.0, spinal cord; splc, splanelmoeoele.

corpuscles have been observed to make their appearance suddenly in large numbers in the circulating blood. The sudden setting free of large numbers of corpuscles is possibly due ‘to an epidemic of mitosis in the mesenchyme cells, as it is well. known that the onset of the mitotic process frequently induces a retraction of the processes of the cell.-body and its assumption of an approximately spherical shape. ’

It is not proposed to trace out in this volume the further development of the blood corpuscles—-—how the originally similar indifferent V1 ORIGIN OF THE HEART AND VESSELS 367

corpuscles become differentiated with further development into specialized strains—Erythrocytes or Red corpuscles and the various types of Leucoeytes; for an account of this in Lepidosiren the student may be referred to the beautiful memoir by Bryce (1905).

The mode of origin of the vascular system in the holoblastic vertebrates in general seems to resemble in its main lines that described above. The chief question that has given rise to discussion is one which in the opinion of the present writer resolves itself very much into a question of mere verbal expression—namely whether it is more correct to state that the first rudiments of the vascular system (or parts of them) are derived from mesoderm or from endoderm. When it is borne in mind that the mesoderm of the Vertebrate is essentially a derivative of the endoderm, it will be realized that the questioll is of minor importance whether or not special portions, such as rudiments of the vascular system in particular cases, lag behind the main 1nesoderm_ in their separation from the endoderm, so as to originate fron1 the latter directly instead of from the already differentiated mesoderm.

On1c1N or THE lI1«:An'r IN MEROBLA IC VEIm::s1iA'1‘Es.——The heart 111 its earliest stages shows in mere » astic vertebrates generally a set of conditions quite similar to those met with in holoblastic forms. In Elasmobranchs (Fig. 176, A) the first obvious rudiment of the heart is in the form of a number of cells of irregular shape which make their appearance on each side of the foregut, between it and the splanchnic mesoderm, from which latter they are apparently derived. As the foregut separates off from the endoderm of the yolk the irregular row of these cells shifts downwards and towards the mesial plane, so as to form a single elongated group underlying the foregut 1 (Fig. 17 6, B). The individual cells unite together and form a syncytial mass containing vacuolar spaces (Fig. 176, C, ens). Finally (Fig. 176, 1)) this elongated mass assumes a tubular form, its vacuoles coalescing and increasing in volume to form a wide cavity, while the protoplasm becomes thinner and forms the endothelial wall. The myocardium (mo) comes to surround the endothelial tube in exactly the way described for holoblastic forms.

In the Sauropsida again the phenomena are similar. To take as an example the Fowl: about the stage When two or three segments areformed isolated cardiac cells begin to appear on each side between the endoderm and splanchnie mesoderm. These cells increase in number and form a longitudinal tract on each side, the two converging and meeting anteriorly. As the foregut becomes constricted off the two cardiac strands come together from before backwards in front of the yolk-stalk though even at the eight-segment stage they are not completely fused. The endothelial rudiment now forms a loose syncytial spongework with large meshes containing


1 Ruckert (1888) believes them to be reinforced by cells derived from the ventral endoderm of the foregut. 368 EMBRYOLOGY OF THE LOWER VERTEBRATES C11.

fluid. The myocardium develops as in the other forms already mentioned.

ORIGIN or 'rm+: l’ERIPHERAI. BLOOD-VESSELS IN THE MERo13LAs'r1o VEl{'1‘l¢3BRA'l‘ES.-——ln those Vertebrates which have meroblastic eggs the concentration of yolk in the highly modified ventral endoderm accentuates the need of an efiicient transport system by which this food material may be taken up and conveyed to the yolkless and actively developing parts of the embryo. ln accordance with this we find that such vertebrates show a precocious development of a rich network of blood-vessels over the surface of the endoderm or yolk and this vitelline network allbrds admirable material for the study of the earliest stages in the development of peripheral bleed-vessels.

it will be convenient to consider in some little detail the early development of the vitelline network in the Fowl, material for the study of which is easily obtainable and which further has been worked out in detail by numerous investigators. Riickert has furnished an excellent modern account (1906) a11d upon it the following description is based: for fuller detail reference must be made to the original.

The first signs of bloed—vessel formation make their appearance extremely early, at a stage when the primitive streak is present but hardly any trace, or only a small stump, of the so-called head-process at its front end. The mesoderm has at this stage spread slightly beyond the edge of the pellucid area, especially posteriorly. Round its posterior edge the mesoderm assumes a mottled appearance owing to the development in it of small cell condensations the first trace of the blood-islands as they were called by Pander. These bloodislands are sometimes arranged in two separate series one on each side but more usually they form a U-shaped arrangement parallel and in close proximity to the posterior limit of the mesoderm.

In a slightly later stage the mottled area containing bloodislands -— the vascular area — is of a U-shape, extending through about the posterior half of the extent of the mesoderm. The bloodislands are less conspicuous in front, gradually fading away, as they do also on the side next the primitive streak. They are most strongly marked towards the external margin of the mesoderm and here they, as well as the whole sheet of mesoderm, are being added to by delamination from the endoderm of the germ-wall. The bloodislands, the first rudiments of blood-vessels, are simply thickenings and condensations of the mesoderm. They at first have the appearance in sections of occupying its whole thickness but later it is seen that each blood-island is roofed over by a layer of unmodified mesoderm -—demareated apparently by a simple process of splitting-olf of the superficial layer of cells.

As development goes on, the area of vascular rudiments spreads inwards into regions of the mesoderm which have been for some time completely separated from the endodérm by a well-marked split. VI ORIGIN OF THE VESSELS 369

Blood-islands developing in such regions are therefore clearly derivatives of the mesoderm and there is no possibility of the endoderm playing a direct part in their formation as might be the case peripherally in the region of the germ-wall.

The vascular rudiments become joined up by strands of cells to form a network and this network gradually spreads inwards, its extension being brought about by a progressive differentiation in situ from the mesoderm : there is no actual sprouting inwards of the already formed strands of the network as is suggested sometimes by the study of whole blastoderms and as was once supposed to take place.

Of the network of cell strands which traverses the rudiment of the vascular area the bulkier portions give rise to masses of‘ blood corpuseles surrounded by an endothelial wall, the more attenuated portions to endothelial tubes without any corpuscles in their interior. In the former case the superficial layer of the cells forming the bloodisland becomes raised up from the main mass of cells, fluid accumulating beneath it in spaces which are at first isolated but later become continuous. The flattened cells which are raised up represent the endothelial wall while the main mass of cells left behind represent developing corpuscles. It is to be noted that the endothelial wall separates from the mass of corpuscles first below ('i.e. on the side towards the yolk) and laterally, so that after fluid has accumulated in the rudimentary vessel the mass of corpuscles still remains attached to its, as yet undifferentiated, roof. The narrower strands between the main blood-islands and also all those in the pellucid area, except sometimes a few near its posterior end, give rise simply to endothelial tubes containing fluid plasma. As the circulation begins the masses of embryonic corpuscles gradually break up, first in the region of the sinus terminalis} the individual corpuscles being whirled away by the current and carried to the heart and thence through the circulation.

The origin of the vitelline network has also been investigated in Elasmobranchs (especially Torpedo) by numerous workers. It agrees in its main features with what occurs in the Fowl.

As regards the peripheral vessels in general, of the Vertebrata, we may say that they take their origin as chinks within the n1esenchyme filled with a clear fluid secretion (plasma). These chinks are at their first appearance in some cases clearly intercellular while in others they at first have the appearance of intracellular vacuoles. As has already been pointed out in dealing with connective tissue (p. 292) this difference though at first sight impressive loses most of its apparent importance when regarded critically. In this particular case the protoplasmic masses in which the vacuoles appear are as a rule multinucleate and it is clearly impossible to draw a sharp line of morphological distinction between spaces in such masses with

1 The topography of the vascular area will be found illustrated later in the special chapter on the development of the Fowl.

voL. II 2 n 370 EMBRYOLOGY OF THE LOWER VERTEBRAT IS on.

partially broken down walls, and the ordinary intercellular spaces of syncytial embryonic connective tissue.

GENERAL CONSIDERATIONS REGARDING THE MORP F rue

RATE EART.-—- may )6’. regarded as apr1m1 ivec aracteristic

o oo -vessc s t rat their walls are contractile, peristaltic waves of contraction serving to propel the blood in their cavities. It is usually the case however in the more complex animals that this contractility becomes concentrated in one or more localized portions of the vessels, known as hearts, in which the vessel becomes much enlarged and its muscular coating thickened and rhythmically contractile.

In the craniate Vertebrates there is one heart present and it represents an enlarged portion of the Von tral vessel in the region immediately behind the gills. During ontogeny the heart still repeats the archaic evolutionary phase in which it was tubular in character. As development goes on the primitive heart, or cardiac tube, shows rapid increase in size within the pericardiac chamber of the coelome in which it lies. This chamber is relatively small in size and in the lower, fish-like, Vertebrates is bounded by rigid unyielding walls. The confined nature of this space in which the heart has been evolved has, by imposing restrictions upon it during its increase in size, exercised a profound influence upon the modelling of the vertebrate heart. It is therefore desirable to have a. clear idea of the general relations of the heart to the perieardiac cavity, during its increase in size, before attempting to study its development in detail in the various groups of Vertebrates.

The portion of vessel originally included between the anterior and posterior limits of the pericardiac cavity will be referred to here as

the primitive heart or cardiac tube. As development proceeds the increase in size of the primitive heart reveals itself in (1) increase

in length and (2) increase in diameter.

(1) As regards the former, the cardiac tube is at its posterior and anterior ends-——where it enters and leaves the pericardiac cavity respectively—firmly embedded in the tissues of the pericardiac wall. These ends being consequently in the lower, fish-like, vertebrates rigidly fixed in position, it has of necessity come about that the cardiac tube, while in the course of evolution it has increased in length, has lost its original straight form and has been thrown into a system of bends or kinks which have had an important influence upon the structure of the fully evolved heart. This bending process is repeated, though with obscuring of some of its detail, during ontogeny and it is an interesting morphological problem to endeavour to unravel the details of the process from the data of comparative anatomy and embryology.

Apparently the primary flexure of the cardiac tube is represented by a simple loop or bulging towards the right side of the body, which is visible in the embryos of most Vertebrates during early stages of heart development. VI MORPHOLOGY OF THE HEART 371

With increasing growth in length of the cardiac tube this simple curvature becomes converted into a double flexure the heart taking on a S-shape. Of the two curves which 1nake up the 5 one which has its concave side towards the head represents the original loop, while the other which is convex towards the head has developed in the portion of cardiac tube lying posterior to the primary loop. Of these two curves the one last mentioned, that which is morphologically posterior, is in an approximately vertical plane. The anterior or primary curve on the other hand shows much variation in position in different Vertebrates. While on the whole it still bulges towards the right side, as did the primary loop, the portion of it formed by the originally headward section of the tube comes in many cases to lie ventral to the other limb of the curve. In other cases this, originally anterior, portion of the tube lies for a time dorsal to the other, as is the case in balamamtlm. The difference will be appre ciated by comparing the

relative Positions of 0 and V Flo. 177.-—-A, diagram to illustrate the llexure of

- - the cardiac tube in the adult I.epz'u’o.s-[rem as seen

In F138’ 184’ A’ and 178' from the ventral side. The portion of thelliagram 01 3:11 the IOWCT Vicrtf?‘ above the horizontal line represents the conus:

}_)1-a,t',es in which the per}- the portion below the horizontal line would

- _ - - represent the rest of the heart on the assumption cardiac space 1S Sun bounded that this portion of the cardiac tube possesses a

by rigid illextensillle Walls similar curvature to that of the conus. Longill} iS the gI'011p Of Lung-fishes tmlinal lines drawn along the tube mark the

that Shows the heart.’ at originally dorsal (1)), ventral (V), right (R), and ‘ left (L).

the highest level of evolu- B I I I L t d I b L It - - , - s rows t ne spira wis mg pro ueec y s rang] ening non‘ Anll In Lorrelatlon out a tube possessing the same llexure as the ‘L-onus

  VVO filld that portion in diagram A.

in these fishes the kinking

of the cardiac tube attains its maximum. In a fully developed Lepidosiren (see below, pp. 376-378) the anterior portion of the cardiac tube (the “conus arteriosus ”) has developed a further 372 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

flexure in addition to those already described. The nature of this flexure is shown in the upper portion of Fig. 177, A, which represents diagrammatically the eonus of Lepiclosiren as seen from the ventral side.

The extreme anterior end of the tube, being fixed firmly at its exit from the pericardiac cavity, retains its primitive morphological position: its originally dorsal side is actually dorsal.

Traced backwards the tube is seen to become sharply bent upon itself in a headward direction, in such a way that the side of the tube which was originally on the left side comes to be ventral, as is indicated by the finely dotted line L in the figure. Tracing the tube onwards a second sharp flexure is found and the tube resumes its antero-posterior direction. This second flexure involves a complete reversal of the tube. Its originally right-hand edge (indicated by the coarsely dotted line 13), which had come to be dorsal as a result of the first flexure, is now ventral.

The changes in the position of the tube caused by the two fiexures may be summed up by saying that the half of the tube which was originally dorsal, and which remains dorsal at its anterior or headward end, has come to be situated on the right side at the posterior or ventricular end of the portion of the tube now under consideration (eonus). Similarly the half of the tube which at the headward end is ventral, has come to be at the ventricular end on the left side.

The lower half of the diagram represents the portion of cardiac tube which gives rise to the main part of the heart and it is to a certain- extent hypothetical, inasmuch as it does not rest on a complete series of observations, but it is clear that the morphologically right side of the cardiac tube, which is topographically ventral in the middle part of the figure, has to get back to its original right-hand position at the hinder end of the cardiac tube (which like the front end is firmly fixed in position), and it is reasonable to infer that the flexure of this portion, which gives rise to the atrium and ventricle, would be found, were its unravelling possible, to be symmetrical with the anterior flexure already dealt with.

It is interesting to take such a model as that represented in Fig. 177 and subject the eonus portion to a process of straightening out——such as would happen in nature if the eonus were to shrink in length, its anterior and posterior ends remaining fixed. The result is shown in Fig. 177, B. The eonus assumes a twisting in a right-handed spiral through three right angles. In the Amniota it will be found that the representatives of the eonus of the Lung-fish~ the roots of the great arteries, pulmonary and systen1ic—as they pass headwards from the ventricular part of the heart, twist round one another in just such a spiral.

(2) As regards the increase in diameter of the cardiac tube, it is characteristic that this does not take place equally throughout. At VI MORPHOLOGY OF THE HEART 373

certain levels the increase in diameter is much less pronounced than it is elsewhere, with the result that the tube appears to be constricted at these points wl1ile it bulges out between them. This development of a series of dilated portions of the heart-tube is the first step in its segmentation into a series of chambers. Of these chambers there are typically in the lower vertebrates four——sinus venosus, atrium, ventricle and conus arteriosus.

Allusion must be made in passing to an unfortunate confusion of nomenclature which is apt to prove a stumbling-block in the way of the student who is trying to ‘get his ideas clear regarding the morphology of. the heart.. The name conus arteriosus was first used, so far as the comparative anatomy of the lower Vertebrates is concerned, by Gegenbaur (1.866) who used it to designate the structure lying between ventricle and ventral aorta in Elasmobranchs and Ganoids, and characterized by its possessing a muscular, rhythmically contractile wall and by its containing longitudinal rows of pocket valves. 'l‘he name’ was introduced in order to accentuate the supposedly fundamental difference, already suggested by Johannes Muller (1845), between the structure in question and the bulbus arteriosus of Teleostean fishes. This latter is not provided with striped muscle in its wall, it is not rhythmically contractile: in other words it does not form physiologically a. part of the heart. Objections, and quite valid ones, have been raised against the use of Gegenbaur’s name from the side 01' Human anatomy, it being pointed out that the “conus” of the lower fishes corresponds rather with the “ bulbus ” of the human heart. Human anatomists working at the embryology of the vertebrate heart in consequence commonly use the_,na1ne bulbus cordis for the part oi the heart under discussion. Gegenbaur’s name however has come to be so universally used by comparative anatomists in reference to the heart of the lower vertebrates as to indicate the desirability 01 using it in a work on comparative morphology such as this. It will be understood then that the name conus arteriosus is used in this volume as equivalent to what is by many writers termed bulbus. cordis,‘ without prejudice however to the question whether or not Gegenbaur was justified in his belief that conus arteriosus and bulbus arteriosus are fundamentally distinct structures.

ELASMOBRANCHII.-—Tl1e Elasmobranch heart passes through the typical early stages, first as a straight tube (see Chap.,XI.), then as a tube which bulges towards the right side, and then as a tube with the characteristic S-shaped double flexure already alluded to (Fig 17 8). The three limbs of the 5 during further development become converted into (1) atrium with sinus venosus, (2) ventricle, and (3: conus arteriosus. The well-marked constriction which demarcates atrium from ventricle forms the auricular canal. The progress

1 I avoid in this book using the term trwncus arteriosus as it is unnecessary and

is liable to cause confusion owing to the want of precision with which it is commonly used. 374 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

towards the condition of the fully developed heart is marked mainly by the increase in relative size of the atrium and ventricle. Whereas however the increase in the size of the atrium takes the form mainly of a mere process ..of dilatation, that of the ventricle is accompanied by a much more marked thickening of its wall. This is brought about by the inner surface of the niyoeardium forming numerous projections into the lumen which, becoming more and more pronounced and interlacing and fusing with one another, form eventually a spongework and encroach considerably on the ventricular cavity round its periphery. The eiidocardiuin fits closely over the surface of each of these inyoeardiac trabeculae.

The physiological iiieaning of the formation of the trabcculac during the evolution of the ventricle probably lies in the fact that a bundle of muscle which has for its function the pulling together of the ventricular wall can carry out this function more efficientl if it runs straight between its two ends, in ot)lIier words if it is in the position of a chord to the curve of the ventricular wall rather than simply a portion of that curve.

Attention must now be directed to a very characteristic and important proliferation of the endocardiac cells which makes itself "apparent in particular regions. In the eonus such proliferation takes place along the course of four longitudinal lines,- giving rise to cells 1,.“178___,l.“,0Smg0mW which lie in the space b(?:l)W€eI1.0Ild0S3iLl‘dl11II1

.1m10p..,e,,t of ti... ;...;.,.., and myocardium. As this proliferation goes 0fAcrmt/_w7a8 see1_1i'r<>mtlur on the endocardium is eventually made to

S1‘;;3'6‘()A”°"1i“"°}“ bulge into the lumen as four . prominent

M m_mm_ F “mm endocardiac ridges. In 1icant7zw,s (GegenW; .M, Sim, .v,e“(')_s_l'l_\,; V, baur, 1894), one of the four ridges——-that which ventricle, is ventral in p,osition——is reduced in size.

_ . _ In the auricular canal sinii.lar endocardiac prohferatious take place, one upon the headward and one upon the tailward wall respectively of the canal. Herc also each causes a prominent bulging of the endocardium into the cavity——-the atrioventricular cushion (anterior and posterior).

Both the ridges of the eonus and the atrioventricular cushions constitute a valvular apparatus in that, by the contraction of the niyocardium lying outside them, they can be jammed together so as to occlude the lumen into which they project. In both cases, as development goes on, they undergo metamorphosis into a purely mechanical and automatic valvular apparatus. In the eonus each ridge becomes excavated into a number of pocket valves (“semilunar ” valves), the cavities of which open in a headward direction. Greil and others explain these cavities as being produced simply by the backward pressure of the blood but it is advisable to regard VI HEART OF ELASMOBRANCHS 375

such simple mechanical explanations of developmental phenomena with S1].Spl(:1UI1. Similarly the atrloventricular cushions become excavated on their ventricular side and form the two atrioventricular

valves of the adult. A pair of laterally placed valves also develop

FIG. 179.-—Vie\\'s of l.l|4,- hc:u‘t. of 1.4‘/n'«I...gu'ren as seu1'1 from the nlorphologically n.-ntral side. (ll and (,9 afta-.1‘ J. Hol_n-rtson, 1913.)

A, stage :52; I3, .s‘l...-1;,-er :51 ; ('.‘, stage 3-3. «I, atrium ; (‘, «onus :u-t-uriosils; 1.4-, In-ft a-uricle; 1211., right auriclc; I", \'«-lltl‘irln.

at the opening of sinus into atrium but according to extant descriptions (Rose, 1890) arise merely as infoldings of the heart wall.

DIPNo1.—-The development of the dipnoan heart has formed the subject of a careful and exhaustive study by Robertson (1913) and her account of the (lm-“cl.0p1ne1'1t of the heart in Lepidoshen forms the basis of the following description. 376 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The first peculiarity to be noticed in comparison with the heart of the Elasmobranch is correlated with the fact that the head and anterior trunk region of the embryo are bent downwards and closely applied to the surface of the yolk. As a consequence, the pericardiac space is reduced to a flat chink, the point of entrance and the point of exit of the cardiac tube being in close proximity to one another at its upper end. The result is that the cardiac tube as it grows in length assumes the form‘ of a flattened loop, first V-shaped (Fig. 179, A) and later Z5-shaped, the apex of the loop being directed in a ventral direction, and the originally posterior (tailward) limb of the loop (at) coming to lie to the left of the anterior limb (C). The cardiac tube becomes demarcated into the same set of chambers as in the Elasmobranch-——sinus venosus, atrium formed from the posterior limb of the cardiac loop, ventricle formed from the apical portion of the loop, and conus formed from the greater part of the anterior limb. The dilatation of the walls of the several chambers is not uniform. In the ease of the atrial wall the dilatation is most marked dorsally and more especially laterally: the posterior wall on the other hand lags behind in its growth and the result is that the sinu-atrial and atrioventrieular openings remain comparatively close to one another (compare adult condition as shown in Fig. 180). Similarly in the case of the ventricular portion of the heart the increase in size is mainly on the ventral and lateral sides, the dorsal wall lagging behind so that the communication between atrium and ventricle and that between ventricle and conus also remain in close proximity (cf. again Fig. 180).

After the demarcation of the chambers there come about two important changes in the general form of the heart-—the first is the assumption of bilateral symmetry on the part of the ventricle, correlated with a rotation of the ventral side of the ventricle towards the animal's left side. The other consists of a very marked increase in the length of the conus which, owing to the fixation of its two ends, is made to assume the characteristic double flexure already described and illustrated (see also Fig. 179, 0).

DEVELOPMENT or SEPTA IN THE HEART. —- By far the most important feature of the Dipnoan heart, as compared with that of the Elasmobranch, is that now, for the first time, there comes about that separation of the heart into an arterial and a venous half, which is so characteristic of the higher Vertebrates. In Lepidosiren this separation is inaugurated at a relatively early stage of development (stage 27)—-at a time when the cardiac tube, as yet, shows no indication of a division into chambers—-by proliferation of the cells of the endocardium on its outer surface. This takes place along a. line which passes along the posterior wall of the U-shaped heart from the left side of the sinu-atrial opening, through the auricular canal down towards the apex of the cardiac loop. As this proliferation goes on it causes the endocardium to bulge into the lumen so as to form a prominent ridge traversing the hinder wall of atrium and v1 HEART OF LEPIDOSIREN 377

ventricle. It is of morphological interest to notice that this atriaventricular ridge extends from its dorsal end not directly ventralwards but towards the animal’s right side, so that, if the ridge in question be taken as marking an originally longitudinal line along the wall of the cardiac tube, it indicates that this part of the cardiac tube has undergone a process of twisting like that of a left-handed screw, in other words a twisting of the kind which might be expected on the hypothesis that the flexure of the atrioventricular portion of the heart was originally that suggested in the discussion on p. 372.

A second endothelial proliferationtakes place along the atrial wall facing that on which the atrioventricular ridge has developed. The projection formed in this way grows towards and fuses with the atrioventricular ridge to form the atrial septum which divides the atrium into a larger right and a smaller left auricle. Owing to the left-handed position of the atrioventricular ridge at its dorsal end the sinus opens into the right auricle. The pulmonary vein as it develops comes to open on the left face of the septum, z'.e. into the left auricle.

The ventricle becomes similarly divided into a right and a left chamber1 the foundation of the septum consisting of the atrioventricular ridge already mentioned. I11 this case however Robertson does not describe any endocardiac proliferation mls-ct-mls to the ridge but says the septum is completed by muscular trabeculae growing towards and eventually fusing with the edge of the ridge.

The conus arteriosus is characterized by the development of a series of longitudinal endocardiac ridges similar in nature to those of Elasmobranchs. A conspicuous difference in detail is that each ridge is markedly discontinuous, the portions situated in the anterior and in the posterior section of the conus developing independently. We may take it that the ridges were primitively in the Vertebrata longitudinal and continuous and the secondary discontinuity visible in Lung-fishes and also in the higher Vertebrates may be associated with two probable causes: (1) interference with the development of the middle region of the conus by the flexure into which it is thrown, and (2) the tendency, as seen in Elasmobranchs and Ganoids, for the terminal members of the longitudinal rows of valves to become enlarged relatively to the rest. There can be no doubt that the longitudinal ridges as we see them in the Lung-fishes and the higher Vertebrates are revertive rather than persistent primitive features. In other words the ancestors of these Vertebrates passed through the phase of evolution in which each ridge had become converted into a row of pocket valves. This seems clearly indicated by the fact that the latter condition holds in modern Elasmobranchs and primitive Ganoids. But if so then the tendency for the terminal valves of the row to be specially developed in that earlier phase of evolution may show itself

1 The separation does not normally become quite complete in the adult either in the case of atrium or ventricle. 378 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

on reversion to the ridge condition in more or less great suppression or diminution of the middle portion of the ridge.

In the anterior section of the conus four longitudinal ridges develop, situated respectively on the right-hand side (1), dorsally (2), on the left-hand side (3), and ventrally (4). This anterior end of the conus retains as already explained its primitive position and we shall therefore always refer to the four ridges according to the position they have in this undisturbed portion of the conus as Right, Dorsal, Left and Ventral respectively, the adverb morphologically being understood before the adjective in each case. The right and left ridges make their appearance first and they alone become prominent, forming thin shelf-like structures which project right in to the centre of the cavity so that their edges overlap. For a short distance at the extreme anterior end they become fused together so as to form a continuous septum. The left ridge is comparatively short, tapering off posteriorly, but the right extends back through the anterior and middle section of the conus. At the point of ilexurc between middle and posterior sections there is a break during early stages but later on the ridge becomes continuous witl1.a portion of ridge which projects from the ventral wall of the posterior section of the conus. There is no reason to doubt that this is really part of the same morphological structure as that with which it is in line in the anterior section of the conus and we shall therefore term it the posterior portion of the right ridge. The whole of this right ridge forms what is often called the spiral valve of the conus.

The dorsal and ventral ridges of the anterior section of the conus are later than the lateral ridges in making their appearance and soon disappear again. The ventral ridge is especially feebly developed.

In the posterior section of the conus the right ridge——now ventral in position—is alone well developed. The other three appear as rudiments, they are at no time prominent and they become resolved into vestigial pocket valves which may still be detected in the adults. This latter fact justifies the conclusion already reached that during the ancestral history of the Lung-fishes a stage was passed through during which the conus was provided with longitudinal rows of functional pocket valves, in other words that the primitive ridges seen in the conus of the modern Lung-fish are revertive rather than persistent. VALVES OF THE HEART.——The sinu-auricular opening is guarded on the right side by a valve. This develops out of the inpushed fold of the cardiac Wall in the constriction between sinus and atrium. The atrioventricular opening is guarded by a highly characteristic bevelled plug (Fig. 180, AV.p) which when the ventricle contracts is pulled downwards so as completely to occlude the opening. Developmentally this plug arises as a thickening of the atrioventricular ridge. This ridge, which, as already indicated, VI HEART OF LEPIDOSIREN 379

forms the common foundation of auricular and ventricular septum, traverses the auricular canal, projecting into it from behind so as to give the atrioventricular opening a horse-shoe shape. It is the part which lies above (dorsal to) the opening which becomes thickened and eventually assumes a cartilaginous character to form the plug. The plug is to be regarded as the homologue of the posterior atrioventricular cushion of Elasmobranchs but Robertson failed to find any trace of an anterior cushion.

The conus in the completely developed state is characterized by the absence of the functional valvular apparatus found in its homologue in other Vertebrates. On the one hand the endocardiac ridges, functional in the young Elasmobranch or Ganoid, are no

I

Fm. 180.-—-Heart of an adult Lepi«.losiren with the right side removeal. (After J. Robertson, 1913.)

AV.p, atrinventricuhw plug; ¢i'.'v, corona:-_v vein (cul'.); d.(j'. ducts of (Juviur; pm, pulnum:u'_\' vein; -p.V.4:, pu.~;t.orio1- \'t‘l'l{1('.‘l\'{l. at its opening into the sinus \'¢-.n0.\'uS: .~'..-L atrial sq-pt.um ; .~'.l', \'eut1'icul:11' h'l!])l-lllll; .~-. 1', sinus \'a-.nosus (its opening into the rigI1t:uu'ix-lo- illulit-:|lI'!l lay an :n-1-nu‘); .-p, .~:piI':Il \':l1\'4!: Ill, V1, aortic arches out near their ventral ends.

longer in a condition to fulfil their original function and on the other the pocket valves are here vestigial.

MUSCULARIZATION or rm: HEAnr.——Bud-like projections from the myocardium grow into the cavity of the heart, meet together and in the ventricle form a muscular spongework of the same type as that seen in Elasmobranchs. In the case of the ventricle numerous trabeculae arising in this way converge upon the free edge of the atrioventricular ridge and become continuous with it. As development goes on this spongy mass of trabeculae undergoes condensation arid acquires the solid character of the fully developed septum. The septum is continuous dorsally with the atri'ove11tricular plug and it forms a muscular apparatus by which the plug is pulled down so as to fit into and close the opening. The myocardium of the auricular 380 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

canal and of the conus never forms a spongework but remains as a compact layer of muscle. In the case of the conus this muscular coat is very feebly developed in the middle and cephalic portionsin which fact we may probably recognize a degenerative feature seeing that in the Elasmobranch the conus musculature is well developed right to its front end. The meshes of the ventricular spongework, as development goes on, come to spread somewhat round the auricular canal and round the ventricular end of the conus, so that each of these structures has the appearance of being drawn into the ventricular cavity.

SAUROI*sI1)A.~——'l‘hc most exhaustive account of the development of a Sauropsidan heart is that dealing with Lucerta by Grreil (1903) and upon it the following description is based. ln its early stages the heart passes through the familiar tubular form and becomes bent, first bulging in a simple curve towards the right and then assuming a double S-like curvature just as in the Elasmobranch. About stage 17-18 the constriction of the heart into sinus venosus, atrium, ventricle and conus becomes apparent—the three last mentioned chambers bulging outwards between the grooves which limit them. The atrial portion does not in these early stages take up the purely dorsal position seen in the Elasmobranch or Lung-fish but remains for a time well to the left.

The conus, in its early stages, much reduced in relative size as compared with that of the Elasmobranch, undergoes a marked increase in length, which causes it to assume a bayonet-shaped curvature in which we may see a reminiscence of the sharp double flexure seen in the conus of the Lung-fish. In the Lizard however this curvature of the conus is merely temporary. As development goes on the increase in length of the conus instead of being more pronounced than that of the heart as a whole becomes less so with the result that between stages 21 and 26 the anterior flexure of the conus becomes pulled out and replaced by a right-handed spiral twist.

DEVELOPMENT or SEPTA.-—Tl1e septation of the heart is inaugurated by the appearance of localized proliferation of the endocardiac lining. In the auricular canal, which runs in an antero-posterior direction rather than dorsi-ventrally, owing to the atrium lying anterior to the ventricle instead of dorsal to it as was the case in the Lung-fish, there develop two endocardiac cushions, one dorsal (posterior), the other ventral (anterior). Of these the ventral or anterior one which was not apparent in Lepidosiren is well developed and is continued as an endocardiac ridge round the anterior (headward) wall of the atrium on to its roof (compare Fig. 183, C, was). As development goes on this projects more and more prominently into the cavity of the atrium and forms the main part of the septum between the two auricles. By about stage 26 it has grown halt’-way across the atrial cavity, and by about stage 29 it reaches the auricular canal. While in a sense the atrial septum is now complete it is not so physiologically VI HEART OF LACERTA 381

as secondary perforations have made their appearance in the septum so as to keep the two auricular cavities in free communication. The two endocardiac cushions of the auricular canal become joined together by a bridge of endocardiac tissue which forms the free edge of the auricular septum. This is followed by a complete fusion taking place between the middle parts of the two cushions, so that the atrioventricular opening becomes completely divided into a larger right and a smaller left portion. The, at first, thick mass of tissue which separates these two openings becomes gradually converted into a thin plate, situated in the plane separating atrium from ventricle, and therefore perpendicular to the plane of the atrial septum. This plate is divided sagittally into a right and a left half by its line of attachment to the septum. The free edge of each half is concave and projects freely into the corresponding auriculoventricular opening—forming the mesial or septal valve of that opening.

In the meantime a new endocardiac cushion develops on what were the right and left sides of the auricular canal. These also become thin flaps and form the lateral auriculoventricular valves. It will be noticed that there have developed round the original atrioveutricular opening few proliferations of endocardium—--the same number as was found in the conus of the Lung -fish and as will be found in the conus of the Amniota, thus supporting the idea that there are four longitudinal endocardiac ridges potentially present throughout the cardiac tube of the higher Vertebrates though they may become actually apparent only in the conus region.

In the Lizard the atrioventricular ridge, which was so conspicuous in the Lung-fish, has practically become reduced to the portion lying within the auricular canal—the dorsal (posterior) endocardiac cushion. The ventricle is undivided.

The conus on the other hand undergoes a complete and somewhat complicated process of septation. This is inaugurated by localized proliferation of the endocardium to form longitudinal ridges. As in the Lung-fish these arise discontinuously there being distinct anterior (headward) and posterior rudiments. Anteriorly the normal four ridges develop, the dorsal and ventral appearing in this case at an earlier stage (17 or 18) than the lateral ones. Towards the ventricular end two ridges first make their appearance in a dorsal (ridge B, Greil) and ventral situation (A, Greil) respectively. Of these the ventral one becomes eventually continuous with the right-hand anterior rudiment. It clearly corresponds with the similarly situated ridge in the hinder portion of the conus of Lepirlrisiren and like it is to be interpreted as the hinder portion of the morphologically righthand ridge. The ridge mls-ft-vie to that just mentioned, here dorsal in position, would similarly represent the hinder portion of the morphologically left-hand ridge. Later on a small and transient ridge (0, Grreil) makes its appearance on the right-hand wall of this hinder portion of the conus and this would represent the hinder portion of the morphologically dorsal ridge. 382 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

As development goes on the wall of the conus becomes changed in histological character, its striped muscles become replaced by smooth and in general it takes on the ordinary features of arterial wall so that it resembles a portion of the ventral aorta rather than of the heart. As may be seen in a living embryo this histological change is accompanied by a physiological one, for the rhythmic contractions of the heart are seen now to extend forwards as far as the anterior limit of the striped Inuscle but no farther. Altogether the superficial ‘appearance is just as if the ventral aorta (“ truncus arteriosus ”) were extending backwards at the expense of the conus, and the word truncus is frequently used to include the whole as far back as the limit for the time being of the smooth non-striated muscular wall. It must however not be forgotten that in the strict morphological sense all that part of the heart is conus which corresponds to the conus of Lepvldosiren. 'l‘he special criterion which identifies it is


Fm. 18l.—~—l)iagraminatic transverse sections through conus of I.a.ce~rta ( A) and Gallus B to show the endocardiac rid res and the iocket valves. la

1, morphologically right ridge; 2, dorsal; 3, left; and 4, ventral. a and I2, problematieal ridges discussed in text; 12, pulmomuy cavity ; 8, main systemic; (.5, left systemic cavity.

the appearance of double flexure or the resultant spiral coiling during its development. The muscular coating, so characteristic a feature in the vertebrates below the Amniota, is associated with a definite type of functional activity: in the Amniota that type of functional activity has disappeared and with it the characteristic type of wall. The ventral aorta is in its hinder portion, where it becomes continuous with the front end of the conus, divided1 into a dorsal (pulmonary) and a ventral (aortic) cavity by a horizontal septum and this is prolonged backwards along the wall of the conus by the right and left ridges. Of these the right is very large, it projects across the lumen and gradually fuses with the left ridge (Fig. 181, A). This ridge (3) is low and double and it is with its dorsal portion,

  • i.e. the portion next the dorsal ridge, that the fusion takes place.

By the spreading backwards of this process of fusion of the right and left ridges the horizontal septum of the ventral aorta becomes prolonged back as a septum in the conus——no longer horizontal

1 See below, p. 393. VI’ HEART OF LACERTA 383

however but spirally twisted owing to the twisting of the conus already mentioned. Parallel with and preparatory to this process of fusion the distal ridge rudiments spread backwards, pursuing a spiral course and thus making evident the spiral twisting of the conus as a whole. It is interesting to notice that the line of insertion of the ridges, and therefore of the septum formed by their fusion, becomes marked on the outer surface of the conus by a distinct incision--a preliminary step towards the complete splitting of the conus in the plane of the septum which takes place in Birds and in Mammals.

In addition to the dividing of the cavity of the conus into a pulmonary and an aortic portion in the manner just described there takes place also, in the Lizard, a splitting of the aortic portion into two parts, corresponding to the right and left halves into which the systemic portion of the ventral aorta is divided} The septum separating these becomes prolonged backwards at its hinder end, on the one hand, into the ventral ridge of the conus (Fig. 181, A,‘4) and on the other into a quite similar ridge developed upon the surface of the septum which separates the aortic from the pulmonary (Fig. 181, A, 1). These two ridges facing one another across the aortic cavity gradually extend backwards and undergo fusion just as in the other case so as to form a complete septum dividing the aortic cavity into two (S and l.s).

In this waythen the original conus becomes replaced by a set of three tubes twisted spirally round one another, forming the roots of the two systemic aortae and of the pulmonary artery, still however enclosed in a common wall.

VALVES on THE HEART.———-The right and left valves which guard the opening from sinus into atrium are formed simply by the exaggeration of the fold of the cardiac wall which delimits these two chambers from one another. The origin of the auriculoventricular valves has already been described. The pocket valves of the systemic aortae and pulmonary artery are derived from the endoeardiac ridges of the conus as in the Elasmobranch. According to Langer (1894) the outer valve in each of the three vessels (pulmonary artery, left systemic aorta, right systemic aorta) are derived from the Dorsal, Left and Ventral endoeardiac ridges (2, 3 and 4) respectively, while the inner valve in all three is derived from the hypertrophied Right ridge (1), which with its outgrowth takes part in the formation of all three vessels (Fig. 181, A).

The question as to whether or not the pocket valve is formed from the extreme ventricular end of the conus ridge, or whether on the other hand a considerable portion of this end of the conus with its contained ridges becomes incorporated in the ventricle as maintained by Langer and Greil does not appear to the present writer to be satisfactorily settled. It is advisable that the point should be re-investigated upon abundant material.

GALLUS.-—-(Figs. 182 and 183.) The most detailed investigations

1 See below, p. 393. 384 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

of the development of the Bird's heart are those of Greil, of which unfortunately there is available so far only the abstract given by Hochstettcr (1906). As we should expect from the close genetic relationship between Birds and Reptiles there is a close correspondence between the general features of the development of the heart in the two cases. It will suflice then to draw attention to the more i1npu1‘l):'mt points in which the development of the Fowl’s heart has

FIG. 182.—Il1ustrating the development of the heart in the fowl. (After original drawings by (_i1'cil.)

at, atrium; h.u..f, bulho-auricular fold :'«-_, l-onus; Lu, ll-ft zuu-iule; Li, left im|mnin:u«- urlo-ry; I.-,2, left pulmmnat-_v; I.l-', left. \‘l'ntl'i('l4-; r.vI, ri;_'l1l Ill1l‘l(,‘ll'f, r.'i, 1-iglnt. lllll()llllll{l.tl'. :u-Lory; r.p, right. pulmonary; r.l-’, l'i;.',‘l1t vent:-ii.-la-; .-..l, s_ysl.<-urn-:u.1-t:.a.

been found to differ from that of Lace-rta. So far as external form is concerned the most striking difference is that the sinus venosus loses its identity as a distinct chamber of the heart. It becomes as it were incorporated in the right auricle, all except its left portion which persists as the cardiac end of the left duct of Cnvier or anterior Vena Cava.

An important advance upon the condition in Lacerta is found in the division of the ventricular part of the heart into a right and a VI HEART OF BIRDS . 385

left ventricle. This (li_visi<.m. comes about in a somewhat complicated fashion the chief points in which, judging from Greil and Hoolistetter’s descriptions and figures, appear to be as follows. The ventricular portion of the cardiac tube is at an early stage encroached upon by the deep (“ bulbo-auricular ”) fold which separates the conus

Fm. lRfi.--~--St.:1._r_ges in the development of the heart of the Fowl, viewed from the right Shite. 'l‘l1crigllt wall of the heart has been removed in each case. (After original ilI'it\\'lll;:_'S lay (‘nu-.il.)

.1, 1.’, proxini.-:1. «_-nals oi‘ l'l(l".."'I‘S of eonus; uI.._-. atrial .-.«-.pl.un'1; h.n,.r, bulbo-auricular ri«l_:_-,-e ; a._-, r-onus arteriosus; i.-v./; llll.(‘i'\‘l‘lIl..l‘iC|ll:ll' 1'<.n':I.m«-u; r'.r..~', tralu__-<-.ul:u- portion of ventricular septum; La, left auriele; p, pu|mcnar_\- ;u-1..-,-_y; -r.u, ri_v_,--ht. auriclo-; r.r.c, rut. surface of endocardiae tissue of atrioventricular opening; 2-.1’, cavil _y oh-i;_,rl'it ventricle : .-, .~:ystennic aorta ; V, ventricle ; 1, ‘2, 3, conus ridges.

from the atrial part of the heart (Fig. 182, A, b.at.f). The encroachment of this fold gives the ventricular part of the tube a squat

U-shape. As the ventricle dilates the extent of the encroachment becomes reduced and the fold may now be called the bulbo-auricular

ridge of the heart-lining (Fig. 183, A, b.a,.'r). During the fourth

day this becomes extended tailwards along the ventral wall of the VOL. II . 2 0 386 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

ventricle (Fig. 181?», B, C, b.a..'r) the ridge and its extension forming the rudiment of the anterior portion of the ventricular septum. The remaining and larger portion of the septum arises otherwise, from a local exaggeration of the muscular trabeculae which in the Bird, as in lower forms, sprout into the ventricular cavity so as to convert the peripheral portion of that cavity into a sponge—work. This spo11gc—work becomes exaggerated in the prominence and thickness of its trabeculae along a plane marked on the external surface of the heart by a distinct groove-——thc interventricular groove (Fig. 182, (J). This trabecular part of the septum (Fig. 183, O, i.'v.s) is at first loose and spongy but it gradually becomes condensed, at first along its thickened free edge, and loses its spongy character. It gradually extends forwards and becomes continuous on the one hand with the bulbo-auricular portion and on the other with the septum of the auricular canal. The ventricular cavity is now divided into a right and a left chamber except at its anterior end where there remains an interventricular foramen (Fig. 18;}, 1), lt will be realized that but for the presence of this foramen the blood could not circulate, as the only means of exit from the ventricle the opening leading into the conus——lies completely on one side (right) of the original bulbo-auricular fold and, therefore, of the ventricular septum of uhich the fold in question forms a part. As a matter of fact this interventricular foramen never disappears, though it loses its right to that name, for it becomes continued as a groove over the surface of the mass of endocardiac tissue lying between it and the conus. Eventually this groove becomes overgrown by its edges and converted into a tubular channel, continuous on the one hand—— through the original interventricular foramen——-with the cavity of the left ventricle and on the other with the systemic or aortic cavity of the conus. rfhis tubular channel persists in the adult condition——as the communication between left ventricle and systemic aorta. _

Finally, before leaving the intcrventricular septuln, it has to be mentioned that dorsally (Fig. 183, 1)) it becomes continuous with the bridge of endocardiac tissue which divides the atrioventricular opening into a right and left half, the ventricular side of this bridge growing out to meet the trabeeular part of the septum. The atrial side of the bridge is continuous with the atrial septum, which develops here as in Lacerta, and the result is that the main part of the heart is now divided into two halves, the left auricle opening into the left ventricle and the right auricle opening into the right ventricle (Fig. 183, E). It is to be noted however that secondary perforations appear in the atrial septum (Fig. 183, E, at.s)—so as to allow the systemic blood which enters the right auricle from the sinus venosus to reach the left auricle, and through it the_left ventricle, without having to traverse the pulmonary circulation during the period before the lungs are functional. VI HEART OF BIRDS 387

of its development the ventricular portion of the heart undergoes a certain amount of rotation, the right ventricle becoming displaced somewhat towards the left side, ventrally to the left ventricle. The result of this is to undo to a small extent the spiral twist of the oonus.

'l‘he conus of the Fowl develops typical endocardiac ridges, here however only three in number, and the individual ridges retain a more nearly primitive condition in that in early stages they are not so completely divided into two distinct rudiments, while in later stages two of the three are obviously continuous. One of these ridges is clearly the morphologically Right. Here again it is Inuch enlarged (Fig. 181, B, 1) and grows right across the cavity to form a complete septum’ between the pulmonary (7)) and systemic portions of the cavity.

Regarding the identity of the two other ridges there is some doubt. They are identified by Grreil as the Dorsal (2) and Left (3) while the Ventral (4) is supposed to have disappeared. It appears to the present writer however that the possibility should be considered whether they do not together represent the Left ridge, with which in Lacerta the free edge of the enlarged Right ridge comes in contact and which in the latter animal shows an incipient division into two parts by a longitudinal groove.

’l‘he septum formed by the enlarged Right ridge follows a spiral course, its line of insertion being indicated by a spiral groove on the outer surface of the conus. In the Bird this groove gradually deepens into a slit which splits the septum into two halves and as a consequence divides the oonus into two separate vessels which course spirally round one another———the roots of the pulmonary arteries and the systemic aorta respectively. N o vestige of a septum subdividing the aortic cavity has so far been described.

V ALvES.——l’ul1nonary artery and systemic aorta are each provided -with three pocket—valves at their ventricular end. These arise in the manner indicated in Fig. 181, B. Each vessel receives a valve split off from the enlarged Right ridge. The pulmonary and the systemic cavities receive further a valve split off from the endocardiac thickenings marked a and b respectively. Following Grreil these would be attributed to the Dorsal and Left ridges, while accepting the alternative interpretation suggested above they would both be referred to the morphologically Left ridge. There remains a third pocket-valve in each cavity. That in the systemic cavity no doubt represents the otherwise missing Ventral ridge, while if a and 12 together represent the Left ridge then the third poeket~ valve of the pulmonary cavity would represent the Dorsal ridge. As there is no reason to doubt the reliability of a pocket-valve as evidence of a once existing endocardiac ridge we should be driven—— if we reject the explanation here suggcsted—-to assume the former existence of an additional ridge between the Right and the Dorsal and there seems no justification otherwise for doing this. 388 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The pocket-valves are stated not to develop at the extreme hinder limit of the ridges, the septum stretching back beyond them to become continuous with the interventricular septum.

In the right auriculoventricular opening the inner or septal valve is not developed, the ventricular septum fusing with what in Lacerta becomes converted into the valve in question.

The main features of heart development having been illustrated from these three dilfercnt groups, Elasmobranchii, Dipnoi and Sauropsida, it will be convenient now to indicate the more important peculiarities which have been detected in other groups of the lower Vertebrates. It should be understood however that in the case of several of these, such as Cyclostonies, Ganoids and even Amphibians, apart from Urodeles, our knowledge is still fragmentary.

In I’ol_2/plerus (_ Graham Kerr, 1907) the cardiac tube when in the form of a loop shows a similar displacement to that which occurs in fM])li([0-5'?:'}‘(’-7'!»-—-lille lower end of the loop being pushed forwards in front of the yolk. In this case however the displacement has gone farther than in the Lung-fish so that the cardiac loop is completely inverted-its apex being directed forwards, while the ventral aorta passes OH’ in a tailward direction. A similar displacement occurs in Teleosts.

'l‘he conus of Ganoid fishes shows the usual endocardiac ridges which become converted into longitudinal rows of pocket-valves as in Elasmobranchs. In Poly/pm-us these ridges are six in number, alternate ones being much reduced in size, with the result that in the adult three rows of large pocket-valves alternate with three rows of small ones. In all these fishes the endocardiac ridges and their resultant rows of pocket-valves run straight along the conus and there is no reason to doubt that this is the primitive condition. To determine the primitive number of the ridges in Fishes more research'is needed although there is little doubt that four was the number present in the primitive Tetrapocls.

In the Teloostcan fishes we find in place of the conus arteriosus the structure known as the aortic bulb. As already indicated (p. 37 this is distinguished from the typical conus by well-marked histological and physiological difi'erenees. And it is frequently regarded as being morphologically a part not of the heart but of the ventral aorta.

If however we take, as we are probably justified in doing, the point of exit from the pericardiac cavity as being relatively fixed and as marking the headward limit of the cardiac tube or primitive heart, then it becomes clear that the aortic bulb, lying as it does within the pericardiac cavity, is really a portion of the primitive cardiac tube and of that part of it which lay between the ventricle and the ventral aorta-——in other words the conus arteriosus. What has happened in the evolution of the. Teleostean heart is in all probability entirely analogous with what has taken place in the ‘ curve lies 111 a nearly vertical plane

VI HEART 389

Amniota, namely that the conus arteriosus has gradually lost its power of rhythmic contraction while pawl passu its myoeardiac coating of striped muscle has degenerated and its primitive histological characteristics have been replaced by others resembling more closely those of the ventral aorta.

.During ontogeny it would appear from Hoyei-’s work on Salmo (1900) that the conus in the embryo possesses the characteristic features——a layer of striated muscle in its wall, and longitudinal ridges (two in number) projecting into its 1uu1en——and differs from that of an Elasmobranch merely in ' the fact that these features do not extend throughout the whole of the distance between the ventricle and the anterior limit of the pericardiac space, but only through about the posterior half of that distance. In the adult the two ridges are represented by the two pocket-valves.

In Urodele amphibians (Salamandm—-Hoehstetter, 1906) the heart during the period when it is in the form of a tube with an S-like curvature is conspicuously different in appearance from that of the Vertebrates already described, owing to the fact that the two curves of s the 8 lie in different planes from those which they occupy elsewhere. The morphologically posterior or tailward curve lies here nearly iii the

horizontal plane While the anterior FIG.184.—-Views of developing heart of ' Salamamlm as seen from the ventral side. (After Hochstetter, 1906.)

——the limb of the curve which will become conus dorsal to the at,‘atriu111;c,eonu's‘u.rte;.ios11s;d.(.',.duc.t ventricular portion, so that it is ;’if,,,(,’,,“,V.,',‘,i,’,,;,,:f,";‘ mm cm’ 8")’ hidden in a View of the heart from

the ventral side (Fig. 184, A). Special interest is lent to this curvature of the atrioventricular portion of the cardiac tube by the fact that it reproduces accurately the type of curvature which we inferred as being present in this portion of the vertebrate heart in our general discussion of its morphology (compare Fig. 184, A, with lower portion of Fig. 177).

As development proceeds the left-hand end of the ventricular portion, z'.e. its actually headward end, swings ventrally and tailwards so that the long axis of this portion of the heart comes to be perpendicular to the sagittal plane of the body (Fig. 184, B). As the ventricular part of the heart shifts backwards the conus becomes

visible in a view from the ventral side. The backward shifting 390% EMBRYOLOGX or THETLOWER VERTEBRATES on.

continues‘ imtil the ventricle comes to lie ventral -to the sinus (Fig. 184, 0) instead of well in front of it as it did originally. The ventricle is now on the tailward side of the atrium which bulges out on each side, its ventral wall fitting closely to the dorsal side of the conus. The sinus becomes marked oft‘ from the atrium by a constriction, which deepens most markedly on the right side so that tl1e sinuatrial opening becomes displaced towards the left. It is to be noted that during these stages in development the portion of the ventricular wall lying on its concave (anterior) side, undergoes relatively very slight increase in size. The result is that the ventricle bulges in a tailward direction and the openings by which it comniunicates with atrium and conus respectively remain relatively closely approximated together. ‘ i

The atrial septum arises as a ridge or fold of endocardium which,

as in the Sauropsida, projects into the lumen from the anterior (head— i

ward) wall. The concave free edge of this grows towards the atrioventricular opening while its base of attachment spreads, on the one hand ventrally until it becomes continuous with the anterior atrioventricular ciisliioii, and on the other dorsally and backwards, to the left of the sinus opening and to the right of the pulmonary vein opening, till it becomes continuous with the posterior (tailward) atrioventricular cushion. The conspicuous openings in the atrial septum known to exist in adult Urodeles though not irr‘Anura are secondary perforations. e t

The conus develops discontinuous ridges. Of these there are four anterior riidiments of which the Dorsal and Ventral develop first and the Right and Left later. Of posterior rudiments only three have been described but that the missing one is at least sometimes present is shown by the adult arrangements of the resulting pocket-valves in rliiiereiit Amphibians. Thus four pocket-valves have been observed in the posterior circle..i;»n specimens of Strait, Nectwrus, the Axolotl, and Salmnandm (Boas). In the anterior circle cases are known of one of the four valves being reiiuced in size (Sz"ren, Axolotl, Triton, Sa.lama./ndm), vestigial (Pqlpa), or_:§i.g6ne entirely (Proteus, Rana). Other cases occur in which an additional valve makes its appearance through one of the original ones becoining split (Nectwrus). All these variations in the pocket-valves of the adult are of importance in relation to the embryonic ridges which the valves represent. Details will be found in Boas (1882).

Of, the anterior rudiments the right-hand ridge (1) is prolonged backwards as thevspiral fold which projects across the lumen and disides it iinpepfectly into an aortic and a pulmonary cavity.

In some cases the s iral fold apparently makes an abortive attempt’

to pursue the course of development which it went through in the ancestral fish-like form, as it segnients up into a row of little knobs each of which, we may take it, represents a pocket-valve (Tm'ton pwnctatus, '1‘. cmlstatus). In other cases (Necturus, Ooecilia‘) the spiral fold has in the adult completely disappeared (Boas- J-R823. ’ s -VI HEART OF ' REPTILES 391

The hearts of Reptiles in general agree closely in their developmental features with that of Lcwerta. The most important variations are seen in the Crocodiles (Hochstetter, 1906*‘). The conus here is- of interest in that it still repeats with particular clearness the sharp double flexure seen in the .l‘)ip1'ioar1 (F 185).

The ventricular portion of the heart becomes completely divided" into a right and left ventricle by a septum which is formed for the most part from trabceular projections of’ the m'yoeardi'um but in part also from the endoeardiac bridge which divides the atri.oventricular opening as in the Bird. _ This septum becomes quite complete, the interventricular foramen which exists for a time closing up and the interventricular septum becoming continuous. with the aortic septum of the conus. This is rendered a possible physiological arrangement in the crocodile by the fact that here the opening from the ventricle into that cavity of the conus which is continuous with the right systemic aorta, is farther to the left than in Lizards, While on the other _hand the right atrioventricular opeuin g is farther to the right. The result is that the opening of the right systemic aorta... and the leftauriculoventricular opening lie to the left of the s_ep'tun1;while the openings of the left aorta and pulmonary artery together with . _ . the right auricuhiventricular opening lie to F“"' 185';“’“H°“"t"l“{"br"°“'°

-_ 7. _ __ w o , Crocouhle. (Alter H0('-llthe right of the septum. buch diiierences Stetter’19O6*.) in the position of the ventricular openings of the great arteries are regarded by Hoolistetter as due to varying degrees of i11c01'po1‘ati01.1 of the obliquely placed conus into the ventricle. i

The foramen of Panizza, that re1narkable_ communication which exists in the crocodile between the two systemic aortae close to their point of exit from the ventricles, arises as a secondary perforation of the aortic septum comparatively late in development just before the closing of the last remains of the interventrieular foramen. _

The splitting up of the coffiis into independent vessels remains as a rule in the Reptiles, as in Lacerla, in an incipient condition, indicated merely by a slight grooving of the surface. In Ophidia however the superficial groove becomes -so deepened as to split the conus completely‘ into an independent aortic and pulmonary root as in Birds. _ , .

Bassing in review the broad features of lieartdevelopliient in the lower Vertebrates the following general principles seem to emerge :(1) The primitive heart or primitive cardiac tube is that portion of the ventral or subintestinal vessel included within the limits of the pericardiac coelome.

(2) Annular segments in the wall of th.i§,i.§,,t11sbe lag behind in increase of diameter so that the tube beg‘oi;1e§f;»§§pnstricted into a


Ir, conus arteriosus; I-", ventricle. 392 EMBRYOLUGY OF THE LOWER V ERTEBRATES cu.

series of chambers~——si1ms venosus, atrium, ventricle and conus arteriosus——-the originally peristaltic waves of contraction tending to become reduced at each constriction so that the chambers come to contract in series.

(3) The primitive valvular apparatus consists of a series of longitudinal ridges. These are best marked in the conus where they were originally at least four in number. The presence of six in 1’ol;z/pterus and the occurrence of more than four rows of valves in various other relatively primitive Ganoids and Elasmobranchs suggests that the number may have been greater. Individual ridges may in various forms become reduced or disappear, while in other cases apparently an increase in number may take place. The ridges show very generally a tendency to develop from two distinct rudiments, an anterior one and a posterior one, but the continuity of the rows of valves in the lower fishes indicates the probability that this discontinuity is secondary. In the portions of the heart behind the conus the1'c is no complete series of ridges like those in the conus though it is possible that vestiges of such ridges are represented by the endocardiac cushions and septal rudiments.

(4) Using the ridges of the conus as marking morphologically longitudinal lines it is seen that the conus has in the Lung—1ishes undergone a double llexure of such a kind as to produce ‘wlfen straightened out a right-handed twist of the conus through approximately three right angles. 3

(5) In tctrapodous Vertebrates the conus shows a similar righthanded twist and this is adequately explained by the relative reduction in length which the conus has undergone if it be assumed that there was an ancestral condition resembling that of the existing Dipnoan. ...

(6) From the Lung-fishes upwards the originally right-hand ridge of the conus becomes hypertrophied and, either alone or by fusion with its mls-d-m's, forms a longitudinal septum dividing the cavity of the conus into two parts——pulmonary and aortic——twisted spirally round one another. "

(7) Correlated with this process is a tendency for the wall of the conus to lose its striped muscle and its rhythmic contractility. This process takes place from before backwards and the portion which has suffered this change, and the wall of which has assumed the ordinary arterial character, is in common usage included under the name truncus arteriosus.

(8) The atrioventricular part of the cardiac tube undergoes a double flexure similar in nature to that seen in the conus of the Lung-fish but of such a kind as would if straightened out give a left-handed twist.

(9) The valves and septa of this part of the heart originate in the endocardiac cushions and in Leptdosiren atrial septum and ventricular septum are at first continuous with. one another.

(10) In the evolution of the valves of the heart there has come VI HE ART AND ARTERIES 393

about a substitution of automatically acting pocket— or flap-valves for the original endocardiac ridge or cushions which for their functioning were dependent upon a complex and fallible neuromuscular mechanism.

ARTERIAL SYs'rEM.—-— -The conus arteriosus is primitively prolonged forwards into the ventral aorta which gives off on each side the series of aortic arches. The most important e.volutionary change wl1iel1 the originally simple tubular ventral aorta undergoes is a process of splitting whereby the stream of blood to the lungs is separated from that to the tissues generally. This splitting is seen for the first time in the Lung-fishes. Here a dorsal portion of the cavity is separated oil’, continuous behind with the pulmonary cavity of the conus and ending blindly in front by its floor meeting its roof. The anterior termination of this cavity is ust in lront of the point of origin of aortic arch V, and this arch along with arch VI, owing to the fact that they branch oil’ from the ventral aorta somewhat dorsally, open from the cavity in question. The horizontal partition which forms the floor of this dorsal pulmonary cavity begins in ontogeny as a rudimentary ingrowth on each side between the origins of arches IV and V. The two rudiments grow back, fuse, and form a horizontal partition continuous at its hinder end with the Right and Left ridges of the conus which, as already explained, form the floor of its. pulmonary cavity.

A’ similar splitting off of a dorsal, pulmonary, part of the ventral aorta occurs in air-breathing Vertebrates in general.

As regards the remaining, or systemic, portion of the ventral aorta, the main point to notice is its tendency to split into two separate halves in its anterior portion, a process correlated probably with" economy of material, allowing as it does the. origins of the aortic

- arch'e.s to be displaced outwards so as to shorten these arches. This

splitting or bifurcation of the ventral aorta spreads backwards for a variable distance, commonly to about the level of aortic arch III or IV. ' .

In the Reptiles there comes about an independent splitting into two lateral halves of the aortic cavity posterior to the region of the bifurcation just alluded to. A vertical septum grows backwards from the anterior lip of the opening into aortic arch IV of the left side and becomes continuous with the septum separating the two systemic cavities of the conus.

When this aortic septum is formed the ventral aorta in its hinder portion contains three cavitics—a dorsal, pulmonary, leading to aortic arches V and VI of both sides, a left ventral leading to aortic arch IV of the left side, and a right ventral leading to aortic arch IV of the right side together with the paired anterior part of the ventral aorta and the aortic arches springing from them. Each of the three cavities is continuous behind with the corresponding cavity of the conus. In the majority of Reptiles (not in Lacerta) the separation of these cavities is followed by splitting of the septa between them 394 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

so that the ventral aorta is resolved into three distinct vessels forming portions of the common pulmonary artery and of the right and left systemic aortae respectively. THE AORTIO Anenns AND THEIR DERIVATIVES.-——F1‘o111 the. ventral aorta there are given elf on each side a series of half-hoop-shaped aortic arches which pass in a dorsal direction, between successive visceral clefts, to open eventually into the dorsal aorta. In the region where it receives the aortic arches the dorsal aorta is frequeiitly paired (forming the aortic roots) either temporarily or throughout life. This assumption of the paired condition may not improbably be of similar significance to that of the ventral aorta 'o'.e. have to do merely with the eeonomizing of material. In any case the precise extent of the paired condition does not appear to be of any great morphological importance.

As regards the aortic arches themselves the following general features are to be noted: (1) that they develop in order of position from before backwards in agreement with the general principle of development of the vertebrate body and (2) that individual arches tend to become r