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Kerr JG. Text-Book of Embryology II (1919) MacMillan and Co., London.

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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.

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 reduced in size in correlation with diminution of functional activity. Thus the mandibular and hyoid arches having lost or at least greatly diminished their respiratory activity even in the lower ﬁshes we find a corresponding disappearance or reduction of their aortic arches. ’

An important point to notice is that where the particular visceral arch carries a true external gill (Crossopterygians, Lept'd0s'£ren and Protopterus, Urodele and some other Amphibians) the aortic arch passes out as a loop into the external gill. The aortic arch is in fact in these forms during early stages, before the gillclefts are perforated, the vessel of the external gill. Such relations on the part of vessels of the fundamental morphological importance of the aortic arches are not to be dismissed lightly as modern adaptive modiﬁcations. They appear to indicate that the archaic function of the aortic arch was to supply the external gill with blood. _

As the external gill ceases to function a short circuit is formed at its base, through which the blood passes directly to the dorsal part of the arch without traversing the external gill. In Urodeles, according to l\Iaurer, the short-eircuiting vessel sprouts downwards from the dorsal limb of the arch but in Lepidosrlren Robertson ﬁnds it arising simply by the enlargement of pre-existing chinks. Thus the deﬁnitive aortic arch, in those cases in which an external gill is for a time present, includes a portion secondarily intercalated in its

course and derived from the short-circuiting vessel.

In the typical ﬁshes, where respiration is carried on by the wall of the gill-cleft, there becomes intercalated in the course of the aortic arch a respiratory network of capillaries, so that the arch is divided into a distinct ventral (afferent) and dorsal (efferent) portion. In such a ﬁsh as Lepidosiren, where the respiratory activity of the gills ‘VI ARTERIAL SYSTEM 395

is comparatively small, the aorticareh can be traced throughout as a distinct channel ; while in the Amniota, where the walls of the gill clefts have completely lost their respiratory function, the respiratory network never develops at all.

FIG. l86.—~—Scheme of aortic arches and their «lerivatives_aS seen from the Velltml Sid}?The parts of the original scheme which disappear dui-mg development aie shown in

pale tone. A, complete unmodiﬁed series of aortic arches; B, arrangement in a Urodele; 0, 111 ‘I Reptile: I), in a Mammal.‘ A, dorsal aorta; a.r, aortic root; mi, aiiastomotic vessel; ac, common carotid;

(LB, duct oi'Bota1lus; d.c, dorsal (internal) carotid; 'i, innoniinate artery; 512'’: left l’”l'“0"‘"'y artery 3 L3, left systemic; yr, pulmonary; 'r.p, right pulmonary; 5'. systemic 30ml; 3. Sllbcla-Via“ 31‘t9l‘)’ § v.n, ventral aorta; um, ventral (external) carotid ; 1, 11, etc., aortic arches. ‘

The longitudinal vessels with which the aortic arches are con nected are prolonged forwards as the carotid arteries which supply

the head with blood. The Ventral aorta is prolonged forwards as the ventral, or external, carotid while the prolongation forwards oi the aortic root forms the dorsal, or -internal, carotid. Of the alternative 396 EMBRYOLOGY OF THE LOWER VERTEBRATES CH’.

names dorsal and ventral (Mackay, 1889) are to be preferred to internal and external, for the latter though in common use are less precise. During the development of the young individual there is laid down a general scheme of aortic arches and associated vessels agreeing with that just described, and the processes of modiﬁcation whereby there becomes evolved out of this the complicated and very different arrangement of the great arteries of the adult afford material for one of the most fascinating chapters in vertebrate

Fm. 187.-—Illustrating modification of the carotid arteries, correlated with elongation of the neck region.

A, Varanid Lizard ; B, Grass-snake (’l"mpi«Ionolus); }).f'., primary carotid. (Other letters as in Fig. 186.)

embryology. The general lines of these processes are best illustrated by an outline of what happens in the group Reptilia.

The arrangement which the main arteries assume in adult Reptiles shows much variety. The relation which the adult arrangement in the more important Reptilian types bears to the primitive scheme may he gathered from an inspection of Figs. 186, C, 187, A, B, and 187A, C3 Through the conspicuous differences in

1 It must be remembered that in the actual animal the various bends and turns of the vessels tend to become strai htened out. For example arch III becomes simply a portion of a straight interna carotid artery. In the diagrams the original curvature of the arches is retained for the sake of clearness. VI ARTERIES OF iREP'.I‘ILESi 397

detail there can be seen general agreement in the fate of various aortic arches and of other parts of the primitive arterial scheme. Thus the ventral aorta is continued forwards to form the paired ventral (“external”) carotid arteries (12.0) while the aortic roots similarly extend forwards as the dorsal (“internal”) carotids (al.c). Aortic arches I and II disappear. Arch III persists as the root of the dorsal carotid while the portion of ventral aorta behind it, when

Fm. 187A.—-Illustrating modiﬁcation of the carotid arteries, correlated with elongation of the neck region.

C, Crocodile; 1); Bird; c, coeliac artery; .s-2, secondary subclavian. (Other letters as in Fig. 186.)

paired, is the common carotid (0.0). Arch IV is the Systemic arch which sends the blood to the hinder portions‘ of the aortic roots (am) and thence to the dorsal aorta (A). Arch V is reduced, appearing only as inconspicuous and transient vestiges during development. Of arch VI the proximal portion becomes the root of the pulmonary artery (rap and l.p) while its dorsal portion disappears.

The chief diiferences in detail are as follows: those affecting the

carotids will be given more fully later on. In Lizards and Chelonians the dorsal part of arch VI persists as

I 398 l EMBRYOLOGY or THE‘ LOWER VERTEBRATES

a duct of Botallus forming a connexio

A.

FIG. 188. —-Illustrating the modiﬁcation of aortic arches III-VI" during ontogeny in Scyllium, according to Dohrn.

A, Dorsal aorta; of, afferent branchiul; cm, unastornotic dues frol-H

vessel; cf, efferent branchial; -v.A, ventral aorta; 'v.c, visceral clefts; I1 I, IV, V, V’ I, aortic arches.

C11.

11 between the pulmonary

artery and the aortic root just as is shown in Fig. 186, B (LLB) for the Urodele amphibian. In other cases it may persist as a ligainentous vestige, as is the case on the left side in Tropidovzotus.

As a rule the portion of aortic root lying between arches III and IV disappears during , development but in most Lizards (not in Chaineleons and Monitors) i.t persists in the adult, so that in a dissection arches III and IV appear to run into one another peripherally.

In Monitors (Varanidae), in correlation with the elongation of the neck, arches Ill and IV hecorne widely separated from one another and the intervening portion of ventral aorta shows a corresponding lengthening both in its paired (common carotid) and its unpaired (primary carotid) portions.

In Birds (Fig. 187A, 1)) arch IV completely disappears on the left side and with it the portion of the left aortic root lying posterior to it. Consequently in the adult Bird there is only a single systemic aortic arch and it passes down the right side of the body.

ELASMOBRANCIIII.-— In the Ichthyopsida, as we should expect, the departthe primitive scheme are less pronounced; VI ARTERIAL SYSTEM 399

they are mainly in details. In the Elasmobranchs perhaps the most conspicuous of these is to be seen in the relations of the efferent vessels, each of which emerges, not from an ordinary aortic arch traversing a gill septum, but from a vascular loop surrounding a gill-cleft, the general arrangement being that shown in Fig. 188, 0. According to Dohrn (1886) this arrangement comes about in the following way. As the Walls of the elefts form lamellae and develop respiratory activity, two branches grow downwards, one anterior and one posterior, from the dorsal end of each aortic arch (Fig. 188, A, V1). These branches become connected together by cross bridges as shown in the ﬁgure (cm). The aortic arch now undergoes reduction and eventually becomes obliterated, just ventral to the point where the two branches are given off (Fig. 188, A, arch lll), so that the arch is now divided into two distinct parts-— a ventral afferent and a dorsal elferent—the latter prolonged ventralwards into the two branches. Of these the posterior branch of each pair becomes somewhat reduced in size, it develops a secondary connexion at its upper end with the efferent vessel next behind and loses its connexion with its original efferent vessel. The result is that, after this has happened, each efferent vessel again possesses two branches at its ventral end but these, instead of passing into the same gill septum, pass into two adjoining septa one in front of and one behind the intervening cleft (Fig. 188, B). Eventually the. ventral ends of each pair of branches become joined so that the cleft is now surrounded by a complete efferent loop-— the loops of successive c1eI'ts being connected together by a single persisting anastomotic vessel (Fig. 188, C).

In C’/olamg/doselmtlms the modification of the aortic arches just described does not take place.

The number of aortic arches corresponds with that of the visceral arches and is normally six.

In correlation with the presence of the pseudobranch on the posterior face of the mandibular arch in Elasmobranchs the first aortic arch in these ﬁshes is well developed but its primitive relations with the arterial scheme become much obscured owing primarily to the development of large new afferent and efferent channels connected with the pseudobraneh, which carries in its train the reduction of both the ventral and the dorsal portions of the original aortic arch.

1n the case of the second aortic arch, correlated with the fact that the anterior face of this visceral arch has lost its respiratory function, there is developed only a single, posterior, efferent downgrowth instead of two as is the case with the arches farther back. A Wide anastomosis between this and the ﬁrst aortic arch

just below the spiracle provides the secondary afferent vessel to

the pseudobranch which as already mentioned supplants the primitive afferent vessel formed by the ventral portion of the ﬁrst arch.

Q-If 400 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

CYeLos'1‘oMA'1'A. -——In the Lamprey it should be noted that according to Dohrn (1888) an aortic arch corresponding to aortic arch l of Gnathostomata makes its appearance and then disappears again. In Myxinoids the most important feature is that in them the number of aortic arches reaches its maximum for Craniata——-up to 14 in Jidellostovna.

CROSSO1’TERYGII.———Our knowledge is in this case very incomplete. The chief peculiarity (Graham Kerr, 1907) is that, correlated with the large size of the external gill belonging to arch II, which forms the sole respiratory organ during early stages of larval life, aortic arch II makes its appearance relatively early and the development of the other aortic arches is postponed. Distinct vestiges of aortic arch I make their appearance. The succeeding aortic arches rema.in small for a prolonged period. Aortic arch V I becomes much enlarged in its ventral part in correlation with the fact that it supplies the pulmonary artery.

AC'I‘IN01"l‘ERYGII.——-I11 the Teleostean ﬁshes and in the Ganoids that approach must nearly to them (Lepido.steu,.s'—-—I*‘. W. Muller, 1897; Am'ia—-—-Allis, 1000) complicated changes, which need not be detailed, take place in arches I and II in relation with the blood supply of the pseudobranch which in these ﬁshes (p. 159) comes to lie on the inner surface of the operculum.

DIPNOI.-—-I11 Lepidosiren (Robertson, 1913) aortic arches I and II never become complete well-developed vessels: they are vestigial and their ventral portions do not appear to develop at all. The remaining four aortic arches are well developed, each passes out into an external gill and in each an intercalary piece becomes developed to short-circuit the blood-stream at the time the external gills atrophy. In 0’emtodus and P9-otupterus efferent downgrowths make their appearance as in Elasmobranchs but they remain connected with the dorsal portion of their own aortic arch and do not undergo fusion at their ventral ends so that the condition in the adult departs less from the primitive than it does in the Elasmobranch.

AMPHIBIA»-—In U rodela the arrangement of aortic arches (Fig. 186, B) closely resembles that in Lung-fishes. Arches I and II are reduced: the latter in fact is according to Maurer (1888) no longer to be detected at all in the case of ’['7~zJt0n. Arches III, IV and V are prolonged outwards into external gills and in each case a short-circuiting piece becomes intercalated as the external gills lose their functional activity. According to Maurer the intercalary portion makes its appearance as a downgrowth from the dorsal or efferent limb of the aortic arch but this may perhaps be doubted in view of the fact that in Lung-ﬁshes the corresponding piece of vessel develops by the widening out of pre-existing chinks (Robertson, 1913).

AMNIO'1‘A.—-Arch IV along with the aortic root into which it passes forms the main systemic aorta on each side. That of the left side is connected, in correlation with the spiral twist of the VI ARTERIAL SYSTEM 401

conus and its derivatives, with the 'r'£_qht side of the ventricular cavity in proximity to the opening of the pulmonary artery. With the. separation of the two ventricles the arch in question remains connected with the right ventricle. With increasing efliciency of the pulmonary circulation the venous blood of the right ventricle would he drawn off more and more to the pulmonary artery and, correlated with this, we ﬁnd in those Sauropsida in which metabolism is most active and respiration most eﬁicient that this fourth arch on the left side with its aortic root disappears completely (luring development, leaving only the single right-hand arch and

VNIII

aczz 0?.

4/I ’ V)

Fm. l89.——lll()0(l-Vessels of (Jroeodile of stage 55-56. (After Hochstetter, 1906*.) Arteries are shown in outline, veins black.

u.I'.r, :mte1mr cardinal vein; .l.;, :lm't:(' root; /mt, b:1.~.Il:u artery; ¢l.a-, (l()l:~.'ll em-ot,ul ; oi, otocyst; p,«-.4‘, 1u)sfL'I‘I0l'('ardInal win; 1.)). right pulmonary an Lt-I) : ear. venttal e.'uot.d ; HI-\ I. aoilic an-hes.

root to form the proximal part of the systemic aorta. (Birds, Fig. 187 A, 1)). '

Arch V, in the Anmiota, appears only transiently and so greatly reduced in size as to have completely escaped the notice of the earlier investigators. Hence in Rathke’s classical scheme of the aortic arches which is given in the older text-books only ﬁve arches are shown, the posterior one being called the fifth. With our present-day knowledge of the homology of the lungs of Amniota with those of Orossopterygians and Lung—ﬁshes, such a scheme is clearly erroneous, as it would involve the pulmonary artery, which is certainly the same vessel throughout, taking its origin in the Ainniotes from the ﬁfth and in the Ichthyopsida from the sixth aortic arch. So without any special embryological data we should

VOL. 11 2 D 402 EMl%RY()L0(l‘rY OF THE LOWER VERTEBRATES CH.

be justified in believing that in the Ainniota an aortic arch has disappeared in front of the last one. The persisting vestiges of this ﬁfth arch, which are now known to occur commonly in the embryos of the Amniota, we"e detected ﬁrst by van llemmelen (1886) in Reptiles and Birds. A good example of such a vestigial ﬁfth arch is seen in the embryo of the Crocodile (Fig. 189). ‘

The cause of the reduction of this ﬁfth aortic arch is probably to be recognized in the fact that it receives its blood from the pulmonary cavity of the conus and of the ventral aorta (Graham Kerr, 1907*‘). As a consequence of this, during the evolution of the lungs as the main organs of respiration a larger and larger proportion of the blood in the cavity mentioned has become drawn off to the lungs, leaving less and less for arch V, with the natural result that the latter has become reduced to the verge ol'disappea1'ance.

Before leaving the subject of the aortic arches it is necessary to point out that their diagrammatic arrangement as shown in Figs. 186 and 187 is commonly much obscured in the adult. For during development there occur not merely the disappearance of large. portions of the original scheme of arches and the straightening out of the unnecessary curves, but also other complications. The chief of these are due to the longitudinal vessels———ventral aorta or aortic roots-——lagging behind in their growth in length. This leads, according to the position in which it takes place, to the crowding together of the ventral or the dorsal ends of consecutive aortic arches, and their mere approximation may he succeeded by actual fusion so that two or more arches may come to have a common root emerging from the. ventral aorta, or a common terminal portion opening into the aortic root.

PULMONARY AR'1‘En\'.——The pulmonary artery makes its ﬁrst appearance in Crossopterygian fishes (1’0l_7/ptelrus) as a branch from arch VI towards its ventral end which passes to the lung and adjoining parts of the pharyngeal wall. Throughout the series of lung—breathing Vertebrates it develops similarly as an outgrowth of the sixth aortic arch. A result of the main blq rod-St1'(‘a.1n of this arch passing olfinto the pulmonary branch is that the dorsal part of the arch lying beyond the point of origin of the pulmonary artery becomes as a rule reduced in size, forming the duct of Botallus. Except in the case of certain Reptiles (p. 397) the duct cl‘ Botallus becomes in normal individuals of the Amniota completely obliterated soon after birth.

In the Lung-ﬁshes the point of origin of the pulmonary artery is displaced up to the dorsal end of arch VI where it is fused with arch V. Thus arch V is able to carry blood directly to the pulmonary artery and correlated with this it does not undergo the reduction in size which has taken place in the Amniota.

In Lepiclosiren and Protopterus an important development of the pulmonary artery takes place inasmuch as its area of distribution extends on to the lung belonging morphologically to the other side of VI ARTERIAL SYSTEM 403

the body. Thus the right pulmonary artery comes to supply the dorsal side of both lungs, and the left artery the ventral side of both lungs. Each lung in other words receives a supply of blood from. both pulmonary arteries and this illustrates an initial step towards the condition in Am/in, where the right and left sides of the air-bladder— the homologue of the right lung—-are supplied with blood directly by a typical right and left pulmonary artery. ln the Actinopterygian ﬁshes apart from Amid. the pulmonary artery has disappeared entirely from development and the air-bladder receives its blood-supply by se.condary connexions with the dorsal aorta and its branches.

CAROTII) ARTERIES. As has already been indicated the great longitudinal arteries ventral and dorsal aortae-—-are prolonged forwards into the region of the head as the carotid arteries——ventral and dorsal. Of these the latter, receiving as it does, in the ease of the more primitive Vertebrates, blood which has been oxygenated by passing through the gills, becomes the more important and is responsible for supplying blood to the brain. .

The ventral carotids are found from the Lung-ﬁshes onwards—— perhaps in correlation with the reduction of the first two aortic arches. They are known eonnn only under the name external earotid( = lingual artery of Amphibia) and supply blood to the ventral side of the head, though cases are known amongst animals no longer having functional gills (certain Mammals) in which they take over the bloodsupply of the brain also.

lt will be convenient to consider first the carotids as they occur in the development of the Amniota. The simplest condition is found in an ordinary Lizard (Lacerm) where they are seen as apparent prolongations forwards of the aortic root and of the ventral aorta respectively. This simple arrangement becomes in otherAmniota modified during the course of development i11 different ways, of which the following are the chief. In various Lizards ag. Chameleons and Monitors (and the same holds for the great majority of Amniota ——Fig. 187, A) the portion of aortic root between aortic arches III and IV disappears during development, the consequence being that arch III comes to form the posterior portion of the internal carotid artery, becoming drawn out in the process of growth so as to be in line with the front part of that artery derived from the aortic root. The paired part of the ventral aorta from which the third arch was given off now becomes the common carotid artery (ac). The unpaired portion of the ventral aorta, from which the common carotids spring, is known as the primary carotid and in the long-necked monitors this becomes much elongated, the growth in length of the neck taking place in the region between aortic arches Ill and IV (Fig. 187, A).

In the European Grass-snake (Tropidonotus, Fig. 187,- B) an anastomosis is formed between the two internal carotids just behind the head (Fig. 187, B, an) and, correlated with this, the right common carotid as a rule disappears except for a small branch at its hinder 404 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

end which supplies the Thyroid gland (O'Donoghue, 1912). The blood-supply of the head-region therefore passes to it entirely by the persisting left common carotid (c.c). In Snakes other than ’1'rop7ld0notus considerable variety exists in the condition of the common carotids. Thus amongst the Boidae the two arteries may remain of approximately equal size (Bria) or on the other hand the left may be reduced (Pg/tIa.0'n.).

In Chelonians and Crocodiles (Fig. 187A, C) the growth in length of the neck takes place in the region in front of arch 111 so that here it is the portions of the carotid arteries in front of this level which undergo elongation. In both of these groups an anastomosis forms in the head-region between dorsal and ventral carotids and, correlated with this, the main blood-stream tends to pass to the head by the dorsal carotids, the ventral vessels becoming to a less (Crocodiles) or greater extent (Chclonians) reduced in size (van Bemmelen, 1887; Mackay, 1889). In the Crocodiles a still further modiﬁcation takes place, inasmuch as the two dorsal carotids become for a considerable, part of their length fused together into a single vessel, and following upon this aortic arch lll of the right side atrophies, so that here as in '/'7"0})id07L0tus, though for a different reason, the main blood-supply of the head comes from the left side.

In the Birds the condition closely resembles that of the Crocodile. Here also the ventral carotids become reduced —in this case to the point of complete disappearance——in the neck region as a (‘.O11b'e(_111OIlC0, of an anastomosis with the dorsal carotids in the head. Here also the. enlarged dorsal carotids approach one another on the ventral side of the vertebral column. In those birds which depart least from the primitive condition in this respect (Ostrich, Emu, Casuari, Tinamus, Penguins, Divers, Gulls, Plovers, Snipe, Rails and their allies, Fowls, Pigeons, Ducks, Ihises, Storks, Herons, Cormorants and some Gannets, Birds of Prey, Parrots, Hornbills, Motmots, Goatsuckers) the two deﬁnitive carotids merely lie in proximity to one another. I11 many birds however they become fused together into a single vessel over a great part of their length and in such a case there may be no further modiﬁcation (certain Herons such as the common Bittern, some Cockatoos, some Uannets), or, as is the general rule, this fusion is followed by the disappearance of the third aortic arch of the right side just as was the case in the Crocodiles (Rhea, Apterg/ac, Grrebes, Quails, some Cockatoos, Capitonidae, 'l‘oucans, Hoopoe, Meropidae, Trogons, VVoodpeckers, most Swifts, Humming birds a11d Passerine birds). I11 a few cases on the other hand it is the third aortic arch of the left side which becomes reduced to a small vestige (l3‘l-amingo) or disappears entirely (.E'upod0t'is—-— the African Bastard).

Amongst the anamniotic Vertebrates typical dorsal and ventral carotids are present in Amphibians a11d Lung-ﬁshes. The arrangement in Urodeles is illustrated by Fig..186, B: the chief peculiarity to be noted is that the posterior portion of the external carotid (12.0) VI - ARTERIAL SYSTEM 405

has disappeared in the adult, the persisting anterior portion receiving its blood through a new anastomotie channel (cm) from the third aortic arch. The posterior portion of the “external carotid” or “lingual artery ” of the adult Amphibian is really constituted by this new development.

The connexion of this newly developed portion of vessel with arch III is just at the point where the short circuit is formed between the afferent and efferent parts of the arch, and in Lungﬁshes (.Lep'id0si7*em——Robertson, 1913) the blood-supply for the external carotid comes to it, for a time during early stages, from the dorsal end of arch III or from the aortic root, through what seems to be a precocious development of this same short-circuiting channel. In this case the vessel in question is at first simply continued from its dorsal origin forwards into the external carotid: it is only later that it communicates with the ventral or afferent end. of arch III so as on the one hand to form the short circuit, and on the other to permit the blood to pass to the external carotid from the ventral aorta.

In the more typical ﬁshes the ventral carotid is not yet present. The dorsal carotids of the two sides develop an anastomotic connexion beneath the base of the skull so to form with the aortic roots a complete “ cephalic circle” which shows characteristic differences in different Teleostean ﬁshes (Ridewood, 1899).

INTE'RSE(_T}MENTAL Anrsmes or TIIE BoI)Y-WALL.-—-'1‘he dorsal aorta gives off on its dorsal side paired arteries which run out into the body wall between the Inyotomes. One of the most important features of this series of intersegmental vessels is that for a time during early stages of development they provide the blood-supply to the limb rudiments.

The main artery of the fore-limbw-the subclavian artery-— appears during the stages in question to be simply a prolongation into the limb rudiment from one of these intersegmental arteries»not necessarily the same artery of the series in different types of Vertebrate, or even in different developmental stages of the same Vertebrate. Thus in Lacerta it is said to be the seventh intersegmental artery (van Bemmelen, Hochstetter, 1906) which becomes the subclavian artery and in the Fowl the ﬁfteenth (eighteenth or nineteenth if the three or four intersegmental vessels in the headregion are ineluded—-—Hochstetter, 1890). In the Duck, Rabl (1907) found that during the ﬁfth day of incubation the subclavian varies from the eighteenth to the twenty-ﬁrst intersegmental artery and that in some cases two or even three such Vessels may pass out into the limb rudiment at one time.

Probably we may take it that the‘ general principle at work is this——-that the limb, as it became shifted along the side of the body in the course of evolution, received its blood-supply from successive intersegmental arteries as it came to be opposite to them, and that during ontogeny there takes place an imperfect repetition (bf this process.

The fact that the pectoral limb is supplied with blood. by an 406 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

intersegmental artery raises the question whether or not this is to be regarded as the primitive mode of blood-supply. For if this question he answered in the affirmative We should be confronted with a11 important point which would have to be borne in mind in all speculations as to the evolutionary origin of the limbs of Vertebrates. As a matter of fact, however, recent investigations tend to answer this question in the negative.

In the Chick (Evans, 1909) the limb rudiment in its earliest stages is traversed by an irregular network of blood spaces and this receives its blood-supply directly from the dorsal aoi ta by a number of slender ehannels——it may be as many as ten or eleven on the right side where they are commonly most numerous. These vessels are scattered irregularly over an antero—posterior extent of from three to ﬁve mesoderm segments and they take their origin from the dorsal aorta quite independently of and considerably ventral to the intersegmental arteries. As developmeiit proceeds a few of these supply channels those. which happen to be most nearly intersegmental in position———beco1ne relatively larger and ﬁnally a single one, at about the level of the eighteenth intersegmental artery, becomes especially enlarged and carries the main stream of blood to the limb rudiment while the others gradually diminish in size and eventually disappear. The persisting enlarged Vessel becomes the subclavian artery and secondarily its origin from the aorta becomes displaced in a dorsal direction until eventually it arises by a common root along with the intersegmental artery of which it now appears to form a branch.

In the Duck similar observations have been made so we are probably justiﬁed in stating that in the earliest stages of ontogenetie development the blood-supply of the pectoral limb is not metameric, and that the relation with the intersegmental artery observed in slightly later stages is a secondary acquii-ement.

In most vertebrates the subclavian artery which arises in the manner above. described (primary subclavian) persists throughout life. In Birds however a cross connexion develops between the primary subclavian just at the base of the limb, and the ventral end of the third aortic arch. This cross connexion gradually increases in size while the proximal part of the primary subclavian, arising from the aortic root, becomes correspondingly reduced and eventually disappears entirely. The result is that the permanent artery of the fore-limb in the adult branches off’, not from the dorsal aortic root but from the deﬁnitive ('z3.e. morphologically “intcrnal”) carotid, close to its hinder end. . A similar substitution takes place in Chelonians and Crocodiles (Fig. l87A, O) and this is the explanation, ﬁrst given by Mackay (1889), of the otherwise puzzling fact that in certain Vertebrates the subclavian artery passes out ventrally to the vagus nerve (“secondary subclavian ”) instead of dorsally as it does normally.

The iliac artery to thehind limb has also been traced back in at least Lizards and Birds to the series of intersegmental arteries but vr ARTERIAL SYSTEM 407

here again it would appear from later investigations that the deﬁnitive artery is merely a surviving and enlarged representative of a number of original supply vessels (Evans, 1909).

’tepresentatives of the series of intersegmental arteries are recognizable in various Vertebrates in the hinder part of the headregion. Thus in Lacerta (van Bemmelen, Hochstetter, 1906) three have been detected in the head-region. Of these the first two disappear while the third becomes prolonged forwards immediately beneath the brain to become continuous with the internal carotid at the level of the mid-brain. These prolongations fuse together in the mid-line and form the basilar artery.

A similar basilar artery continuous with the internal carotids is of common occurrence in Vertebrates though the relations of its paired fore-runner to the series of intersegmental arteries differ in different forms.

The vertebral artery of Sauropsida is in its origin intimately related to the interserrmental arteries. Thus in Lacertrr. the intersegmental vessels posterior to the subclavian become connected together by a longitudinal anastomotic vessel, which persists and forms the cervical portion of the vertebral artery. In Snakes the two similarly-arising vessels apparently usually undergo fusion together in their anterior portion while farther back the unpaired condition is reached by the disappearance of the rudime.nt on the right side (Hochstetter, 1906).

The arterial blood-supply of the urino-genital organs is also generally provided by branches from the intersegmental arteries of the embryo.

M1«JsEN'1‘E1tIC Au'1‘E1uEs. The digestive tract receives a varying number of branches from the dorsal aorta which may be at first paired and undergo fusion secondarily or may be impaired from the beginning. In those vertebrates which have a bulky yolk—sae a pair of these are precociously developed as vitelline arteries. In some cases there is a remarkable relation between the chief mesenterie artery (coeliac) and the prone] )ll1'OS. Thus in L6})’id08'I:I'(3'Il. a connexion becomes established between the blood-spaces of the. right pronephros and those of the gut wall, and the branch of the dorsal aorta which supplies the right pronephros persists as the root of the definitive eoeliac artery. In such Teleostean ﬁshes as the. Trout the connexion with the pronephric arterial supply is only temporary, a new anastomotic channel arising farther back between dorsal aorta

and mesenteric artery which remains as the deﬁnitive root of the latter vessel.

VENOUS SYsTEM.——-As an example of the development of the

venous system in a holoblastic Vertebrate we will take that of Lep1Idos'£1'e7t (Robertson, 1913)}

‘ For comparison with Lcpidosirmz, an account of the development of the vascular system of Uemtodus will be found in Kellicott (1905). 408 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.vI

The ﬁrst part of the venous system (Fig. 190, A)—indeed of the, vascular systen1——-—to take definite form consists of the two vitelline veins, which pass tailwards ove.r the anterior surface of the yolk (stage 24). Anteriorly they become conjoined to form the heart while ,‘posteriorly they are continued into the rudiments of the vitelliue network (stage 24-25). The ap]»earanee ol' the vitelline. veins is followed almost immediately by the development of a longitudinal venous channel on each side anteriorly, superlicial to the aortic arches ——the anterior cardinal vein. At its hinder bud the anterior cardinal is continued into a set of venous spaces in the region of the pronephros (pronephric sinus) and onwards behind this as the posterior cardinal vein.

The pronephrie sinus is continued along its oute.r edge, by a number of channels, into the vitelline network of venous spaces, lying in the, splanchnic niesoderni over the surface of the yolk. In this network a conspicuous channel becomes apparent, leading from the anterior end of the ]_)l'()ll£’1)lll'lC sinus outwards to the vitelline vein, and so, by way of the anterior part of the vitelline Vein, to the heart. This vessel so constituted, which at iirst makes a wide sweep over the lateral surface of the yolk, is the Duct of Cuvier (Fig. 190, b, (L0). As development goes on the Ducts of (luvier becoine greatly shortened and at the same time widened until e\'e.ntually they form in the adult very short wide channels for the conveyance of the blood from the cardinal veins into the sinus venosus (see. Fig. 190, b and e and d).

The posterior cardinal vein on each side ap])ea1‘s about stage 24 in the form of spaces along the course of the archinephrie duct which become joined up so as to form two longitudinal Vessels running parallel to the duct, one on its mediodorsal the other on its ventrolateral side, the two vessels being joined round the duct by numerous anastomoses (Fig. 190, h and e, ;u.c.v).

These. posterior cardinal veins accompany the archinepliric duets throughout their length and just in front of the cloaca are joined by the hind ends of the bifurcated subintestinal vein (see below) and of the dorsal aorta. The vessel formed on each side by the union of these three elements is continued back last the cloaca and unites with its fellow of the opposite side to form a vessel lying iinmediately beneath the post-anal gut. This vessel is to be interpreted 1norpho_ logically as a post-anal ])o1‘tion of the sul )lllbC‘Sl}l]l{1]. vein and as will be shown later it is destined to become the caudal vein of the adult.

It will now be convenient to trace. out the subsequent fate of what may be called the dorsal venous system, consisting primarily of the anterior and posterior cardinal veins.

POSTERIOR CARDINAL V1«11Ns.——’l‘he posterior cardinal Vein Was left in the form of a pair of vessels, an inner and an outer, lying close to the archinephric duct and connected together by numerous anastomoses. As the opisthonephros develops between these channels they consequently co1ne,over a considerable part of their length, to be VI . VENOUS SYSTEM 417

blood-stream passes through these communications into the posterior cardinals and the latter take on the appearance of a direct forward prolongation of the caudal vein.

Dorsally there develops on each_ side a cardinal trunk which swells out into a great irregular sinus (pa) in the region of the pronephros, On its outer side branches pass from the pronephric sinus into the general vitelline network. In this network a specially wide channel develops, starting from the pronephric sinus and sweeping outwards over the yolk to join the lateral’ vitelline vein and so reach the heart. This channel, which becomes gradually more and more sharply defined, is the duct of Cuvier (Fig; 193, A, (L0). The pronephric sinus is continued backwards into the posterior cardinal vein. This is at ﬁrst distinct from its fellow but at an early stage fuses with it to form a median inter—renal vein (Fig. 193, B, 7I7')—--the fusion being foreshadowed by the development of anastomotic conncxions between the two veins while still separated from one another by a distinct space (Fig. 193, A, an). Towards the cloaca the posterior cardinals taper off and are connected by irregular anastomotic ehannels_ with the subintestinal vein, as already mentioned, and also with the dorsal aorta. Eventually, as -already indicated, the inter-renal vein and the caudal vein form a continuous vessel.

During the later stages a striking asymmetry becomes apparent in the anterior, unfused, portions of the posterior cardinals——the left becoming greatly reduced as compared with the right (Fig. 193, D). The main blood-stream thus passesforwards on the right side, and upon this side a special direct channel develops on the ventral side of the pronephros, through which the blood-stream is able to reach the duct of Cuvier without passing through the tangle of pronephric tubules. The asymmetry affects also the ducts of Cuvier-— showing itself ﬁrst in that of the left side becoming relatively shorter than its fellow, which retains for a time its wide sweep over the surface of the yolk (Fig. 193, B and C, d.(/"9. Eventually it too becomes shortened and its calibre becomes considerably greater than that of the left side (Fig. 193, D).

From the pronephric sinus a branch (Z/2:) develops about stage 30 which passes dorsalwards and then backwards beneath the lateral line nerve-—the lateral cutaneous vein. This, a large vessel about stage 33, becomes reduced to an insigniﬁcant vestige later on.

The anterior cardinal vein runs along the side of the head region, passing through the angle on the ventromesial side of the otocyst, between the latter and the brain-wall. At its front end the vein dilates into a large sinus, which gives off irregular branches to the mesoderm of the head. At an early stage an anastornotic channel makes its appearance on the outer side of the otocyst continuous anteriorly and posteriorly with the anterior cardinal. When this channel has been established (Fig. 193, A, La) the blood-stream from the head divides in front of the otocyst and passes backwards, part

VOL. 11 2 E 410 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

separated from one another by a considerable space in which the renal organ lies. As this happens the inner components of the two posterior cardinals become approximated and eventually undergo fusion with one another to form an inter-renal vein (Fig. 190, d, 'i9'.'v). In _Lep1Idosi'ren this fusion is only temporary and the two components again recede from one another—remaining, however, connected by a small number of anastomotic vessels (Fig. 190, e). There now takes place a severance of the continuity of the inner component at its hinder end (Fig. 190, e,"*), and a little later a similar severance of the outer component at its front end. The physiological result of these interruptions of continuity is that the blood from the caudal region now reaches the opisthonephros entirely by way of the outer component, it then passes through the substance of the kidney and is draine.d away entirely by the inner component. In other words the outer component and its backward continuation has now become the renal portal Vein.

While these changes are going on in the opisthonephric region of the posterior cardinal the portion of that vein in front of the opisthonephros takes on the form of a single channel, the original inner component becoming enlarged while-the outer component becomes reduced and eventually disappears. The rightand left posterior cardinal veins in this region in front of the opisthonephros become connected by numerous transverse vessels (Fig. 190, d). A short circuit now becomes established by which the, blood from the right posterior cardinal can pass direct to the sinus venosus through the substance of the liver (Fig. 190, d, 12/0.0).

This short-circuiting channel is of great morphological importance. It constitutes the intrahepatic or headward section of the posterior (“inferior”) vena cava, which in the higher vertebrates becomes the largest vein in the body. lts appearance here is followed by two important results. (1) The main blood-stream from the kidney region tends to pass to the heart, more and more by this direct channel, which in correlation with this becomes larger and larger. (2) The portion of right posterior cardinal vein lying behind its junction with the intrahepatic channel becomes correspondingly e11la.I'ge( l.

These two components together constitute the deﬁnitive posterior vena cava, in which vessel therefore. we recognize two fundamentally distinct portions, an anterior or intrahcpatic, and a posterior or cardinal. A secondary result of the diversion of the blood-stream from the right posterior cardinal vein through the hepatic component is that the anterior portion of the former vein, lying in front of the junction of the two components, becomes relatively reduced in size. It loses its continuity with the rest of the. right posterior cardinal and persists as the small vein shown at 12 iii Fig. 190, e.

An important clue to the origin of the. posterior vena cava in phylogeny is given by the condition seen in the adults of existing L1.1ng-ﬁshes. The opisthonephros in these vertebrates retains its VI VEINS OF LEPIDOSIREN 411

primitive elongated form, extending far forwards in the splanchnocoele, and the iront end of the right opisthoncphros is in immediate apposition to the tip of the liver which is also situated dorsally and on the right side. It is no doubt the approximation of the tips of these two organs, still persisting iii the adult Dipnoan, which paved the way for the establislnnent of direct continuity between their vascular networks and the consequent short-circniting of the renal blood through the hepatic vci11 into the heart (Graham Kerr, 1910)

The ladder-like connexions between right and left posterior aardinals in the region anterior to the inter-renal vein gradually disappear in turn from before backwards (Fig. 190, d and e).

ANTERIOR CARDIN AL V EIN s.——Apart from relatively less important details, the chief change which comes about in regard to the anterior cardinal is the diversion of the blood-stream about the level of the otocyst into a more laterally placed channel called the lateral cephalic vein, which eventually becomes intercalated in the course of the anterior cardinal, and to all appearance forms simply a portion of that vein (Fig. 190, (1, L0). The anterior cardinal vein at ﬁrst passes back along the side of the head region (following the course shown by the dotted outline i11 Fig. 190, d), ventral to the otocyst and internal to the posterior cranial nerves, to join the front end of the pronephric sinus. Presently (stage 30) a branch of the anterior cardinal vein makes its appearance and extends backwards external to the ganglion of the seventh cranial nerve, bending inwards and n-joininlg the anterior cardinal between the eighth and ninth cranial nerves. A little later (stage 31) the vessel forming the outer side of this loop becomes prolonged back and forms a second loop external to the ninth cranial nerve and rejoining the main Vessel between nerves IX and X. Finally (stage 31 + ) a similar extension backwards occurs external to the vagus, rejoining the anterior cardinal vein just behind it. ’l‘he lateral cephalic vein develops from the outer portions of these three vascular loops, the development of each of the three segments being followed by the atrophy of the corresponding section of the original anterior cardinal, except in the case of the most posterior section which persists as a short wide vein opening along with the posterior cerebral vein (ce”) into the deﬁnitive anterior cardinal.

VENTRAI. VENOUS SYSTEM.-—In addition to the dorsally placed cardinal veins there. exist certain important veins situated more ventrally and developed in relation with the vitelline veins. The vitelline veins spread backwards on each side as a wide vessel consisting of an enlarged channel of the vitelline network which covers the whole surface of the yolk. Posteriorly they unite in the. midventral line to form a very short subintestinal vein in front of the an11s (Fig. 190, O, S.’t.’I)). Later on the space bounded by the two vitelline veins becomes bisected by a median ventral vein which looks like a prolongation forwards of the subintestinal vein and 412 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

is known by the same name. (Fig. 190, D, s.z';v”). It will be realized that this anterior section of the subintestinal vein is developmentally of a different nature from the posterior portion for it is formed by a short-eircuiting of the blood-stream through the vitelline network, while the posterior portion represents rather the conjoined hinder ends of the paired vitelline veins (Fig. 190, O and D). This difference in development is no doubt purely secondary and we may take it that the later condition, where the subintestinal vein is continuous right forwards to the heart, represents the really primitive condition of this vein in evolution. The continuity of the subintestinal vein at its front end with the heart is brought about in ontogeny through blood-sinuses which make their appearance in the liver (stage 3]).

The right vitelline vein and the anterior, secondarily formed, portion of the subintestinal vein now gradually disappear (about stages :52 - ‘%5). The left vitelline vein ceases to form a continuous channel over the surface of the liver to the heart, so that the blood in it is forced to traverse the system of blood sinuses within the liver. 111 other words the whole of the blood which streams forwards in the subintestinal vein is diverted along the persisting left vitelline vein into the network of blood spaces in the liver. Subintestinal vein and left vitelline vein have thus come to constitute the hepatic portal vein. 'l‘he latter becomes complicated by a branch sprouting out from the front end of its subintestinal portion. This branch spreads round the alimentary canal along the line of the spiral valve, fusing with the subintestinal vein at each point of intersection (Fig. 190, F). As the liver increases in length a special supply channel from the portal vein lengthens out along its left side, giving off numerous branches into the liver substance (Fig. 190, F). From this the blood drains by numerous efferent vessels into the intrahepatic portion of the posterior vena cava.

CAUDAI. VI«:IN.——’.l‘hc post-anal portion of the subintestinal vein was left (p. 408) at a stage when its anterior bifurcated portion was continuous on each side not only with the pre-anal portion of the same vein but also with the dorsal aorta and the posterior cardinal. As development goes’ on the first two of these connexions disappear so that the post-anal subintestinal vein is new continuous anteriorly only with the posterior cardinal. VVith the atrophy of the post-anal gut, it comes to lie immediately beneath the caudal portion of the dorsal aorta and is now known as the caudal vein, its anterior forked portion forming the binder ends of the renal portals.

The veins which have been described constitute the main trunks of the venous system; in the later stages of development a number of other important vessels appear which are indicated in the diagrams. The subclavian vein (Fig. 190, d, .9) appears about stage 31 + leading from the pectoral limb into the pronephric sinus. As this sinus atrophies the point of opening of the subclavian vein comes to be situated on the anterior cardinal vein (Fig. 190, e, s). From the subclavian vein a small lateral cutaneous vein (Lo) passes back VI VENOUS SYSTEM 413

in tl1e body-wall. An inferior jugular vein (7}.j) passes back from the head-region, lateral to the pericardiae cavity, and opens into the anterior cardinal close to its hind end and on its ventral side. In later stages the po_int of junction of the inferior ji1g11la1' with the anterior cardinal comes to be shifted relatively forwards, as was the case with the suhclavian. Farther forwards the anterior cardinal is joined by an anterior and a posterior cerebral vein (ce’ and ca”) from

d.C.

A B C

D

Flu. 191.—Diagrams illustrating early stages in the development of the venous system of Elasmobranchs according to Rah] (1892) and Hochstetter (1906).

«.0. v, anterior cardinal vein; om, fused portion of \-'it'..ellim- veins behind liver; rl, posii.ion of cloaca; 4l._('_', duct oi‘ Uuvier; h..:', hepatic vein; p.(_'.'1‘, posterior cardinal vein; s.i.'v, subintestinal vein; ruv, vitelline vein ; :0/..s'.v, main vein from yolk sue.

the inside of the head. At the hind end of the system rectal (7') and pelvic (pl) veins open into the renal portal. The former appear to be the persistent remains of the anastomotic branches which in early stages connected the hind end of the subintestinal vein with the posterior cardinal.

ELASMOB_RANCIIII.——It is instructive to compare with the development of the venous system in a holohlastic vertebrate the corresponding phenomena as they occur in the Elasmobranchs, the lowest of the nieroblastic gnathostomes. An inspection of Fig. 191 brings out the most conspicuous difference, one that could be foretold a priori, 414 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

namely that, in agreement with the general principle «that animals with a large supply of yolk tend to show a precocious development of the vessels on the surface of the yolk, the vitelline veins and their derivatives make their appearance relatively earlier as compared with the cardinals than they do in Lepirlomlren.

Here again the first of the main trunks to make their appearance are the vite.lline veins, which by their fusion anteriorly form the heart (Fig. 191, A, 12.12). The most conspicuous difference in the next subsequent stages is that the two veins are for a considerable period strongly asymmetrical (Fig. 191, B), the right becoming greatly “reduced, and only the left being commonly traceable back into the subintestinal vein (1’r7Istiurus——Rab1, 1892 ; Accmth'ias——Hoffmann, 1893). There can be no reasonable doubt that this is to be looked upon as a secondary modiﬁcation of a symmetrical condition of the two veins as seen in Le}7'id0s’iTe7b, and probably the impelling factor which has brought about the modification is to be recognized in the fact that the main channel for draining away the blood from the surface of the yolk-sac is situated on the left side and opens into the left vitelline vein (Fig. 191, 3/.s.v).

The unpaired mesial prolongation forwards of the subintestinal vein between the two vitelline veins which was so conspicuous in Lepidoselren has not been noticed in Elasmobranchs.

Both vitelline veins break up into a network as they traverse the liver, the parts of the veins in front of this network persisting as the hepatic veins of the adult (Fig. 191, l), /my). The portion of right vitelline vein behind this network is for a time much reduced, the left alone serving to supply the network with blood. Immediately behind the live.r, and in front of the yolk-sac vein, the two vitelline veins are fused together for a short distance into a single vessel (Fig. 191, C, an), and the same is the case again from about the level of the pancreas backwards where they form the subintestinal vei11, though it should be noticed that there still persist during early stages of the development of the subintestinal vein in Elasmobranchs distinct traces of the paired condition, the vein consisting of two parallel components situated on each side of the mid-ventral line of the intestine. These components soon become connected together by numerous anastomoses and eventually fuse completely to form a median vessel except where this is prevented by the presence of the cloaca.

The main vein from the yolk-sac (3/.s.-v) joins the left vitelline vein in the region in front of the pancreas and presently the portion of left vitelline vein behind the opening of the yolk-sac vein disappears. It thus comes about that the blood from the subintestinal vein passes forwards to the liver entirely through the right vitelline vein---in contrast with Lepidosiren where it did so by the left vitelline vein.

Cardinal veins and duct of Cuvier make their appearance, the chief difference from Lepidosdren being the comparative shortness of VI VENOUS SYSTEM 415

the duct of Cuvier which does not take the wide sweep over the surface of the yolk that it does in Lepitlosrkren during early stages. This difference is related to the fact that here the first rudiment ‘of the duct of Uuvier opens into a portion of the vitelline vein which has fused witl1 its fellow to form the hind end of the cardiac tube, while in Lep'izl0s'i7'e7t it opens much farther back, the result being that in Le_p'i(l0.‘i'r6'n a considerable stretch of free vitelline vein becomes incorporated in the definitive duct of Cuvier (compare Figs. 191, C and 190, b).

As in Lepwlclosirevi. the caudal vein becomes continuous with the posterior cardinals and loses its continuity with the pre-anal portion of the subintestinal vein. The inter-renal vein however develops here simply as a forward extension of the caudal vein according to Rabl. A number of anastomotic vessels connect up the inter-renal vein with the “posterior cardinal ”—-—the equivalent of the external component of this vein in Lep'idos'£7'en. The posterior cardinal now becomes obliterated behind the anterior one of’ theseanas tomotic vessels while the inter-renal becomes separated off from the caudal vein so that the whole blood-stream from the latter has to pass through the kidneys to reach the inter-renal. The latter vein splits into a pair of vessels eventually, thereby revealing that the inter-renal vein here is homologous with that of L6p'icl0si}'c7l in spite of’ its difl'erent———no doubt secondarily modified-—mode of development.

The anterior cardinal vein here again becomes in part replaced by a lateral cephalic vein.

In the lower vertebrates in general we may recognize the same main trunks as occur in Dipnoi and Elasmobranehs, with differences in detail. The following account gives an outline sketch of the development of the venous system in the various groups, the outline being filled in more fully in the case of 1’ol3/pte7"u.s on account of the very archaic character of this ﬁsh.

CYeI.0s'1‘0MA'1‘A.———ln the Lamprey, according to Goette (1890), the pair of vitelline veins appear first, spreading backwards on either side of the liver rudiment and meeting behind in the unpaired and much dilated subintestinal vein (Fig. 192, A). The vitelline veins break up into a network in the liver b11t on the left side the posthepatic section of vitelline vein disappears, so that the hepatic portal vein is formed by the subintestinal and right vitelline vei11 (Fig. 192, B and ())—somewhat as in the Elasmobranch, and unlike Legnldosiren where it is the right vein which disappears. 'l‘he vein of the “spiral valve.” of the intestine arises comparatively late, at the time of metamorphosis according to Goette, and on the opposite side of the gut from that on which the subintestinal vein lies. This latter is no longer ventral but high up on the right side, owing to a rotation which the gut has undergone.

The anterior and posterior cardinal veins present the peculiarity that they open at first separately into the vitelline veins. Later they become fused together to form the duct of Cuvier. Eventually the 416 EMBRYOLOGY OF THE LOWER VERTEBRATES on.

left duct of Cuvier disappears completely (Fig. 192, C) the blood from the left cardinals passing to the right side by a new anastomotic vessel which develops ventral to the dorsal aorta (Fig. 192, C, an)—-an arrangement presenting a remarkable analogy with what happens in certain Mammals.

CROSSO1’TERY(‘}1'I.——-II1 these archaic Teleostonies the main features of the development of the venous system have been investigated in P0lypterus—the less specialized of the two surviving genera (Graham Kerr, 1907).

In the earliest stage described there is a well-developed subintestinal vein which in front breaks up into a vitelline network.

Fm. 19‘3.-—-l)evelopment of veins in P(’tI'()uJ_7/2-;'n-N. as seen from the ventral side. (Al'1'er Goette, 1890.)

c_z.r.r, anterior c-ardinal vein; rm, anastomotic vein ; (l.l', duct. of (juvia-r; hm, lmpatic veins; Ii, outline oI.']i\'e1'; 1.11:9, left vitelline vein; p.rt.:', posterior ca1'tlina.l win; r. 121', right vitelline VI-in; s.i..a_-, suhintr-stinal win. Portions of the venous trunks which disappear during,‘ ontogeny are shown by dotted 0llhlllll'h' ; the outline of the liver is shown in IS by an into-rrnptetl line.

This drains into a pair of lateral vitelline veins which unite in front to form the heart. Postcriorly the subintestinal vein bifurcates to pass on each side of the cloaca and then joins again to form the post-anal. subintestinal vein. In its double cloacal portion, and also in front of this, wide communications pass between the subintestinal vein and the posterior cardinals. The ultimate fate of the subintestinal vein differs in its pre-cloacal and post-cloaéal portions. The former loses its identity in that it becomes entirely resolved into, portions of the vitelline network. The post-cloaeal portion becomes converted into the caudal vein in the normal fashion as already described for Lepid0s'i'r(m. It has already been mentioned that wide communications were established between the paired cloacal portion of the subintestinal vein and the posterior cardinals. With the breaking up of the pre-cloacal part of the vein the main VI VENOUS SYSTEM 417

blood-stream passes through these communications into the posterior cardinals and the latter take on the appearance of a direct forward prolongation of the caudal vein.

Dorsally there develops on each side a cardinal trunk which swells o11t into a great irregular sinus (gm) in the region of the pronephros. On its outer side branches pass from the pronephric sinus into the general vitelline network. In this network a specially wide channel develops, starting from the pronephric sinus and sWee.ping outwards over the yolk to join the lateral vitelline vei11 and so reach the heart. This channel, which becomes gradually more and more sharply deﬁned, is the duct of Cuvier (Fig. 193, A, (1.0). The pronephrie sinus is continued backwards into the posterior cardinal vein. This is at first distinct from its fellow but at an early stage fuses with it to form a median inter-renal vein (Fig. 193, B, 7}/-)——the fusion being foreshadowed by the developme.nt of anastomotic eonnexions between the two veins while still separated from one another by a distinct space (Fig. 193, A, cm). Towards the eloaea the poste1'io1' cardinals taper off and are connected by irregular anastometic chan11els_ with the subintestinal vein, as already mentioned, and also with the dorsal aorta. Eventually, as -already indicated, the inter-renal vein and the caudal vein form a continuous vessel.

During the later stages a striking asymmetry becomes apparent in the anterior, unfused, portions of the posterior cardinals——the left becoming‘ greatly reduced as compared with the right (Fig. 193, D). The main blood-stream thus passes forwards on the right side, and upon this side a special direct channel develops en the ventral side of the pronephres, through which the blood-stream is able to reach the duct of Uuvier without passing through the tangle of pronephric tubules. The asymmetry affects also the ducts of (}uvier—— showing itself first in that of the left side becoming relatively shorter than its fellow, which retains for a time its wide sweep over the surface of the yolk (Fig. 193, B and (J, d.(/'). Eventually it too becomes shortened and its calibre becomes considerably greater than that of the left side (Fig. 193, D).

From the prenephric sinus a branch (lxz-) develops about stage 30 which passes dorsalwards and then backwards beneath the lateral line nerve——tl1e lateral cutaneous vein. This, a large vessel about stage 33, becomes reduced to an insigniﬁcant vestige later on.

The anterior zardinal vein runs along the side of the head region, passing through the angle on the ventromesial side of the otocyst, between the latter and the brai11-wall. At its front end the vein dilates into a large sinus, which gives off irregular branches to the mesoderm of the head. At an early stage an anastomotic channel makes its appearance on the outer side of the otocyst continuous anteriorly and posteriorly with the anterior cardinal. When this channel has been established (Fig. 193, A, Le) the blood-stream from the head divides in front of the otocyst and passes backwards, part Fm. 193.——Devc1opmeut of dorsal venous systeiii of 1"(')ly}Jte‘l"lt8_, as seen from the dorsal side.

Stages 27-29 (A); 31 (B): 33 (C); 30 mm. larm (D and E). a.c.w, anterior cardinal vein; an, aliastomotic vein; (1.! ‘, duct of Cuvier; hm, hepatic vein; h.('p.*v.('), pmzteriol‘ vena cavn; i.j, inferior jugular ; 1'7‘, inter-renal vein; l.c, lateral coplialic; I.«v,1ato,ru1 cutaneous; 31, pulmonary vein; p.c.v, posterior cardinal; pn, pronepin-ic sinus; s, .~:ubc1m'ian Vein ; ’l'h, thyro d.

418 CH. v1 VEIN S OF POLYPTERUS 419

on its outer and part on its inner side. Eventually the inner channel becomes constricted across and ﬁnally completely severed at its hinder end so that the whole blood-stream passes back by what was the outer channel. The inner channel persists as a small vein which opens at its front end into the deﬁnitive anterior cardinal. We see then that here, as in the Lung-ﬁsh or the Elasmobranch, the “ anterior cardinal” vein of later stages has intercalated in its length a segment of lateral cephalic vein.

Au inferior jugular vein on each side ('23./') drains the. blood from the ventral side of the head into the. duct of (juvier, and development goes on these become asymmetrical, the left becoming greatly reduced while the right forms a large vessel, triﬁd at its front end where it receives the blood from the thyroid (Fig. 193, D, T/1,).

'J‘he liver rudiment, at first a portion of the general yolk-mass, is at this stage supplied with blood by the portion of the general vitelline network which extends over it. The. portal vein develops simply as an enlarged channel of the network on the left side of the hepatic rudiment. There is no information as to whether this is really the persisting left lateral vitelline vein as we might expect: nor is it known whether the hepatic vein is derived from the prehepatic portion of this same vein.

An important feature is that there becomes established an anastomosis between the, blood-vessels of the liver at the hinder end of that organ and the inter-renal vein, so that a portion of the interrenal blood - stream becomes short - circuited direct to the heart through the hepatic vein, which in correlation with this forms a wide channel throughout the length of the liver (Fig. 193, E, h.(p.'r.c)). This enlarge.d hepatic vein is of morphological importance as it is clearly the equivalent of a posterior (or ‘‘inferior’’) vena cava. In the. Crossopterygian this vessel takes a step in evolution beyond the condition in Lung-fishes, inasmuch as its posterior portion becomes denuded of liver substance, so that it runs for a considerable distance through the splanchnocoele as a naked vessel.

Another important vein which makes its ﬁrst appearance in the (lrossopterygian is the pulmonary vein. The hepatic vein, which towards its headward end lies on the dorsal side of the liver, comes into close contact with the pharyngeal floor in the region of the glottis, and venous spaces developing in the mesoderm sheath of the pharynx come to open into it. These are apparently the forerunners of the pulmonary vein. In the 30-min. larva, as in the adult, there is a main pulmonary vein which still opens into the hepatic vein on its dorsal side (Fig. 193, D, 1)). In addition to this main pulmonary vein, formed by tl1e. fusion of two branches coming from the ventral side of the two lungs, there is a small accessory vein coming from the dorsal side of the root of each lung and from the adjoining parts of the pharyngeal wall. These also open into the hepatic vein just in front, and on each side, of the opening of the main vein.

TEl.EOS'1‘El.——A1I10ngSt the Teleostean ﬁshes We ﬁnd the same 4'20 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

venous trunks laid down as in the Lung-ﬁsh or Elasmobranch. There is within the group considerable variability but the variations are as a rule derivable from a primitive type like that of Lepidosiren. Thus in Salmo the subintestinal vein bifurcates in front into the two paired vitelline veins while in numerous other Teleosts (Esom, Bclone, S3/ngnathas, Iﬂppocampas, Gobius) it passes forwards into a median unpaired vitelline vein. Each of these conditions is obviously derivable from that illustrated by Jjepidosiren, by the disappearance, on the one hand, of the median, and, on the other, of the paired vitelline veins.

AMPnmIA.—-In S'a.l(mtu.n¢l7'a, (Uhoronshitzky, 1900) two lateral vitelline veins are described, the right comparatively small in size. Behind the liver rudiment they lie close together near the midventral line and passing forwards they diverge, passing one on each side of the liver rudiment to unite in front of it and form the hind end of the heart. The two veins undergo fusion behind the liver to form the subintestinal vein and in front of the point of fusion the right vein disappears so that, as in the Lung-ﬁsh, all the blood passes to the heart round the left side of the liver. The mesenteric vein develops as a branch of the right vitelline vein close to its front end and after the disappearance of the greater part of the right vitelline vein the mesenteric is seen replacing it as the right limb of a horseshoe-shaped arrangement of veins which embraces the «liver rudiment from in front. The portion of the vitelline veins in front of the mesenteric breaks up into the hepatic network. The vitelline vein shrinks to an inconspicuous vestige while the mesenteric becomes relatively large and forms the hepatic portal of the adult.

The posterior cardinal veins (Hochstetter, 1888) run alongside the archinephric ducts, which they more or less surround, to the region of the pronephros where each dilates to form a large pronephric sinus. I11 the opisthonephric region the vein forms two main channels an inner and an outer (Fig. 194, B, op), the former eventually undergoing fusion with its fellow to form an inter-renal vein which becomes later the renal portion of the posterior vena cava. The outer channel, as in the Lung-fish, becomes continuous with the caudal vein to form the renal portal, while at the front end of the opisthonephros it loses its connexion with the part of the vein lying farther forwards. A venous connexion is established between the front end of the inter-renal vein and the tip of the liver, and the channel which so arises, commencing behind in the inter—renal vein, traversing the substance of the liver and ending in the hepatic vein, forms the posterior vena cava (Fig. 194, 0, 10.0.0). In later stages the liver tissue disappears over the greater part of the posterior vena cava so that it passes naked through the splanchnocoele.

The anterior cardinal vein with its intercalated section of lateral cephalic persists as the internal jugular vein of the adult. The Duct of (juvier also persists in the adult and is now termed the

anterior vena. cava. VI VENOUS SYSTEM 421

The anterior abdominal vein arises as a pair of small veins in the ventral body-wall. These unite in front in the region of the liver and open into the left duct of Cuvier according to Hochstetter. Later the two veins fuse into a single unpaired vessel except at their hinder ends where they become connected with the renal portal vein on each side. Anteriorly the opening into the duct of Cuvier becomes replaced by an opening into the hepatic portal vein.

d.C. Jill. act». /2 a. pn. si 11.. par op ’ (‘U A B C 0

Flu. 19-1.—~Development of main venous trunks in Salanuuzdra according to Hoclistctter (1888).

u.r.u, anterior ('.'1r(linal vein; c.v, vandal win ; ¢I.(', duct of Cuvwr; hm, hepatic \cm ; op, opisthonephros; p.r‘.v, p0St~l‘I‘l()l cardinal vein; p.'v.c, posterior wna vava; pn, prenephric sinus; r.;p, renal portal; s, subclavian ; s.’i.I', subinte-stmal wm ; 3.1‘, sinus venosus.

SAUROPSIDA.—-—Here we find the same main venous trunks as in the lower groups and, in correlation with the large quantity of yolk, the ventral or vitelline system of veins develops preccciously as compared with the dorsal or cardinal system. .

Lacerta, which has been investigated in detail by Hochstetter (1892), may be taken as an example of the Reptiles. The first veins to make their appearance are the lateral vitelline veins which pass forwards on the dorsal side of the yolk-sac, converging in front to form 422 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the hind end of the heart. The two vitelline veins become connected by a transverse anastoniosis dorsal to the gut and just behind the dorsal pancreatic rudiment (Fig. 195, B). The parts of the veins in front of this anastornosis break up into a network in the substance of the li.ver, and the two networks become continuous with one another (Fig. 195, C). The two vitelline veins now form another anastomosis

Fm. 195.—-Diagrams to illustrate the development of the. ventral part of the venous system in Lacertu u._(/zll-is as seen from the ventral side. (After Iiochstetter, 1892.)

ab, abdominal vein; «LC, duct or (fuvler; :1.-v, ductus venosus; ant, alimentary canal: 1.0.11, left all:1.lll}(')l('. win; If, liver; 1.1.‘.-v, left vitelline vein; 7;, portal vein; 1». l‘.(.', posterior vona, cm-a; mull, right alluntoic vein ; -r.'v.-r, right vitelline vein.

with one another farther back and ventral to the alimentary canal (Fig. 195, C). The right vitelline vein becomes reduced and ﬁnally obliterated in the region in front of this ventral anastomosis so that the whole blood-stream passes forwards to the level of the dorsal anastomosis by the persistent left Vein (Fig. 195, D, l.v.v). In the region anterior to the dorsal anastomosis the left vein now diminishes in size and ﬁnally disappears, ﬁrst in the region behind the hepatic VI VEIN S OF LACERTA 423

network (Fig. 195, D) and then in the region in front of the network (Fig. 195, E). Results of these changes are (1) that the hepatic network receives its blood supply by a single afferent vessel——the hepatic portal vein—-which curves round the gut and is derived in great part from the left vitelline vein——and (2) that its blood drains away to the heart by a single efferent vessel~—the hepatic vein——derived from the front end of the original right vitelline vein.

At a comparatively early stage in development a direct channel becomes established, by the widening out of the venous spaces along the middle of the hepatic network, so that a considerable proportion of the blood is able to pass forwards from the portal vein through the liver without actually traversing the network itself. This cha.nnel——the ductus venosus (Fig. 195, G, 11.1)) persists till nearly the period of hatching but then becomes obliterated so that all the portal blood has to traverse the hepatic network.

The posterior vena cava makes its appearance as a gradually widening channel through the hepatic network towards its right side (Fig. 195, D, E, [).'I7.6). This portion of the liver becomes prolonged backwards as a slender lobe ensheathing a prolongation of the blood channel mentioned. This prolongation fuses at its tip with the tip of the right opisthonephros, continuity becomes established between the venous spaces of the two organs and finally, as in the Amphibian, the liver tissue disappears ove.r a large stretch of the slender lobe already mentioned so that the vena cava is now for a considerable length free from either liver or kidney.

At an early stage a branch of the vitellinc vein develops close to its front end. This is the allantoic or umbilical vein (Fig. 195, roll and loll). These veins soon become asymmetrical, the left for a time being smaller than the right (Fig. 195, I), E). A little later however the left vein establishes a connexion with the hepatic network (Fig. 195, F); the portion of the vein posterior to this connexion becomes much widened, and the blood-stream from it courses by an enlarged direct channel of the network into the posterior vena cava (Fig. 195, F, G). The blood-stream being diverted through this channel, the portion of the left allantoic vein in front of it shrinks in size and disappears, as does the Whole of the right allantoic vein (Fig. 195, F, G). The result is that there persists a single (left) allantoic vein which drains the blood from the allantois into the posterior vena cava near its front end. The allantoic vein increases in size with the allantois but becomes obliterated at the time of hatching when the allantois is cast off. The mesenteric vein develops as a branch of the portal vein (left vitelline vein) a short distance behind its entry into the liver: it increases in size as the vitellinc diminishes with the consumption of the yolk and eventually it alone persists as the peripheral portion of the definitive portal vein of the adult.

The subintestinal vein is apparently present only in its post-anal portion which persists as the caudal vein of the adult. In front 424 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

of the anus, where the ventral wall of the primitive alimentary canal has become intensely modiﬁed in connexion with the storage of yolk, the subintestinal vein has disappeared from the course of development.

As regards the dorsal venous system (Fig. 196), the two posterior cardinal veins converge posteriorly and become continuous with the caudal vein. The portions in the region of the opisthonephros become resolved into their external and internal components connected by numerous sinus~like spaces and channels amongst the kidney tubules (Fig. 196, A). With the development of a capillary network

A B C

as

C“-"

Flo. 196.——Diagr:un illustrating the development of the dorsal venous system in l.m.-arm according to Hochstetter, as seen from the ventral side.

a..c.'v, anterior cardinal vein ; «-, (-.:audnl vein; d.(J, duct of Cuvier (-—-ant. vena cava); ii, iliac vein ; 1r.c.v, posterior cardinal vein; 7-:.zr.r.', posterior vena cava; .-, subclavian vein.

in the substance of the opisthonephros the larger blood spaces become divided into an afferent set connected with the external component and an efferent set connected with the internal one. The external channel remains continuous with the caudal vein and forms the renal portal vein. The two internal components fuse together in their anterior portion (Fig. 196, B) and become continuous with the intrahepatic portion of the vena cava. Posteriorly they remain separate and lose their continuity with the caudal vein (Fig. 196, B, C). The blood from the kidneys being now able to pass to the heart by the direct route through the posterior vena cava, the portions of posterior cardinal lying in front of the kidneys are no longer required and soon disappear (Fig. 196, C). VI VEN OUS SYSTEM 425

In the head region the anterior cardinal becomes in great part replaced by a lateral cephalic vein in a manner similar to that already described for Lc_p'id0.s"i7'07t.

B1nDs.——In Birds the development of the venous system pursues, as we should expect, a similar course to that already described for Reptiles. Amongst the differences in detail the most striking is that the two vitelline veins become completely fused into a single vessel, the ductus venosus, through the hepatic region, before there are any signs of a hepatic network. This may be regarded as a backward extension of the fusion of the two vitelline veins which gives rise to the heart. The ductus venosus secondarily becomes surrounded by tile. liver rudiment and a network of channels spreads out from it in the liver substance. The allantoic veins behave as in Lmrerta except that a small vestige of the left is said to persist throughout life.

The reduction of the tail region in modern birds has brought with it a modiﬁcation of the caudal vein which is here paired, taking the form of a simple backward prolongation of the posterior cardinal. The main channel of the posterior cardinal runs along the outer edge of the opisthoneppros but later on a slender channel appears along its inner edge—that on the right side being continuous with the posterior vena cava of which it forms simply a backward prolongation. The two inner channels undergo fusion so that the blood from the kidneys can drain away entirely into the posterior vena cava and this is followed as in other cases by the atrophy of the. portion of posterior cardinal lying in front of the opisthonephros. This atrophy extends as far forwards as the subclavian vein which in the Fowl opens into the posterior cardinal vein some distance from its front end. The portion of posterior cardinal lying in front of this point is consequently saved from disappearance and persists as a portion of the deﬁnitive subclavian vein of the adult. It will be. understood that the blood of the outer channel of the posterior cardinal, which reaches it from the caudal vein, from the posterior limb, and from the body-wall, passes entirely through the_ opisthonephric network towards the posterior vena cava, in other words that there is at this time a typical renal portal system.

As the metanephros develops, its tubules are also mixed up with the sinuses connecting external and internal channels of the posterior cardinals, so that it too has for a time a functional renal portal system. Later on however one of the channels through the metanephros becomes enlarged and the blood-stream passes directly through it to the posterior vcna cava without traversing the meshes of the network. A true renal portal system then no longer exists and the reason for its disappearance is no doubt to be found in the fact that the vascular network of the kidney has become connected with the arterial system. Obviously this will give a much more efficient circulation than the original one, owing to the. higher blood pressure in the dorsal aorta and renal arteries than in the renal portal veins which have the 426 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

systemic network of capillaries interposed between them and the heart. Further the quality of the blood supplied in this way to the kidney is better——being arterial instead of venous—-and for both these reasons we can readily understand the tendency in the more highly developed vertebrates for the renal portal system to disappear.

It has already been remarked that the posterior cardinals do not pass back into an unpaired caudal vein as in the Lizard. A vestige however of the unpaired condition may perhaps be recognized in the development of an anastomosis between the two vessels just behind the metanephros. From the transverse bridge so formed a connexion (coccygeo-mesenteric vein) is established with the portal vein in the mesentery.

The anterior cardinal vein together with an intercalated portion of lateral cephalic persists as the jugular vein of the adult bird.

The posterior cardinal vein undergoes in the Bird a curious change of position in relation to the root of the iliac artery which it crosses behind the mesoncphros. At ﬁrst it lies on the ventral side of this artery: then it develops an accessory channel round the dorsal side of the vessel and finally the whole blood-stream passes by this dorsal channel while the ventral one disappears. This affords a

,good example of the way in which a vein may in the course of evolu tion pass an apparent barrier formed by an artery, nerve or other organ.

INTER-SEGMENTAL VE1Ns.-—-In the body wall there develops a series of veins corresponding with tl1c interscgmental arteries and opening into the cardinal veins.

VEINS OF LIMBS.—The vascular network of the limb-bud drains into the posterior cardinal vein. In the Bird (Evans, 1909) the, drainage during its earliest stages is into the allantoic vein. Later numerous cl1an11els arise connecting the network with the posterior cardinal and presumably one or two of these become enlarged and persist as the deﬁnitive veins draining the hind limb.

LYMPHATIC SYsTEM.——The venous system has its obvious roots peripherally in the capillary network of blood-vessels, but it is also provided with a much less conspicuous set of tributary channels which constitute the lymphatic system. This extension of the vascular system retains a lower grade of evolution than the remainder. Its channels are less sharply deﬁned, the lining endothelium over most of its extent having a much feebler development of the backing of connective tissue and muscle which forms the thick wall of the vein or artery. In its peripheral portions the lymphatic spaces may have remained practically in the primitive condition of intercellular chinks of the mesenchyme, while in its central portions, as it approaches the points at which it opens into the ordinary veins, its walls may be Well developed and muscular. The lymphatic system serves to drain oil’ the plasma which has oozed out from the capillary blood-vessels and forms the internal medium bathing the surface of the living cells of the body, and to return it to the blood-stream. VI VASCULAR SYSTEM 427

The ﬂuid is peopledfby amoeboid eorpuscles but is without the red corpuscles which have no power to escape through the walls of the blood-vessels.

Our present knowledge of the ontogeny of the lymphatic system is in great part due to the labours of Huntington and McClure whose papers should be consulted as regards details. It seems clear that as a general rule lymphatic channels develop, later than the blood - vessels, as intercellular chinks in the mesenchyme which become continuous and form deﬁnite channels, the bounding cells becoming converted into thin endothelium.

SPLEEN.-——'l‘he spleen arises in Lepidoszveot and 1’r0topte7'us (Bryce, 1905 ; Purser, 1917), which may be taken as typical examples, in the form of a condensation of the mesenchyme of the gut-wall. Blood spaces soon make their appearance in the rudiment which later becomes intercalated in the course of the main venous channel leading from intestine to liver. Later on the main blood-stream passes to the liver by a direct channel, the spleen now lying on a lateral loop : later still the afferent part of this loop becomes replaced functionally by a new arterial connexion.

The spleen rudiment frequently arises in close proximity to that of the pancreas and this has led to statements that the spleen is actually derived from the pancreas but the probability seems to be that such statements are based upon erroneous observation.

LITERATITRE

Allis. Zool. Jahrb. (Anat.), xiv, 1900.

Bemmelen, van. Zool. Anzeiger, ix, 1886.

Bemmelen, van. Meded. tot de dierkunde. Amsterdam, 1887.

Boas. Morph. Jahrb., vii, 1882.

Bryce. Trans. Roy. Soc. Edin., xli, 1905.

Choronshitzky. Anat. Hefte (A1-b.), xiii, 1900.

Dohrn. Mitt. Zool. Stat. Neapel, vii, 1886.

Dohrn. Mitt. Zool. Stat. Neapel, viii, 1888.

Evans. Amer. Journ. Anat., ix, 1909.

Gegenbaur. J enaiseher Zeitsehrift, ii, 1866.

Goette. Abhandlungen zur Entwickelungsgeschiclate der Tiere, v. Hamburg and

Leipzig, 1890.

Greil. Morph. .laln'b., xxxi, 1903.

Hochstetter. Morph. Jahi-b., xiii, 1888.

Eochstetter. Morph. Jahrb., xvi, 1890.

Hochstettor. Morph. .Tahrb., xix, 1892.

Hochstetter. Hertwigs Handbuch der Entwicklnngslehre, iii, 1906. Hochstetter. Voeltzkows Wissensehaftliehc Ergebnisse, iv. Stuttgart, 1906*. Hoﬁmann. Morph. Ja.hrb., xx, 1893.

Hoyer. Bull. intern. Acad. Sci. Cracovie. (Jomptes rendus des Séances, 1900. Huntington and others. Anat. Record, ii, 1908.

Huntington. Anat. Record, iv, 1910.

Kellicott. Memoirs N. Y. Acad. Sci., ii, 1905.

Kerr, Graham. The work of J. S. Budgctt. Cambridge, 1907.

Kerr, Graham. Proc. Roy. Phys. Soc. Edin., xvii, 1907*.

Kerr, Graham. Proe. Roy. Phys. Soc. Edin., xviii, 1910.

Langer. Morph. Jahrb., xxi, 1894.

Mackay. Phil. Trans. Roy. Soc., B, clxxix, 1889. 428 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. VI

McClure. Aunt. Record, ix, 1915.

Maurer. Morph. .Iuhrl)., xiv, 1888.

Mollier. IIc1't\\ig'.-4 I-[mulbuch «lor Entwicklungslelm-, i, 1906. Muller. Abhand. Akad. Wiss. Berlin, 1845.

Miiller, F. W. Arch. mikr. Anat., xlix, 1897.

O’Donog'hue. I’:-nu. '/.00]. Soc.'Lond., 1912.

Purser. (311zn't. .lo1u'n. Mior. Sci., lxii, 1917.

Rabl, C. Lt-1u-kawts l"vstscln'ift. Leipzig, 1892.

Rabl, H. Arch. mikr. Anat., lxix, 1907.

Ridewood. l’l'0c. '/.001. S00. Lund., 1899.

Robertson. Quart. .lo1u'11. Micr. S(:i., li\', 1913.

R586. Morph. J:Lh1'b., .\'Vi, 1890.

Ruckert. Biol. ()9IItI':1l1)laLt, viil, 1888.

Rlickert. Hortwigs Hamlbuch dvr Entuicklungslohro, i, 1906.

Chapter VII The External Features Of The Body

TIIE preceding chapters have dealt with the ontogenetic evolution of the various organ systems of the vertebrate body. The present chapter will sketch in outline the development of the external characteristics in so far as these have not already been referred to.

(1) DEvEI.oPMEN'r or GENERAL FORM.——'l‘he groups of Ve1‘Lcl)1‘nt('s in which the egg is typically holoblastic will be considered first.

CROSSO1”l‘EltY(:II.———Of the two surviving genera 1’olg/ptcrus alone has been studied (Graham Kerr, 1907) and the main features in the development of its body-form may be gathered from an inspection of Fig. 197.

It will be seen that the head—end of the embryo is the first to project freely above the general surface of the body (Fig. 197, The tail projection soon however makes its appearance (Fig. 197, B) and during subsequent stages grows much more actively in length, the embryo soon assuming a somewhat tadpole-like shape, with a laterally compressed hinder region a11d a rounded swollen anterior region formed by the main part of the yolk-laden egg. Two organs, the cement-organ (0.0) and the external gill (e.g), inake their appearance as slight bulgings of the surface at a very early stage. During subsequent stages the binder, laterally compressed region grows rapidly at the expense of the mass of yolk which becomes consequently reduced in volume and at the same time loses its spherical shape s» that it projects less prominently. It will be noticed that during the later stages the post-anal region grows particularly actively, the anus thus coming to lie at a relatively greater and greater distance from the hinder end of the body and giving rise to a rapidly growing true “ tail” region. During the later stages (Fig. 197, D, E, F) the head undergoes much increase in length, its active forward growth beginning about stage 31. The mouth is at first widely gaping (Fig. 197, E) but by stage 34 the mouth-hinge becomes functional and it can be closed. By stage 36 the anterior swelling due to the yolk has practically disappeared.

ACTINOPTERYGII.—A1nongst the Actinopterygian fishes the Ganoids which still retain the holoblastic segmentation show very

429 430 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

FIG. 197.-——St:1;.:o.s in the «IL-.vclop1'm-.nt of l‘u/_;_/I;/o:'ru.._<»'. (All except A :lft(.!l‘ drawings by lillclgett.)

A, stage 12; B, 23; C, 27; D, 31; E, 33; I", 36; (I, 1:u'\':L 30 mm. in len;.;'t11. (I.,ill|1l. ; mu, cument organ ; E, eye; ¢.-..g, external gill; op, ope:-culum; p.f, pectoral Iin ; y, yulk. (A-E x 11; F x 8.) VII THE EXTERNAL FEATURES OF THE BODY 431

much the same arrangement in early stages as will be seen in the Lung-ﬁshes (l<i0'. 200, A), the dorsal part of the embryonic body being curved round the periphery of the egg. As theembryo increases in length the growth of the posterior end is specially active and the general proportions become very much as in Polypterus. The somewhat tadpole-like appearance of the larva, caused by the persistent spherical shape of the main mass of yolk, is again apparent-especially in Amie and Lepidosteus (Fig. 198, A). As in Poly/pterus

Fm. 12.18. —-Stamges in the development of 1.:-,u'¢1u.s-(ms.

(I, anus; I...-_, widely gaping buccal cavity : «nu, (-elln-11l‘nrg':1n ; .»,». npereulum; p.f, pectoral ﬂu; pl.f, pelvic lin.

the intestinal portion of the alimentary canal rudiment is relatively slender in form, arising by a process of actual backgrowth of the posterior trunk region rather than by gradual modelling of the yolk as is the thick intestinal rudiment in the Lung-ﬁsh. Conspicuous characteristics of the actinopterygian Ganoid larvae are the presence of well—deve1oped cement-organs and the absence of external gills.

In the Teleostean ﬁshes there has come about with the high development of telnlecitliality a great reduction in the angular extent of the e1nlu‘_yuni«-_. rudiment during its early stages. Consequently there is very slight ventral curvature round the. yolk. In the FIG. 199.-—-(U//IIIII!/'11/HIS nljlu/I'4'-II.s'. ilftttl‘ Asshetmx.)

A, seventh day; 13, t;o':nt..h «lay ; (J, l‘uurt;ccnt,h «l:1_y ; I), ago unknown; E, I'o1'Ly-tlnirtl day. 432 CH. V11 THE EXTERNAL FEATURES OF THE BODY 433

Granoids Azrmlco and Lepidosteus the main mass of yolk retains its form for a considerable period, causing a great bulging of the ventral body-wall anteriorly. In the Teleost this is still further accentuated, the bulging forming the yolk-sac which remains prominent even in larvae sufficiently developed to be able to swim actively. An extreme case of the prominence of the yolk-sac is afforded by G’;/mncrclz/us (Fig. 199) where it shows a peculiarly elongated form for a certain period. Cement organs are as a rule absent in Teleostei : so also are external gills though in rare cases the latter have physiological representatives in ﬁlamentous prolongations of the gill lamellae (Fig. 199, B).

Great variety of form exists amongst the larvae of Teleostean ﬁshes, more especially amongst those of pelagic habit. Familiar examples are seen in the pelagic larvae of the Eels—-much compressed from side to side, transparent and colour-less—even the blood being free from liaemoglobin———and much greater in bulk than the innnediately succeeding phase in the life-history. The larvae of the Flat-ﬁshes (Pleuronectidae—Flounder, l’laice, Sole, etc.) are again of special interest owing to the extraordinary asymmetry which they develop. They are at first quite symmetrical and in no way abnormal. The larva swims at this time with its laterally compressed body vertical after the manner of a Bream but later develops the habit of swimming on its side. The side of the head-region which is below now grows more actively than the other so that the head becomes strongly asymmetrical and the eye. of the lower side becomes gradually transferred to the upper, the right and left eyes being now both on the same side of the head. Correlated with this asymmetry in form there comes about a corresponding asymmetry in colour, the chromatophores being collected together on the upper side and giving it its characteristic obliterative colouring, In some genera it is the right side of the body which is above, in others the left——while in a few species it appears to be indifferently the one or the other.

DIPNOI.—-—Both of the dipneumonic L11ng-ﬁsheS——Lq)idos’i7°(m and 1)’I'0t0])t67"’lt8——-ll:tVB been investigated (Graham Kerr, 1900 and 1909; Budgett, 1901). They closely resemble one another and Lepi«lo.yia~en Will be chosen here for description (Fig. 200).

During the early stages of the modelling of the embryonic body (Fig. 200, A) the latter is curved round the egg, occupying about 290° in angular extent. The head-region becomes demarcated as a slight, somewhat lance-shaped protuberance above the general surface of the egg due to the neural rudiment. The branchial region becomes marked at an early stage by a slight elevation of the surface which soon becomes divided by shallow oblique grooves into. the series of branchial arch rudiments. About stage 25 (Fig. 200, B) the tip of the head and the tip of the tail project sharply above the general surface: the external gills (c._q) are now in the form of four distinct little knobs on each side, and the cement organ (6.0) has made its appearance as a crescentic structure on the ventral side curving round the tip of the

VOL. II 2 F RTEBRATES CII.

FIG. 200.-—Stages in the development of Lepidosiren pa.mdo;ca.

A, stage 23; 13,25; 0, 26; D, 28; E, 31; F, 33; G, 35; II, 36. (2.0, cement organ; E, eye; ¢a.g/, external gill; M, mnuth; p.f, pectoral ﬁn; pl. f, pelvic ﬁn; sp.z..~, spiral valve of intestine. (A-FXB; Gx2'5; IIX2.) VII EXTERNAL FEATURES OF DIPN OI 435

head. The posterior part of the body now becomes laterally compressed, it grows rapidly in length and the larva assumes a somewhat tadpole-like form——the apparent “ tail” being at ﬁrst bent ventrally (Fig. 200, O). '1‘he anus is situated close to the tip of this portion of the body, therefore it is, strictly speaking, not tail but rather posterior trunk-region. About this period hatching takes place. The tail-like hinder region now straightens out (Fig. 200, D) and grows rapidly in length, the growth being at ﬁrst mainly pre-anal and the true tail-region developing later. As in Crossopterygians the tail is throughout protocercal. As in I’ol3/pterus again the headregion for a considerable period shows no active growth in length: it is not until about stage 31 (Fig. 200, E) that its growth becomes active and the head-region begins to develop the modelling of its deﬁnitive features. The external gills grow actively in length after hatching: each develops a double row of pinnae along its external margin and eventually all four become fused togetlier at their bases. They reach their maximum about stage 35 and thereafter undergo a process of atrophy resulting in their complete disappearance. The limbs make their appearance about stage 31, each as a little knob bearing a striking resemblance to the ﬁrst stage of an external gill. The cement organ increases in size forming a large cushion-like and very conspicuous organ in the larva of stages 32-34 (Fig. 200, F). Eventually it shrivels up and disappears without leaving a trace behind.

In 1’r0topte7°zLs as already mentioned the general features of development agree very closely with those of Lep'iclos'£7'en.

Of Cerutoclus developmental material was obtained by Caldwell in 1884 a11d by Semen in 1891. Semon’s material has formed the basis of a long series of investigations by himself and others which together constitute an important contribution to Vertebrate morphology (Semen, 1893-1913; 1901*).

While the general modelling of the body shows a general resemblance to that of Lepido.9-iren and P7'otopterus there are certain well-marked differences in detail. Perhaps the most striking of these is that the head-region shoots ahead in its development and grows actively in length so as to project freely in front of the yolk at a much earlier period than in the other genera (Fig. 201, A, B). Again the main mass of yolk undergoes a more uniform process of lengthening so that it assumes a somewhat spindle-like form and allows the body as a whole to become slender and “ﬁsh ”-like,. the “ tadpole” shape due to the persisting spherical mass of yolk in Polypterus or Lepiclosiren being here absent. During the later larval stages the divergence of Oemtodus from the other two Lungﬁshes towards the more typical ﬁsh condition becomes marked by the paddle-like form of the limbs and the much greater development of the median ﬁn round the hinder end of the body. It will be noticed also that two conspicuous features of the young Lep'id0s'i7'en or P'rotopte'rus—the Cement organ and the external gills-are completely absent in (Ieratodus. 436 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

In the Urodela the least specialized subdivision of the Amphihia the evolution of external form closely resembles that of the Lung Fm. 20].——Development of C'emt4)dus_/0'rste'ri. (From Sen1ou——1'n. the Australian Bush.) A, stage 32; I3, 34; C, 38; D, 41; E, 45; F, 48. (Magnillcation about 6,} diameters.)

ﬁshes. An early ‘stage of such a relatively primitive member as Nectums might readily be mistaken for the corresponding stage of I vn

EXTERNAL FEATURES OF AMPHIBIA

Oeratodus and an almost equally striking resemblance is shown by

an Axolotl or Newt about the time of hatching, except that in this case there are the well-developed external gills which were as we have seen absent in Ueratodus though present in the other two Lung-ﬁ.shes.

In the case of the Anura it is perhaps premature to make general statements regarding the diflerences in form which distinguish them from the more primitive Urodela, for different species

differ greatly in the size of the

egg and its richness in yolk and the great majority of them have not as yet had their development worked out.

The head-region projects less prominently, sometimes beingin its early stages quite ﬂattened out on the yolk (Alytes, Phyllomedusa) while in other cases the embryo elongates as a whole there being for a time no marked break in contour between head, trunk and tail. In such cases growth in length may for a time be most active ventrally, so as to cause a curvature of the embryo with its concavity on the dorsal side (Rama). In the later stages the tail-region is highly developed, the splanchnocoele being greatly shortened and widened and the head also very broad giving the

characteristic tadpole type of

larva.

Particular interest attaches to the development of such types of Amphibia as possess heavily yolked eggs. A good example is afforded by the Gymnophiona such as Hypogeophxis (Fig. 202). A conspicuous difference from the condition seen in the Telec SCOIIIBS (1),

FIG. 202.—Embryos of 11;:/pogeopluis rostratus. (After Brauer, 1899.)

A, 15, ()x4_; I) xll. (For 41¢‘-tails of B cf. Fig. 87, In‘, eye; «.3/, (‘Kl-I‘-l'lHll gill ; ul_/ ‘l':u-tory orgzm. 438 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

in the fact that here as in the Dipnoi the embryonic body during the early stages of its differentiation has a much greater angular extent, curving round the mass of yolk cells instead of being restricted to a small extent near the apical pole. Another important point to notice is the well-marked downward flexure of the head during early stages—-—-a feature which has already been correlated (p. 93) with the presence of a large supply of yolk. The active forward growth of the head-region leads to the rounded main mass of yolk being situated well back, just in front of the anal region (Fig. 202, 0), instead of anteriorly as in the tadpole,shaped larvae of Lepidosiren. or Protopterus or the Ganoid ﬁshes.

ELASMOBRANCHII.—-Of the isolated groups of Vertebrates characterized by having meroblastic eggs the most nearly primitive is that of the Elasmobranch ﬁshes. Unfortunately for purposes of comparison we are not, up to the present, acquainted with any member of the group possessing small eggs poor in yolk.

Here as in other groups with typically meroblastic eggs the early rudiment of the body of the embryo——or more correctly of the dorsal portion of the body——extends through a relatively small angular extent, in striking contrast with the 200” or more of a Lung-ﬁsh or of one of the Gymnophiona. As the embryo proceeds with its development it grows actively in length, headwards and tailwards, so as to project freely in a tangential direction, remaining in connexion with the main mass of the egg (yolk-sac) by a narrow yolk-stalk. During the forward growth of the head growth-activity is less pronounced on the ventral side so that the gill-clefts are forced into an oblique position and the head undergoes pronounced cerebral ﬂexure.

AMNIOTA.——In the non-mainmalian Amniotal the ﬁrst point to notice is that although the size of the egg and the absolute amount of food-yolk contained within it are relatively enormous yet the degree of telolecithality is less extreme than it is in the case of the meroblastic eggs of ﬁshes. Consequently the segmentation process resulting in blastoderm formation spreads in an abapical direction past the level at which the posterior end of the embryonic rudiment will be developed: as a result of this the entire embryonic rudiment lies at its first appearance well within the boundary of the blastoderm, instead of its hinder end being coincident with that boundary as is the case in Elasmobranchs.

Here again the embryonic body grows forwards and backwards free from the surface of the egg, the backward growth being much less pronounced than in the case of the ﬁshes in anticipation of the ultimate lesser degree of development of the tail-region correlated with its diminished locomotor importance in the adult.

The most striking feature however is one which may perhaps be correctly expressed by saying that the clogging inﬂuence of the

1 The external features in their development are well illustrated by the developing Bird (see the figures in Chap. X.). VII THE EXTERNAL FEATURES OF THE BODY 439

yolk upon the growth in length of the ventral side of the body is much more marked than in other Vertebrates. The result is a strong ventral curvature of the body. The ventral flexure of the head. in the mid-brain region already seen in the Elasmobranchs and Grymnophiona is here still more marked, but in addition the whole body is strongly curved ventrally to such an extent as to form more than one complete t11rn of a spiral. This curvature is in its incipient stages of great morphological interest as providing a possible explanation of the downward indentation of the blastoderm by the head and tail regions, and their consequent ensheathment in blastedermal pockets, which led eventually to the evolution of the amnion.

The formation of the amnion and the separation of true amnion from false amnion involve as will be gathered from Chapter VIII. a solution of continuity of the somatopleure or original body-wall and probably this has initiated what is perhaps the most striking feature of amniote development—the loss, at the time of batching, of a relatively large proportion of somatopleure together with the allantois.

As a matter of minor detail it should be mentioned that in the case of Birds as compared with the lower Amniota the embryo is distinguished during a long period by the relatively enormous size of the head. It seems reasonable to regard this as in the main an anticipation of the great development of the eyes and optic lobes in the adult.

In the foregoing short sketch the author has confined himself to the main branches of the Vertebrate stem. He has omitted all reference to three different types near the base of that stem, namely Amphz'oams, the Lamprey, and the Myxinoid, which are of great interest in themselves but which are of less importance for enforcing general principles of Vertebrate development. Of the three types the ﬁrst will be found fully described by MacBride in Vol. I. and it will easily be seen how in the general form of body, as in various other characteristics, the young Amphioxus has diverged widely from the more typical Vertebrates. The Myxinoids, so far as they are known from Bashford Dean’s researches on Bdellostoma (1899), appear also to be highly specialized. The Lampreys on the other hand have diverged to a much less extent from the normal. The most striking features during the early development of the body are (1) that the head-region, as Was the ease in C'e7"at0du.~: and Hypogeoplz/is and as is the case also with Bdellostoma, shows a marked activity in its growth in length, the mass of yolk persisting longest posteriorly and (2) that the outgrowth of the tail is delayed until a comparatively late period. A marked negative feature in all three of the types mentioned is the absence of all trace of paired limbs.

(2) THE MEDIAN FINs.—- The primitive Vertebrate, with its segmented musculature arranged along the two sides of the body and its skeletal axis and central nervous system lying at the mesial plane, is clearly a creature constructed for swimming by lateral 440 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

ﬂexure after the manner of an Eel. To secure greater eﬂiciency the body, more particularly its purely motor post-anal portion, becomes compressed from side to side, the compression being most marked near the margin of the body where the thin almost membranous median ﬁn is produced.

In development the median ﬁn arises as a projecting fold of slightly thickened ectoderm into which later on mesenchyme penetrates. 111 what the evidence points to as being the primitive condition this ﬁn rudiment is continuous and extends round the hind end of the body. In such relatively primitive Vertebrates as Orossopterygians, Lungﬁshes, and some Elasmobranchs, it extends forwards on the dorsal side practically to the head-region, while on the ventral side it reaches the anus and may even be continued onwards as a pre-anal median ﬁn, though possibly this has originated in phylogeny independently of the main ﬁn-fold.

In the Lung-ﬁshes the median ﬁn-fold during the course of development never loses its continuous and practically symmetrical arrangement round the tip of the tail. lt retains throughout life the ptimitive symmetrical (protocercal) form. In the Crossopterygians (apart from the anterior portion ofthe dorsal ﬁn which becomes divided up into a series of ﬁnlets) the same holds until a very late stage in development, the tail of the adult becoming very slightly asymmetrical though the term protocercal is usually and justiﬁably still applied to it. A similar protocercal tail occurs in the Amphibian larva while the tail of the adult Newt or Crocodile is simply a protocercal tail in which there is no longer a membranous ﬁn-fold present.

It is however characteristic of the ﬁshes in general that, in accordance with their high specialization as expert swimmers, the median ﬁn during ontogeny loses its homogeneous character——eertain portions of it, probably those portions which are in mechanically the most favourable positions, becoming enlarged while the intervening portions become reduced to the point of complete disappearance. The result is that the place of the originally continuous ﬁn-fold is taken by a series of separate ﬁns—--one or two dorsal, a caudal or tail ﬁn, and on the ventral side, an anal ﬁn. 01' these the caudal fin--most favourably situated of all the series to serve as a propelling organ——becomes specially enlarged. It is also characteristic of the more efficient swimmers that the part of the caudal ﬁn lying on the ventral side of the axis becomes particularly developed. We may probably associate this with the function of rotating the body about its long axis as may be seen in a shark when it seizes its prey. The unsymmetrical condition of the tail so produced is termed heterocercal. When carried to its extreme the tip of the vertebral axis becomes tilted upwards and, so far as external appearance goes, a (secondarily) symmetrical condition is arrived at, as is seen in the homocercal tail of the Teleostean ﬁshes. It will be understood that the protocercal, heterocercal and homocercal conditions merge into one another and cannot be distinguished by any rigid deﬁnition. VII FIN S AND LIMBS 441

They clearly represent successive grades in the evolution of the tail as a more and more perfect organ of propulsion, a process of evolution which has come about independently in the various groups of ﬁshes. Thus even the Lung-ﬁshes—the surviving members of which group possess the primitive protocercal type of tail——during the geological periods when they most ﬂourished showed numerous forms in which there was a highly developed heterocercal tail.

Again the assumption of a sluggish mode of life, or the simpliﬁcation of the swimming movements, is frequently correlated with reversion of the tail towards the. protocercal condition. This is clearly seen in many teleostean ﬁshes, such as the Eels and many deep—sea bottom-frequenting ﬁshes. In such cases all trace of the unsymmetrical condition may have disappeared from ontogeny but there is no room for doubt regarding an ancestral heteroeercal or homocercal condition——-for in their general structure these ﬁshes are highly evolved Teleosts and the group as a whole is characterized by the tail being homocercal.

In the case of’ the surviving Lung-ﬁshes the general archaicism in structure, and more especially the extremely archaic character bf the paired ﬁns, are in favour of the protocercal character of the tail being persistent rather than revertive-—apart from the evidence of embryology which fails to disclose any trace of a pre—existing heteroeercal phase.

(3) THE LI.\ms.—~()nc of the characteristic structural features 01 the Vertebrata is the presence of the two pairs of limbs, pectoral and pelvic. Two main types of such limbs can be recognized—the ﬁn type for swimming and the. pentadactyle or leg type for moving on a solid substratum. As the former is on the whole characteristic of ﬁshes, and as ﬁshes are on the whole more nearly primitive than are terrestrial \'e1‘tcbrates, the idea has naturally arisen and has now attained perilously near to the position of a dogma that the leg type of limb has been evolved out of the fin; and elaborate attempts have been made to deﬁne the manner i11 which this has come about. It is necessary at the outset to emphasize the importance of keeping an open mind upon this question: there exists the possibi1ity———which as will be seen later is not lightly to be. brushed aside—that pentadactyle limb and ﬁn are not in the relation of lineal descent at all but that they have been derived from a common ancestral type of limb differiiig from either.

No limbs exist in Amp/uJo.vus or in the Cyelostomata. There is however a general tendency in Vertebrates which have assumed an eel-like form of body for the limbs to degenerate and disappear, and it is well to bear in mind the possibility that this has happened in the case of both of the types mentioned.

The limb at its ﬁrst appearance in embryonic development forms a little projection from the body surface—a core of mesenchyme enclosed in an ectodermal sheath. In Lapidosirevt or Protopterus or a Urodele it is in the form of a rounded knob identical in appearance 442 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

with the rudiment of an external gill, but in a large proportion of the Vertebrates it is narrower in a dorsi-ventral than in an anteroposterior direction so as to have the form of a short longitudinallyrunning ridge.

As is usual where active increase of surface is about to take place the projecting limb rudiment is foreshadowed by a thickening of the ectoderm and by a condensation of the underlying mesenchyme.

In Torpedo, one of the Rays--fishes characterized by the great

antero-posterior extension of the large pectoral ﬁn——the two limb,

rudiments which are at ﬁrst distinct (Rabl, 1893) become for a time joined together by a transitory ectodermal thickening. This phase, in which the two ﬁn rudiments are as it were parts of a continuous ridge, was the earliest stage observed by Balfour (1878) and it afforded him an embryological basis for the lateral-fold view of the phylogenetic origin of the Vertebrate limbs (see p. 445).

As the limb rudiment develops it shows in many cases characteristic changes in its position. First it shows a movement of rotation. This is well illustrated by the ease of C'e7'at0dus as described by Semen (1898). Here the pectoral limb rudiment becomes rotated in such a way that its originally pre-axial or headward edge becomes dorsal and its originally lower or ventral surface comes to face in a headward direction. In other words, if one could observe the developing left pectoral limb from a point away to the animal's left side the limb would be seen to undergo a clockwise rotation. It results from this that when the fully developed limb is folded back alongside the body its outer surface is that which was originally ventral. A rotation similar in direction though varying in angular extent in different forms occurs also during the development of the pectoral tin in Crossopterygians and Actinopterygians. In Tetrapoda, on the other hand, a rotation of the limb rudiment in the opposite direction takes place—the pre-axial edge becoming ventral. N ot improbably this may be regarded as a secondary modiﬁcation foreshadowing the pronate position of the fore-limb characteristic of terrestrial progression.

The pelvic ﬁn in Ceratodus undergoes a similar rotation but in the opposite direction to that of the pectoral: the left pelvic ﬁn regarded from a point away on the animal's left would be seen to undergo a counter-clockwise rotation. The result is that the originally dorsal surface comes to face headwards or, when the ﬁn is folded back alongside the body, outwards.

The corresponding rotation of the pelvic limb in other ﬁshes and in the lower Tetrapods appears to stand in need of further investigation,

It is clear from the facts of Comparative Anatomy that the paired limbs have undergone extensive shiftings along the surface of the vertebrate body in adaptation to its general form and its method of movement (see below, p. 448). It is of interest-——though not necessary for establishing the fact of such phylogenetic shifting-—to enquire VII DEVELOPMENT OF LIMBS -143

whether any record of it occurs in ontogeny. Obvious evidence which at once suggests itself‘ in this connexion is the presence of abortive muscle-buds in front of or behind those which become incorporated in the definitive limb. These seem clearly to indicate that the part of the body surface superﬁcial to these buds was at one time part of the actual limb. But unfortunately such abortive buds occur both anterior and posterior to the definitive. limb and there is no means of ﬁxing deﬁnitely the time in phylogenetic evolution from which the two sets of abortive buds date. They may date from the same period, in which case they might merely aﬁbrd evidence of the process of narrowing of the limb base which has undoubtedly taken place during the later evolution of ﬁns; or they may date from dill'ere11t periods, in which case the anterior set might be taken as evidence of a backward movement of the limb, and the hinder set as evidence of a forward movement occurring at a dilli-rent period—— movements which again have undoubtedly taken place. In View of the impossibility of determining to what extent the evidence in any particular case is to be interpreted in these two different directions it seems on the whole advisable to leave this muscle-bud evidence on one side.

Other evidence has been adduced from cases where the actual limb rudiment as a whole (’i.6. the projection from the surface of the body) seems to be displaced during development. For example in the ﬁgures illustrating the development of the pelvic limb in Spvlnaa; (p. 207) it will be seen that the anterior limit of the ﬁn is in successive stages of development opposite myotomes 21, 26, 28 and 3]; the hinder limit at the same stages opposite myotomes 30, 38, 38 and 39, and the middle of the ﬁn base opposite myotomes 25, 32/33, 33/34 and 35/36. It seems quite allowable in such a case to speak of the limb as having undergone a backward displacement. In other cases, as that of Sq?//ldzmz. according to Goodrich (1906), ontogenetic development discloses no evidence of such backward migration.

The limb rudiment gradually increases in size and assumes its deﬁnitive form and as it does so it becomes equipped with its characteristic skeletal and neuromuscular arrangements in the manner already described.

l’lIYLOGENETI(-3 ORIGIN or THE LIMBS.-—Tl1e limbs are organs highly characteristic of the V ertebrata. While they exist typically as two pairs, pectoral and pelvic, one or both pairs readily disappear in groups where they are no longer needed. They are particularly prone to disappear in those Vertebrates which assume an elongated form of body and revert to the archaic method of moving by lateral ﬁexure. Thus in Eels the pelvic limb has disappeared, in S3/mbrancltus both pairs. In Lepidosiren, the most elongated Lung-ﬁsh, we see both pairs in process of reduction. We see the same in elongated Urodeles such as A1n.ph'im22a., while in the Gymnophiona both pairs have vanished. In Reptiles we see beautifully how the limbs undergo reduction (O/mlcvldes) and complete disappearance (Amphisbaenidae, 444 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Alngwis) in those various groups of Lizards which have developed an elongated snake-like form. So also in Snakes.

In cases where the limbs are completely gone in the adult it may be possible to observe them during early stages of development. The young Symbranchus (Fig. 203) has for a time huge pectoral ﬁns which it uses as organs of respiration (Taylor, 1914). In Gymnaphiona and Blind Worms (Angu'£s)‘ minute limb rudiments have also been observed in the embryo. In other cases no trace of the missing limbs has been found during early development. In view of this general tendency of the limbs to disappear in Vertebrates which have assumed an eel-like or snake-like form of body it is well, as

FIG. 203-""SU7n.bm-m:/rus -ﬁ‘e..m'm.m'u/am. Larvae showing I-lectoml ﬁ"S' (After Taylor, 1914.)

o.u, opercular opening; s.i.'v, subintestinal vein ; y, yolk.

already indicated, not to assume a dogmatic attitude in regard to such Vertebrates as Lampreys or Hag-ﬁshes. The possibility is not excluded that even these Cyclostomes are descended from ancestors in which limbs were present.

The interesting question now emerges—-1-Iow did the limbs of the Vertebrate originate in evolution 2 Few morphological speculations have excited more interest and more controversy than this. Two main hypotheses have been propounded and each has found supporters amongst the most eminent morphologists. Although in the opinion of the present writer it is no longer necessary to fall back upon either of these views, a simpler possibility having presented itself, a

U The elementary student may be warned not to mistake the rudiments of the paired penes in Snake embryos for limbs ! " VII EVOLUTIONARY ORIGIN OF LIMBS 445

short sketch of each and of the arguments for and against it may now be given. The two hypotheses indicated are the “ Lateral Fold ” hypothesis of Balfour, Mivart, Thacher and others and the “ Gillseptum ” hypothesis of Gegenbaur and his school. Each hypothesis concerns itself with the origin of the paired limbs of ﬁshes -—the ﬁn being regarded as the primitive type of limb from which the pentadactyle limb has been evolved later on.

THE LATERAL-FOLD HYPO'l‘HESIS.——It will be remembered that the mode of development of the median unpaired ﬁns indicates clearly that these ﬁns are simply persisting and exaggerated portions of a once continuous median ﬁn-fold. According to the lateral-fold hypothesis of the origin of the limbs the paired ﬁns of ﬁshes are similarly to be looked on as persisting and enlarged portions of a continuous ﬁn-fold which once extended along each side of the body. The hypothesis rests upon a tripod basis of Embryological, Anatomical and Palaeontological fact.

Balfour in his Development of Elasmobranch ﬁshes (1878) wrote: “Along each side of the body there appears during this stage (G-I) a thickened line of epiblast, which from the ﬁrst exhibits two special developments: one of these just in front of the anus, and a second and better marked one opposite the front end of the segmental 1 duct. These two special thickenings are the rudiments of the paired ﬁns, which thus arise as special developments of a continuous ridge on each side, precisely like the ridges of epiblast which form the rudiments of the unpaired ﬁns.” “If the account just given of the development of the limbs is an accurate record of what really takes place, it is not possible to deny that some light is thrown by it on the ﬁrst origin of the vertebrate limbs. The facts can only bear one interpretation, viz. that the limbs are the remnants of continuous lateral ﬁns.”

Further embryological support to this hypothesis has been provided (1) by the fact that the muscles of the limb are of segmental origin, derived from a number, often a considerable number, of myotomes (see p. 207) and that apparently vestigial muscle-buds have been found both headward and tailward of the series which take part in the muscularization of the deﬁnitive ﬁn, a11d (2) by the fact that the ﬁn type of limb commonly shows a marked narrowing of its base. of attachment during the process of development, the rudiment having during early stages the form of a longitudinal ridge attached throughout its length to the body.

Regarding the evidence upon which the ﬁn-fold hypothesis rests the following criticisms may be expressed. (1) The ectodermal ridge described as connecting the two limb rudiments in Elasmobranchs turns out to be a characteristic not of Elasmobranchs in general but only of Rays (Torpedo) 7I.e. of forms in which there is an enormous, admittedly secondary, extension of the pectoral ﬁn along the side of the body. The Elasmobranchs less specialized in this respect—the

1 = Archinephric. 446 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Sharks and Dog-lisl1es——-do not, so far as is known, develop this ridge.

(2) The fact that the myotomes-—-from which the limb rudiment, like other portions of the body, has to derive its equipment of voluntary muscles———arc themselves metameric and that the skeletal elements necessarily correspond in position with the muscles seems to render it unnecessary to seek any further evolutionary explanation of the tendency on the part of the musculature and skeleton of the limb to exhibit a markedly metameric appearance during early

stages in its development. The occurrence of abortive muscle-buds 111 front of the deﬁnitive

limb is taken-——-quite reasonably-——as evidence pointing to a tailward shifting of the anteiior margin of the limb having taken place, and similarly the presence of abortive buds behind the deﬁnitive limb is taken as evidence of a headward shifting of the hinder margin of the limb. but this shifting of the anterior and posterior margins of the limb may have in evolution taken place either synchronously (13.6. together with a narrowing of the base of attachment of the ﬁn) or at dhferent periods as the limb shifted backwards and forwards as a whole in accordance with variations in adaptational requirements. The present writer sees no convincing reason for rejecting either of these possibilities and if either be possible then the evidence loses its value as support of one view rather than the other.

(3) The narrowing of the limb proximally and its expansion distally is a process which would naturally take place as the ﬁn became more eﬂieient as a propelling organ --—just as in the evolution of a racing oar or paddle with its broad blade and slender shaft-and, accordingly, too great weight should not be attached to the occurrence of such a process during ontogeny in arguing as to the evolutionary 07"£g'i'a of the limbs.

As regards anatomical evidence stress is laid on the exceedingly close structural resemblance in skeleton and musculature between the paired and the unpaired ﬁns. On the other hand it is suggested that, seeing that lateral and paired fins are organs similar in function and built up out of similar muscular elements, a close similarity in their anatomical arrangements may quite probably be merely a case of that secondary convergence of which so many striking examples are known in the animal kingdom.

An ancient fossil ﬁsh, Cladoselache, is brought in to corroborate the view, its paired ﬁns having each a broad longitudinally-running base of attachment and being apparently supported by separate rays without any continuous basal skeleton. But it is pointed out (1) that what signs there are of basal skeleton maybe readily interpreted as representing the axis of a ﬁn of the Ceratodns type laid back against the side of the body (see Fig. 169, E, p. 353), and (2) that the structure of the tail is of a very highly developed and powerful type and that it is most unlikely that a powerful swimmer, such as the highly evolved tail demonstrates Oladoselache to have been, should VII EVOLUTIONARY ORIGIN OF LIMBS 447

have retained its paired ﬁns in a relatively primitive a11d ineﬁicient condition.

Finally there are great physiological dillieulties in the way of accepting the lateral-fold hypothesis. There are no more fundamental characteristics of the Vertebrate body than the arrangement of its longitudinal muscles in segmental masses along each side of the body, a'nd the position of its skeletal axis, its central nervous system and its main arterial trunk in the region of the mesial plane. It is quite clear that such a creature is built for swimming by waves of lateral ﬂexure after the manner of an Amphioxus, a Lamprey or a Lung-fish. Any new swimming organ that became evolved 111 primitive Vertebrates must have had some advantage over, or at least not interfered with, this primitive method of swimming. it is difficult to see how the supposedly ancestral lateral fold could possibly have complied with these conditions. 'l‘l1e suggestion that the lateral fold may have functioned at first as a balancing organ or as a “bilge keel” will not bear examination from the point of view of elementary physics. Rabl suggests that the two lateral folds may have acted primitively as a kind of parachute and that they became muscularized at their anterior and posterior ends, the intermediate portion undergoing atrophy (thus originating the two pairs of limbs). The‘skeletal elements on this view would also develop at the ends of the ridge ﬁrst, and spread backwards (pectoral ﬁn) or forwards (pelvic ﬁn). Thus would be explained the reversal of the position of the anterior and posterior edges of the two ﬁns in 0.9. C'e'mtodu.s-. Such an explanation however fails entirely to meet the diiliculty that there exists not merely an antero-posterior reversal in the structure of the two ﬁns but also a dorsi-ventral one.

THE GILL-sEr'rUM llYI’O’l‘HESIS.-—Tl1iS hypothesis was based by Gegenbaur (1872) on facts of adult anatomy. In some of the Elasmobranclis (1’r1Istt'.s) the central gill ray attached to the branchial arch is enlarged and the rays next to it have come to have their bases of attachment shifted secondarily from the arch 011 to this enlarged ray, so as to produce an arrangement recalling the biserial archipterygium of Oemtodus with its central axis and lateral rays; Gegenbaur suggests that the archipterygium with its limb girdle has in fact been evolved o11t of such an arrangement of rays attached to a branchial arch and.that the limb itself is serially homologous with the gill septum.

In support of this view it is pointed out that branchial arch and limb girdle are each in early stages of development in the form of a continuous curved rod of cartilage; that this becomes usually segmented in the case of the branchial arch but that even ill the girdle it also shows traces of segmentation in some ancient fossil forms (Pleuracanthids, Acanthodians); that in some cases the perichondrium of the pectoral girdle is known to be innervated by that typical branchial nerve the Vagus; that in the lower forms the t7'apez'ius,o11e of the muscles associated with the fore-limb, is innervated 448 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

by the same nerve; and that connected with the ordinary branchial arches there are myotomic muscles as well as splanchnic, so that the basis already exists for a muscularization purely myotomic.

On.the other hand the objection is urged against the (legenbaur hypothesis that it involves a very great shifting of the pelvic ﬁn backwards from its assumedly original position at the hinder end of the branchial region. This objection need not be taken seriously in view of the extensive shiftings of the limbs which are deﬁnitely known to have taken place. Thus in Rays we commonly ﬁnd that the pectoral girdle has moved back to a position in relation to the segmentation of the body far posterior to the position which it occupies in Sharks: in Urodele Amphibians the hind-limb has taken up positions, as indicated by the position of the sacrum, varying between the 14th (ﬂriton palmatus) and 63rd vertebra (Amphvluma means) while in the Anura—where in accordance with the leaping habits it is advantageous to have the attachment of the hind-limb far forward——the sacrum has come to be as far forward as the 9th or even (II;/menoclmlrus) the 6th vertebra 1 : in l’l'esiosaurs and Birds a still more striking backward migration of the pectoral girdle with its attached limb has taken place (any. in the Swan as compared with A7'cltaeopte'rg/at through 14 or 15 segments): and ﬁnally in many Teleostean ﬁshes the pelvic ﬁns have become so shifted forwards along the sides of the body as to attain to an actually jugular position.

The fact that the limb girdles are embedded in the somatopleure while the branchial arches lie in the splanchnopleure has again been raised as a diﬂiculty in the way of accepting the Gegenbaur theory. The diﬁiculty is not so serious as it seems at ﬁrst sight. The chief obstacle in the way of a splanchnopleural organ becoming shifted outwards into the somatopleure is clearly the coelomic cavity—but in the branchial region this tends to be in great part obliterated. As regards blood-vessels, nerves, etc.——-these form by no means insuperable barriers to the change in position of skeletal elements. Such skeletal tissue may; as has already been indicated in Chapter V., spread past a blood-vessel or nerve and if it then becomes absorbed behind the obstacle there is brought about a complete transposition of the two structures. The criticism that the musculature of the limbs is myotomic in origin while that of the branchial arches is splanchnic is provided against by the mixed character of the muscularization of the branchial arches, taken in conjunction with the demonstration that in such a case replacement ofsplanchnic muscle by myotomic may take place (p. 217).

Rabl considers the metameric origin of the muscles etc. of the limb to be enough by itself to undermine the Gegenbaur hypothesis, but it is diﬁicult to see how the musculature could be otherwise than metameric in origin seeing that it has to be derived from the segmentally arranged myotomes.

1 Gadow, in Cambridge Natural Ilistory. VII EVOLUTIONARY ORIGIN OF LIMBS 449

The muscularization of the jugular pelvic ﬁn of Teleostean fishes is admittedly secondary: the limb rudiment becomes muscularizerl by the myotomes to which it happens to be opposite- at the time muscularization begins: but if this fact be admitted it is not open to us to deny the possibility of a similar process having taken place in the successive positions taken up by the pelvic limb in the course of the movements which it has undergone during phylogenetic evolution.

A further objection urged against the Gegenbaur hypothesis is that there have not been discovered, up to the present, any examples of the intermediate stages between gill-septum and limb which must have existed if this hypothesis be a true theory. This objection appears to be a valid one.

Again it is urged that in those Vertebrates which would appear, in this respect, to have retained most nearly the primitive condition (Cyelostomata, Elasmobranchii) the gill-septa are fixed firmly in position and are therefore not likely to become converted into motor organs, which must necessarily project beyond the surface and be freely movable. This objection like the last appears to be a valid one.

It will be apparent from the short sketch which has been given of the two rival views of the evolutionary origin of the limbs of Vertebrates that neither can be regarded as wholly satisfactory. However these hypotheses are old, as the science of Embryology goes. They were designed to ﬁt the data available at the time they were formulated and the great bulk of subsequent work upon this particular problem has consisted in the adducing of new facts which appear conveniently to ﬁt on to those already accumulated by the supporters of one view or the other. In a rapidly advancing science like Embryology however it is advisable to have from time to time a stocktaking of the facts of contemporary knowledge with the object of seeing whether the more extensive body of available facts suggests the same. working hypotheses as were suggested by the facts known at earlier periods or, as is always possible, something quite different. The putting this principle into practice is more conducive to progress and more stimulating to research than the mere accumulation of further facts to support or to confute the working hypotheses of earlier times.

THE EXTERNAL GILL HY1>o'rIn3s1s.——App1ying this principle to the problem of the evolutionary origin of the limbs one ﬁnds an important set of data which were not available to Gegenbaur or Balfour. In their day there was no proper appreciation of the importance of the fact that there existed in three of the less specialized groups of Vertebrates——Urodele Amphibians, Lung-ﬁshes and Crossopterygians——-those organs which have been described in Chapter III. under the name External Gills. The mode of development of these organs is now known in all three of the groups mentioned and the

VOL. 11 2 G 450 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

evidence appears to be conclusive that they are truly homologous throughout.

It has been shown that there is a tendency for the External Gills to become eliminated-—as e.g. in various Anurous Amphibians: it has been shown further that in some of the main groups of Vertebrates in which they do not occur. their disappearance may be accounted for by the evolution of a new physiological substitute-— the vascular surface of the yolk-sac.

Having regard to these facts and to the relatively archaic character of the groups in which they actually occur the conclusion is considered justiﬁable that such external gills are organs of high antiquity in the Vertebrate stem. Further, from their distribution upon the various arches it is inferred that in all probability an external gill was once present upon each visceral arch. But it has also been shown to be probable that the series of visceral cleft_s —-and therefore of visceral arches—was formerly more extensive, extending farther back along the body than it does in existing Vertebrates. It is therefore concluded that in an earlier phase of its evolution

A the phylum whose modern representatives we call Vertebrates was characterized by me. 204. -—'Left side of head the possession of a series of external gills fig?“ ‘:lp](:f":‘vh‘i’:h‘Rm? extending tailwards ‘beyond the limit days pl'eviou.x-ly, .1 piece of reached by the branchial region of existskin from the branchial region 'Verteb1~a,t,gs_

3fa;}t1;_f;fh¢;!'A ;?t*(: *I}?;j,33m11l‘=j:119"1*;;f'; But such external gills are potential

organs of support—as shown by the __-"-P "' ‘”= ‘,“‘il""_"“' 3*”-" (“_""_°' “balancers” of Urodeles (see Fig. 88, p. 157) sltlc), e.g/.t, external gills (pa1a- . . .ee.,) me. «levvloped ——and also potentlal organs of moVement——— the implanted piece of skin; on, as shown by the well-developed muscula"1’°""“"‘“" ture by which they can be ﬂicked backwards. In other words these organs—— and these alone among the organs of the Vertebrata—posseSs the qualiﬁcations which have to be postulated for the evolutionary forerunner of the Vertebrate limb.

In view of such considerations as those just set forth the present writer believes the most plausible working hypothesis of the evolutionary origin of the limbs—having regard to our present-day knowledge-—to be that which interprets them as modiﬁed external gills, belonging to visceral arches farther back in the series than those forming the branchial arches of existing Vertebrates. The limb girdle would on this hypothesis, as on that of Gegenbaur, be interpreted as representing a branchial arch skeleton, the dllfeI‘('.‘.I1("(' from the Gegenbaur view having to do rather with the nature of the projecting limb itself. VII EVOLUTIONARY ORIGIN OF LIMBS 451

Fig. 200 illustrates how close may be the general resemblance between the earliest stages of development of limb and external gill. The same is brought out also by Fig. 204, representing part of a frog larva on which had been grafted a piece of skin from the gill-producing region of another larva. The external gills of the graft have gone on developing and are remarkably limb-like in appearance.‘

The External Gill Hypothesis as to the evolutionary origin of the limbs ﬁts in well with other facts which are n.ow known. In the breeding male Lepidosiren the hind limb regularly and the pectoral limb occasionally (Agar, 1908) take on temporarily the characters of an external gill both in structure and in function (Fig. 205). This remarkable fact——-otherwise a morphological 1nystery—becomes at once understandable on the hypothesis outlined above, as a simple “reversion” towards an ancestral condition. Fig. 205 brings out clearly a further peculiarity of these gill-like limbs-of the male Lepidosiren, namely that the respiratory outgrowths of the limb are

Flo. 205.--Lq2i4lo.w79-en., breeding male showing apparent rever.si«:m of hoth pectoral and pelvic liinhs to the branchial condition. (From a specimen in the Zoological Museum of the University of Glasgow.)

in the case of the pectoral limb attached to its ventral side, in the case of the pelvic to its dorsal side. But it has already been shown that the deﬁnitively ventral side of the pectoral limb is homologous with the deﬁnitively dorsal side of the pelvic limb——the difference in position lming due to the rotation ,in different directions undergone by the llllll.) rudiments in the course of their development. This reversed position of the respiratory ﬁlaments in the two sets of limbs clearly then ﬁts in exactly with the View that they are ancient morphological characteristics of the li.mb which have reappeared in the male Lepidosiren.

The striking resemblance between the pectoral girdle and the branchial arches in some of the more ancient Fishes again ﬁnds its explanation in the morphological identity of the two structures. It is now established that the swim-bladder of Fishes is morphologically a lung, and that the lung is to be regarded as at the least an extremely ancient organ in the Vertebrate phylum. This points to the probability that the early Vertebrates were creatures which clambered

-‘ Ii('.ll:l'(-‘.1l(_i4.‘. slmllltl also be made to Fig. 88 (p. 157) which brings out clearly the rcn1:.n'kahly limb-like eh:n'acter of the Urodele “ balancers.”

Budgctt (]9()‘l) mentions the case of an abnormal Protopterus larva which had failed to develop the pinnae upon one of its external gills. “This bare shaft so much resembled the pm-_-.toi-al limb that the larva appeared to have two pectoral limbs on one side.” 452 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

about amongst the vegetation of shallow water and we may suppose that in this early stage the limb is as of a crude styliform shape such as we see exempliﬁed in the metamorphosed external gill of the Urodele balancers, or in the actual limb of the larva of Lepidosiren.

On this hypothesis the ancestral styliform limb has pursued two divergent lines of evolution. The one of these is found in those Vertebrates which have developed along the lines of becoming specialized for eﬂicient swimming. Here it has become a ﬁn, an early stage of this evolution being represented by the crude paddle of Ueratodus. That this biserial archipterygial type does actually represent an extremely early type in the evolution of ﬁns seems to be demonstrated by two facts taken in conjunction with one another——

1. That this thick and clumsy organ represents functionally a relatively ineﬂicient type of swimming organ as compared with the thin ﬂat ﬁn of most existing Fishes, and 2. That palaeontology shows it to have been a widely distributed type of ﬁn in the early days of the evolution of the main groups of Fishes. It was in fact the predominant type of limb amongst ancient Elasmobranchs, Ganoids and Lung-ﬁshes.

Evidence is not entirely wanting to show how the Crossopterygian type of ﬁn on the one hand (as seen in the existing Polypterus) and the Actinopterygian type on the other (as seen in Amie and other Ganoids and Teleosts) may have been evolved out of the biserial archipterygial type. This evidence cannot be gone into here but so far as Crossopterygians are concerned the student should note the close resemblance of the pectoral ﬁn of the young Polypterus (Fig. 197, E) and of its supporting skeleton (Fig. 169, F) to the modiﬁed archipterygial ﬁn of the ancient Shark Plcumcanthus (Fig. 169, B).

Along the other line of evolution the styliform limb has given rise to the pentadactylc leg with its expanded foot and its characteristic jointing. It is of great interest in this connexion to watch the clumsy movements of a Lepidosiren larva and to note that the hind limb by which the creature pusllvs itself along becomes bent twice upon itself precisely in the way which would give rise to the ankle and knee-joints of one of the lower Tetrapoda. Occasionally the appearance is rendered still more suggestive by the tip ﬂattening out slightly into a foot-like expansion. The observer watching a Lepidosircn larva. performing such movements ﬁnds it difficult to avoid the suspicion that he is witnessing something very like What took place in the early stages of the evolution of the pentadactyle limb. Should this be the true history of the origin of that type of limb it would explain the unsatisfactory and wholly unconvincing results of the efforts of comparative anatomists to derive the skeletal elements of the pentadactylc limb from those of one or other type of ﬁn.

P VII ORIGIN OF LIMBS AND TAIL 453

Embryology offers no explanation of the number of digits being so generally ﬁve. The physiological advantage of the expanded foot being divided up into separate radiating digits is obvious, as is that of the double nature of the adjoining portion of the limb skeleton to facilitate rotation round the axis of the limb. There are also mechanical advantages in there being a central digit with one on each side of it. Possibly the presence of an additional digit outside of these is to be looked on as of the nature of simple reinforcement.

The modiﬁcation of the pectoral limb in the case of Birds for purposes of ﬂight is of great interest, but nothing is known as to the phylogenetic transition from Reptile to Bird in this connexion. To the present writer it seems most probable that the Birds were evolved out of aquatic Reptiles in which the fore-limb was specialized for use in swimming under water, after the manner of existing Penguins, and that the function of aerial ﬂight was evolved directly from such movement under water. On this hypothesis the more or less terrestrial habits of modern Birds would be regarded as a secondary acquirement. _

(4) EVOLUTIONARY ORIGIN or THE TAIL REe1oN.—It is characteristic of Vertebrates that the anus loses its practically terminal position and comes to be situated some distance forwards on the ventral side, the overhanging hinder end of the body forming the tail. This opens up a question of much morphological interestthough one to which we are not yet in a position to give any certain answer—as to the phylogenetic origin of the tail.

It seems clear that the tail arose in ancient aquatic Vertebrates as an adaptation to swimming and on the whole it seems most probable that it came into existence through the gradual migration forwards of the anus upon the ventral side. Such a shifting forwards of the anal opening from the binder end of the body is a familiar feature in many groups of invertebrates where it is associated as a rule with a tubicolous habit and has doubtless for its object the getting rid of excretory products which would otherwise be discharged into the depths of the tube, or burrow, or shell. In the Vertebrate the forward shifting of the anal opening has probably its physiological signiﬁcance in the increasing eﬂicieney of the tail as the main motive organ—the disappearance from it of the alimentary canal, and its surrounding splanchnocoele, being correlated with the conversion of the tissues on each side of the skeletal axis into a solid mass of muscle. Probability is added to this conjecture by the fact that we see what appears to be a continuation of the same process in the most efficient group of modern swimming Vertebrates (Teleostei) where in the most highly developed forms the alimentary canal and splanchnocovle come to be restricted to a relatively small region immediately behind the head, the remaining and main part of the body being entirely “ tail.”

In actual ontogeny the tail region is developed not by the withdrawal from it of gut and splanchnocoele but as an actual outgrowth, 454 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. VII

the hind end of the body continuing to sprout out past and dorsal to the anal opening. It is of course conceivable that in phylogeny the tail arose similarly as an outgrowth of the body dorsal to the anus but this seems in every way less probable than the method of

evolution sketched above.

LITERATURE

Agar. Anat. Anzciger, xxxiii, 1908.

Balfour. Monograph on the Development of Elasmobranc-h Fishes. London, 1878. Budgett. Trans. Z001. Soc. Lond., xvi, 1901.

Caldwell. Jouru. Proc. Roy. Soc. New South Wales, xviii, 1884.

Dean, Bashford. Kupfl'crs Festscl1rif't. Jena, 1899.

Ekman. Morph. .lahrb., xlvii, 1913.

Gegenbaur. Untersuchungen zur vergl. Anat. dcr Wirlielfllierc, iii. Leipzig. 1872. Goodrich. Quart. Journ. Micr. Sci., l, 1906.

Kerr, Graham. Phil. 'l‘ran.s. Roy, Soc. London, B, oxcii, 1900.

Kerr, Graham. The Work of J. S. Blulgett. Ua.1nl)ridgv, 1907.

Kerr, Graham. Kcihcls Normentafcln, iii. Jena, 1909.

Rabl. Morph. Jaln‘l)., xix, 1893.

Semon. Zoologische Fo1'soln111gsreison, i. Jcna, 1893-1913.

Semon. Koibcls Normentafcln, iii. Jena, 1901*.

Taylor. Quart. Journ. Micr. Sr-i., lix, 1914. CHAPTER Vlll

Al)A‘PTA'l‘ION TO ENVIRONMENTAL CONDITIONS DURING EARLY STAGES OF DEVELOPMENT

I. PROTECTIVE ENVELOPES on THE ZY("}O'I‘E.——Tl1e Zygote or fertilized egg is in the Vertebrata as in other groups provided with protective envelopes. Of such we may recognize three fundamentally distinct types which are conveniently designated as primary, secondary and tertiary envelopes respectively. A primary envelope is a cuticular covering of the surface of the zygote: it is therefore produced by the living activity of the protoplasm of the macrogamete or zygote itself. A typical example of a primary envelope is the vitelline membrane which is formed on the surface of the Echinoderm egg in response to the act of fertilization. The “ vitelline membrane ” which covers the surface of the egg of a Bird is commonly looked on as a primary envelope.

A secondary envelope is one which is formed by the activity of the surrounding cells while the egg is still contained in its ovarian follicle. It may be cuticle-like in its nature or it may be composed of cells.

Finally tertiary envelopes are formed by the excretory activity of the oviducal lining, being deposited upon the surface of the egg as it travels down the oviduct. Of such a nature are the complicated protective envelopes which surround the egg of a Bird or Reptile, or the simpler jelly - like investment found in the case of most Amphibians.

Apart from tertiary envelopes the. most conspicuous envelope of the Vertebrate egg is usually what is known as the zona radiata or zona pellucida—-—the former name being given to it from the fact that it is pierced by numerous very ﬁne canals which give it a characteristic radiate appearance when seen in section. These ﬁne canals apparently contain protoplasmic bridges connecting the protoplasm of the egg with that of the follicle-cells which surround it while still in the ovary, and doubtless having for their function the passing in of food-material from the follicle-cells into the egg-cell.

The zona radiata is, as a rule, most conspicuous during early intra-ovarian stages while the egg is undergoing active growth during the storing up of yolk. Later it thins out and becomes less con 455 456 EMBRYOLOGY OF THE LOWER VERTEBRATES on.

spicuous. The zone radiata is usually looked upon as primary in

its nature but this is by no means settled and some competent authorities regard it as secondary. ‘ 4 Outside the zona radiata there may often be found a second envelope which does not show the perforations characteristic of the zona radiate: this also in the case of the large heavily yolked eggs becomes thinned out during the process of growth. Envelopes of this type are specially conspicuous in those Vertebrates 111 which there is no great development of tertiary envelopes secreted by the oviducal wall, ag. Teleostcan ﬁshes. In such cases the

Flu. 206.-—A, cluster of eggs of Btlc/lo.stom.a, attached together by the interlocking of their anchoring ‘filaments ; B, apical portion of the egg-shell. showing the anchoring ﬁlaments projecting from the middle of the separable “lid.” (Figure by Bashford Dean, from The (;’r.rni..lmTcig¢: .Vrttu-ru..l lI'[stm;z/.)

outer layer of the envelope, lying immediately external to the typical radiate layer, frequently shows characteristic modiﬁcations, consisting of closely packed villi or columns composed of cells which swell up and become strongly adhesive, serving to attach the eggs to one another or to a solid object (Roach-———I,euc7Jscus 9'-utvllus, Bleak-——Albu7~mzs, Herring—,—-C'lupea harengus). In the Actinopterygian Ganoids a similar condition is found. In Uemtodus this special outer layer is not found, there being here a tertiary envelope of jelly. ~ In I.cpz'nlosz”re7a._ and Protoptev-us the tertiary envelope is as a rule no longer formed, the eggs lying loose in the bottom of the burrow, though it is of interest to notice that in Lepidosiren the secretion of a jelly-like tertiary envelope round the eggs is still occasionally found as an individual variation. In Petromyzon, as in the \Teleosts alluded to above, a radiate envelope is found inside a villous one which becomes swollen up and sticky on the absorption VI ll EGG-EN V ELOPES 457

of Water. In the Myxinoids the egg is contained in a cl1aracteristic elongated shell provided at each pole with a group of stiff anchoring ﬁlaments ending each in a lobed umbrella-shaped expansion (Fig. 206). The piece of shell covering the germinal pole is marked off by a deep incision from the rest so as to form a lid which is forced ell’ at the time of hatching.

Whatever the true nature of the enve.lopes under discussion, whether primary or secondary, they already exist round the egg before fertilization takes place, and as the substance of the envelope is, as a rule, impenetrable by spermatozoa there necessarily exist one or more openings or micropyles through which the fertilizing sperniatozoon makes its way into the egg. In the Myxinoids one such micropyle is found in the middle of the lid, surrounded by the concentric circles of anchoring ﬁlaments. The presence of a micropyle in Lampreys and in Lung-ﬁshes is not delinitely established. In Lepiclomlren it has been observed that the envelope enclosing the coelomic, and therefore unfertilized. egg is thick and gelatinous while after fertilization it becomes dense and horny. Possibly therefore during the. ﬁrst-mentioned condition it is penetrable by the spermatozoa. In Te.leostean ﬁshes a mieropyle occurs at the apical pole, and so also with Actinoptcrygian Ganoids except that in the Sturgeons there exist a group of openings (5-13 in the Sterlet, according to Salensky) instead of a single. one.

Of the more complicated arrangements of tertiary envelopes found in Vertebrates no better example could be taken than those found in the case of the Fowl’s egg. These will be found described in Chap. X. In Birds in general the envelopes resemble those of the Fowl, differences occurring in details of relative size, shape, and colour of the shell. The “ egg ” (73.0. the zygote with its envelopes) appears to be largest relatively in Apiary/a; where it reaches about a quarter ot' the weight of the parent.

The shape of the shell is impressed upon it by the pressure of the oviducal wall and differences in shape are no doubt due to differences in the peristaltic contraction. Thus the strong contraction of the oviducal muscles which, acting on the headward side of the egg, propels it onward, if combined with comparatively slight contraction on the tailward side of the egg will naturally cause the egg to assume a more or less markedly conical shape-—the end of the egg directed towards the eloaca being broader than the other end. In some cases, as that of eggs laid on bare ledges of rock, this conical shape has probably had a deﬁnite natural selection value, in causing any rolling niovement of the egg to follow a strongly curved path. In other cases where there ' is less marked inequality of pressure on the two poles of the egg the shape is more regularly ellipsoidal.

The eggs of Birds being commonly exposed to light and to view they very often show a characteristic colouring, either throughout the thickness or merely in the outer layer of the shell. In very 458 EMBRYOLO(:‘rY OF THE LOWER VER'.l‘EBRA'l‘ES CH.

.numerous cases the natural selection value of the colouring as a

means of making the egg less conspicuous is obvious. .

In Reptiles the tertiary envelopes resemble those of Birds though in many cases, as in various Lizards and Turtles, there is no deﬁnite rigid shell. On the other hand there may be a certain amount of lime deposited in the outer layers of the shell-membrane. The albumen varies in amount : in Sp}z,e7z0(lo72, it forms only a very thin layer (Dendy, 1899)

In Elasmobranch ﬁshes the egg is again enclosed in a layer of albumen and this in turn surrounded by a shell. The shell is of a horny consistency and is frequently rectangular and pillow-shaped. Characteristic dilfereiiees are found in different genera and species.

Thus in the Skates (Ram) each angle is prolonged so that the egg

Flo. ‘207.——Egg of .\f'cyl/(um, held in position by its four l‘l.‘l.'~‘llI'. lilunu-uts being wound round a plant. (Fi_<_r1|re l»_\' Kopscli, from The (Jam.i;r£¢[_r/+3:.;\'u/um! //-1'.<:lo-ry.)

has an outline like that of a hand-barrow. In Sag/llz'mn (Fig. 207) the prolongations become long spirally coiled anchoring ﬁlaments: in Pmlstiurus two short’ prolongations occur at one end while the other end is simply rounded.

1}‘. MOI)IFICATIONS or THE ENVELOPES AND OTHER ADAPTIVE MODIFICATIONS OCCURRING DURING THE “EARLY DEVELOPMENT OF THE AMPH1BIA.———The Amphibians form a group of Vertebrates which have taken less or more completely to a. terrestrial existence in their adult condition. They have not been able to emancipate themselves entirely from the ancestral aquatic habitat, possibly on account of the feeble development of the horny outer layer of the epidermis. They are still as a rule entirely aquatic during the early stages of their development,the eggs being laid in water and the young animal passing its larval existence in the water.

In a number of cases, particularly in Anura inhabiting tropical regions with a well-marked dry season, very interesting adaptations are found whereby the young animal is enabled to pass a more or less prolonged period out of the water. In the ﬁrst type of these V [II DEVELOPMENTAL ADAPTATION S 459

adaptations we ﬁnd special modiﬁcations of the tertiary envelope which is normally a simple mass of jelly deposited round the egg.

The ﬁrst type of such adaptation is exempliﬁed by various species of If;/locles and by Rana opistltorlon in which the eggs are simply deposited in free air in damp spots, each surrounded by a transparent spherical protective shell. In R. opisthoclon (Boulenger, 1890) the young Frog before hatching develops on the tip of its snout a small conical protuberance apparently used like the egg-tooth of Reptiles and the similar organ in Birds to tear open the egg-envelope. A further interesting adaptive feature. is that the young unhatched Frog possesses on each side of its body a series of vascular ﬂaps of skin somewhat resembling the gill-flaps of an Elasmobranch ﬁsh and apparently functioning as respiratory organs.

In a considerable number of tropical Anura the ovidueal secretion which surrounds the eggs is, at the time of laying, beaten up by rapid movements of the hind feet of the parents into a ﬁne foam or froth with numerous entangled air-bubbles. This may be deposited on the surface of a pool where it floats about like a ﬂeck of ordinary foam with the developing eggs scattered through it (Paludiwla

fuscomacula/u.). At a particular stage in development a digestive

ferment apparently is secreted, probably by ectodermal gland cells, which liqueﬁes the jelly and allows the larvae to drop through into the underlying water.‘ In other cases the mass of foam is deposited in an excavation in the ground, so situated that rain-water readily trickles into it (Engystrmza orale), or merely in a damp spot. In the case of the Japanese 1t’lz.ucoplo.orus (.Polg/pedates) so/Llegelé (Ikeda, 1897) the b11rrow is made in a bank by the margin of standing water and after the mass of egg-foam has been deposited the pair of Frogs make their way out by excavating a tunnel which slopes downwards and opens near the water’s surface. Here again at the appropriate stage. of development the jelly liqueﬁes and the. young larvae are carried down by it into the water.

In the case of Phy/lloznedwsct 11.3/pot-/aonu’rial'£s the process of oviposition was observed by Budgett (1899) in the Gran Chaco. The eggs are deposited during the night, the female clambering up amongst the leaves of a suitable plant by the margin of a pool, with the male on her back (Fig. 208). With their hind legs the. two Frogs bend the margins of a leaf together so as to form a funnel into which the eggs are poured together with the fertilizing sperm. The eggs are enclosed in a mass of ﬁrm adhesive jelly which causes the leaf to

1 It is probable that. such ferments play an important part in softening the eggcnvclopes preparatory to hatching in various animals. Thus in Lcpidosircn the process of hatching is rendered possible by the softening of the egg-shell brought about apparently by digestive ferment secreted by the ectoderm covering the body (Graham Kerr, 1900). The same appears to he the case in Telcosts (Wintrebert, 1912). In Xrmbpus amongst Amphibians :1. similar process apparently takes place and in this case Bles (1905) attributes the formation of the ferment not simply to the diffuse activity of the cctodcrm cells but to the action of a special “frontal gland.” It seems not improbable that the formation of such hatching ferments will be found to occur very generally in aquatic Vertebrates. 460 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

retain its funnel shape. The eggs develop within the jelly up till

Flu. ‘Z08. _I’/1,:/llunuv/ustt l1._c/pm:/u_;m.lr£u.l-is, l'e1u.ale carrying male on her back during oviposition. (After Builgttt, 1899.)

contain no egg in their interior? The eggs are thus protected both above and below by a thick mass of eggless spheres. During the later stages of development the layer of envelope next the surface of each egg becomes greatly distended by the accumulation of ﬂuid within it, the jelly between the eggs meanwhile diminishing in volume. with their huge external gills have thus considerable room in which freely. Eventually the envelope ruptures and the larva hatches. comes to be occupied by a seething mass of tadpoles, ﬂoored and roofed in by a thick mass of jelly formed by the empty Eventually——in from 12-24

spheres.

the stage of a tadpole of 9-10 mm. in length. During this process the jelly apparently liqueﬁes, until only a thin membranous bag containin g watery ﬂuid surrounds each embryo. Eventually the remains of the jelly with its contained tadpoles trickles downwards into the water. of the water has retreated from immediately below the leaf the tadpoles may still make their way for a distance of several inches to the pool by active jumping movements, helped it may be by a shower of rain.

In the allied I’hg/llomedusa sauvagii, from the same neighbourhood, a similar mode of oviposition occurs, though here the nest is composed of several leaves (Fig. 209). Agar (1.909) ﬁnds in this case that both at the commencement and end of oviposition there are laid a large number of spheres of jelly which

'l‘he larvae to move

The nest thus

hours after the bulk of the larvae have

hatched——the jelly begins to deliquesee and the larvae drop down with it into

the water.

FM. 209. - -— I‘/lg/ll4ruu;du.w so I’ ('14;/ii, mass of spawn. (After Agar, l909J

Similar nesting habits occur in other tropical Hylids, e.g. Phyll0 1 In the eommon I‘-‘rocr j.’u-nu. /nu.-~m'-rm-in. a narent] em it ca wsnles ma be formed an I l.l y l. y l y

in quantity in the oviduct before eggs begin 10 enter it (Wezel, 1908). with the normal eggs Agar found 2-3 per cent nl'.s1u:l] eggless capsules.

Interspersed 'l‘hese appear

to be deposited round small solid particles such as fragments of shed epithelium

(Lebrun, 1891).

In C'671.t7'opho7'us, where the left ovary is no longer functional, empty tertiary envelopes are frequently still formed in the left oviduct (Braus, 1906).

If, as sometimes happens, the margin vm DEVELOPMENTAL ADAPTA'J.‘IONS 461

I medusa vllterringvli (von Ihering, 1886), H3/la nebulosa (Goeldi, 1895),

Rltacopltorus 're7Jnwardt'£'13(Siedleeki, 1909). In the last mentioned the eggs are deposited in a mass of foam enclosed in one or several leaves (Fig. 210). At the appropriate time the central portion of the mass liqueﬁes and the colourless tadpoles make their way into this central ﬂuid —«—~ the superﬁcial layer of the mass being hard and dry. Eventually the lower part of the mass softens and the

_1iq_uid containing the tadpoles trickles out on to the ground where

the larvae are able to continue their development in the smallest puddles. s

In the second type of such adaptations the eggs or young are carried about, away from the water, by one of the parents. In the simplest of such cases no structural modiﬁcation of the parent's body is involved. Thus i_n A/,3/tes ribstetrwicans the male draws the strings of eggs out of the cloacal aperture of the female and loops them round his thighs-——the portion of oviducal secretion lying between successive eggs- becoming highly elastic and gripping the thighs tightly. Oviposition takes place on land and the male pays only occasional visits to the water. When one of these happens at the appropriate period the. young hatch in the form of tadpoles while the male parent resumes his terrestrial habits.

In a number of - cases the transport of the young by the parent takes place at a later period, when the tadpole stage has been reached, the larvae adhmhing to the back of the male parent and so being transported from one pool to another (Fig. 211, A). This habit occurs in various species of Iﬂzndrobates and P/b3/U0- p A _ bates (Brandes u. Schoenichen, 1901). 1“‘5,‘,;(,)_:§}g.' "fl£1’"(f)';?’t’f‘;f;El

In the most interesting cases however },,,M,,.,,’ ,p‘.“1‘I;‘f)1t,_\__ ‘(11,;,-W. 5,9,1. the transport of the eggs or young by the lvvki, 1909-) parent is associated with the making use of some particular structural feature of the latter——-either permanent or specially developed for this purpose- In Rhacophorus reticulatus (Uriiiitlxer, 1876) the eggs are carried about by the female, adherent to its ventral surface. In Iig/la goeldivl (Boulenger, 1895) the eggs adhere to the dorsal surface of the female, only in this case the skin of the parent responds to the stimulus afforded by the presence of the eggs and grows up into a slight ledge surrounding them (Fig. 211, B). In Pipe amcricana (Bartlett, 1896) the cloaca of the female is protruded at the time of oviposition as a large spout-lilce 462 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

structure which projects forwards between the dorsal surface of the female and the ventral surface of the male. The eggs pass out one by one through this and are distributed at fairly equal intervals over the dorsal surface of the trunk of the female. The skin now proliferates actively, growing up so as to form highly vascular partitions between the eggs, each of the latter coming to be enclosed in a deep pit. The mouth of this becomes closed in by a dark-coloured operculum, possibly formed of hardened epidermal secretion. Each egg is thus enclosed in a little chamber in which it passes through the early stages of its development, including a modiﬁed tadpole stage, and issues forth eventually (after about 82ldays) as a young Toad.

In another set of Anurous Amphibians the eggs undergo their development in a spacious single cavity

  within the parental body. In Rhivzoderma

darwvlml (Jimenez de la Espada, 1872;
Plate, 1897) this cavity is the enlarged

unpaired croaking sac of the male, into

which the eggs, to .the number of from
5 to 15, are swallowed and from which the
 young issue after completing the tadpole

, stage. In the genus Nototrema the brood

cavity is a special large pouch lying
 beneath the skin of the back, lined by
involuted epidermis and opening to the
exterior just in front of the cloacal aperture.

In different species of the genus there is much difference in the length of time

 during which the developing embryo is
retained within the pouch, the length of

this period being apparently correlated with the size of the egg and the amount 1.~IG_ 21]__A, M1,, ,,,- [:/,.‘,/g[,_,_ of food-yolk stored within it. Thus in hates to-'initatz's carrying tad- N. vizarsupiatum there may be as many as P°‘;";',.?. "B! .f""“‘1f_°f ff’Jt"‘ 200 eggs in the pouch, each measuring ?;':',"',,'1',l.',",,_,.(,'.',i.1_‘ rjV§1;,%_i;"gb' ( er about 5 mm. in diameter (Brandes u. Schoenichen, 1901), and the young make their way out as typical tadpoles which doubtless lead for a time a free aquatic existence before metamorphosis takes place.

In N. omgfemm (Weinland, 1854) the eggs are much larger

(10 mm.) and fewer in number (about 15) and in this case as in the

. allied N. testudinemn and N. ﬂs.s"ipes, which also possess large eggs,

the young go on developing within the pouch until after the period of metamorphosis.

In Noto/mama. an interesting adaptive feature characterizes the external gills. These organs are present upon branchial arches I and II, each consisting of a long slender stalk, which passes at its outer end into a thin highly vascular membrane formed by the fused and VIII DEVELOPMENTAL ADAPTATIONS 463

expanded outer ends of the two external gills. The two membranes so formed, one on each side of the body, are closely applied to the inner surface of the thin egg-envelope. The outer surface of the envelope is in turn in intimate contact with the highly vascular lining of the pouch which sends projecting folds in between the eggs. We have here clearly an adaptive arrangement to minister to the respiratory needs of the developing young, analogous with that provided by the allantois of a Reptile or Bird.

It appears somewhat puzzling that the eggs of Nototrema should come to be contained in a pouch the opening of which is much smaller than the cross-section of the egg. The probability appears to be (Boulenger, 1895) that the pouch is formed in response to the presence of the eggs upon the animal’s back, a ridge growing up round the eggs as in the case of fig/la. goelrlvli but in this case continuing its growth towards the mesial plane until the corresponding upgrowths from the two sides meet and completely roof in the pouchlike cavity. Support is given to this explanation by the condition in N. pg/gznacmn where the opening of the pouch is in the form of a median longitudinal slit, prolonged forwards as a kind of seamvor raphe along which the roof of the pouch readily tears and which presents all the appearance of having been formed by the coming together of two originally separate lips.

It must never be forgotten that such peculiarities of development as have been alluded to in the above--mentioned Anura involve adaptive modiﬁcations on the part of the young individual itself. The most frequent of such modiﬁcations is physiological adaptation, as shown for example by the fact that the transference of the young individual to water before the normal time is commonly fatal. In other cases structural adaptations of a more conspicuous kind are apparent. Thus in 1-’1Ipu. the late tadpole stage, although enclosed within its cell, develops a broad and highly vascular ta.il which doubtless serves for 1'espirato1'y and possibly nutritive interchange with the maternal tissues: again in ]Vutotrema, the external gills show the peculiar modiﬁcation already alluded to. The true external gills are in several cases absent, their function being taken over by the vascular surface of the yolk, while in such a case as Rana opz'.s-thodcm. special new respiratory organs have been developed.

In the various modiﬁcations of development dealt with in the

‘preceding section we have to do with attempts, so to speak, on the

part of isolated members of a particular group of Vertebrates (Anura) to lessen the degree of their dependence upon the ancestral aquatic habitat. Such attempts amongst the existing Amphibia are not altogether successful: the group as a whole remains chained to a watery, or at least humid, environment.

The lower Vertebrates which made a real success of terrestrial existence, emancipating themselves entirely from the aquatic environment, are represented to-day by the Amniota, and it remains now to study their special modiﬁcations of development. 464 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

III. ADAPTIVE MODIFICATIONS IN THE DEVELOPMENT or THE AMNIOTA.-—It is characteristic of many Vertebrates that, associated with the provision of special arrangements for nourishing the young individual, the tiI11e of commencing an independent life on its own account is greatly delayed. In such cases where a considerable proportion of the whole development takes place within the shelter of the egg-shell (or of the parental body) we have to do with what is known as embryonic in eontradistinetion to larval development. During embryonic development the young individual is free from the necessity of ﬁghting and lending for itself; it is to a great extent sheltered from the struggle for existence, and in correlation with this we ﬁnd remarkable hypertrophies and modiﬁcations of various parts of the body taking place which in a free state would render life impossible.

The ﬁrst of these modiﬁcations makes its appearance in the lower, aquatic, Vertebrates in the form of a pronounced bulging of the ventral side of the body. In the more primitive holoblastic Vertebrates this is caused by the great thickening of the ventral endoderm (Fig. 80, E, p. 146), its cells being much enlarged and packed with granules of yolk. Where this distension of the endoderm cells is most marked anteriorly there is brought about the tadpole shape of body as seen in the Ganoids and Lept'do.s7I7vm: or, on the other hand, the distended region may be situated towards the hinder end as in _l-’et7'0r/nyzon, Uemtodas or the Grymnophiona. In such cases as development proceeds the large yolk-cells go on segmenting, the yolk within them is gradually used up, and the Inass of endoderm, becoming Inore and more attenuated, ceases to project beyond the general outline of the body.

In the meroblastic egg, as has already been shown, the proportion of living protoplasm amongst the yolk has been reduced to vanishing point so that except superﬁcially the yolk never segments. Typically it becomes gradually enclosed in the endoderm which spreads over its surface. There is thus formed what is known as the yolk-sac, a structure ‘usually of enormous size as compared with the rest of the embryo. lt will readily be understood how impossible a free active existence would be while there is a large yolk-sac present. The assimilation of yolk and its transport to the actively growing parts of the embryo are brought about mainly by the rich development of superﬁcial blood-vessels forming the vitelline network. In typical Teleosts, ag. Salmonids, the yolk-sac becomes at an early period completely separated from the dorsal part of the endoderm which becomes the functional gut, the yolk absorption taking place entirely by the vitelline vessels.

An important point to be remembered is that the vitelline network though primarily nutritive in function is necessarily also respiratory, gaseous interchange taking place between the blood circulating in its vessels and the medium which bathes its surface. The vitelline network is the primary breathing organ in the great VIII DEVELOPMENTAL AD APTATION S 465

majority of Vertebrates during early stages of development. In cases where the embryo lies in contact with maternal tissues the respiratory exchange takes place ultimately, through the thin intervening layers of ﬂuid or envelope, between the blood circulating in the vitelline network and that circulating in the oviducal lining of the mother. In this way all the necessary preliminary conditions are provided for the evolution of a placenta, and as will be shown later these conditions are actually taken advantage of in some cases and a simple yolk-sac placenta is formed.

In the more highly developed types of yolk-sac the splanchnic mesoderm which surrounds the vitelline vessels sprouts inwards, forming irregular vascular septa which project into the yolk-sac. This modiﬁcation, which brings about a great increase in the assimilatory surface, reaches such a development in Birds that towards the end of incubation these ingrowths form an irregular meshwork of vascular trabeculae traversing the whole of the yolk right to its centre.

Eventually the yolk, whether in the form of a yolk-sac or a mass of heavily yolked cells, is enclosed within the ventral wall of the body. In the holoblastie Vertebrates this comes about as already indicated by the simple spreading of the blastoderm over the surface of the yolk so as completely to enclose it. In the Fowl the spreading of the blastode)rm, and its derivatives the endoderm and mesoderm, round the yolk is never quite completed, there remaining a small circular patch at which the yolk is separated from the albumen only by the remains of the vitelline membrane (of. Fig. 215, am).

Further in the Amniota the region of somatopleure bounding the coelomic space in which the yolk-sac lies becomes converted into amnion and serous membrane (of. Fig. 215, A), and is eventually cast off‘, playing no part iii the formation of the definitive body-wall. The yolk thus lies outside the limits of the deﬁnitive body-wall, projecting through the umbilical funnel which is bounded all round by the stalk of the amnion. Eventually, shortly before hatching, the edges of the umbilical opening are drawn over the yolk-sac in a manner which will be described later (see p. 475). In llacerta mlmlpztm in which the yolk-sac is reduced the remains of it are simply cast off according to Strahl.

The most remarkable of the excrescences adaptive to an embryonic existence are the organs known as Amnion and Allantois ——portions of the embryonic body which become greatly hypertrophied and perform important functions during embryonic life but which are eventually, for the most part, shed about the time of birth or hatching and play no part in the formation of the body of the adult.

AMNION.—-The most nearly primitive subdivision of the Amniota is the group Reptilia and we accordingly turn to it and more especially to the Ohelonia, which have been worked out by Mitsukuri (1891), to provide a foundation for our description.

VOL. II 2 II B

Fro. ‘.212.—-Chelonian blastodenns illustrating the development of the amnion. (A and C after Mitsukuri, 1891.)

A, ('Iemm.ys; 13, tfhelzudm; C, Ulemmys. a, amnion with neural rudiment seen indistinct.1_v through it; a.u., edge of amniotic ﬂap; a.r, amniotic tunnel; c.g, cephalic groove; f, inconstant fold which is sometimes present; 31.1‘, gm-‘:trul8I' rim ; m._f, medullary fold ; gm, proamnion with head of embryo showing through it. *

466 CH. VIII DEVELOPMENT OF AMN ION 467

In Chelonia the iirst indication of amnion formation appears at a stage like that represented in Fig. 212, A. The future body of the embryo, indicated by the medullary folds, lies ﬂat on the surface of the egg, extending out all round into the blastoderm. The ﬁrst sign of the amnion is produced by thefront end of the medullary plate coming to dip downwards so as to form a deep slit or groove (Figs. 212 and 213, c._q) curving tailwards on each side as seen from above. '1‘he posterior wall of this slit forms the anterior limit of the head of the embryo while its anterior wall forms the rudiment of the amnion (Fig. 213, me). The portion of blastoderm in front of and to the side of the head of the embryo is as yet two-layered, the Inesoderm not yet having spread into it, and it follows that the amniotic ru(:1in1ont is also two-layered. This region of the blastoderm,

FIG. 213.——-Sagittal section through the head end of a Chelonian embryo. (After lVIit'sul<m'i, 1891.)

t.l..(:, anmiot;-iv ml;_-,-u : mgr, cephalic groove; earl, 4'-I.-t.m1r-1'-n1 of medullary plate ; end, oudoderm.

which is still without mesoderm and which in this case forms the amniotic rudiment, is termed the proamnion. As development proceeds the head end of the embryo increases in size and as it does so it dips more and more downwards so as to deepen the cephalic groove or slit in front of it. While this is going on there takes place active growth of the ectoderm along the sharp edge of the amniotic rudiment (Fig. 213, a.e) i.n such a way that this edge becomes prolonged backwards as a solid ﬂap, covering over the body of the embryo from before backwards. This amniotic ﬂap continues to grow tailwards, its growing edge concave and prolonged backwards on each side (Fig. 212, B, a.e), until it reaches the tail end of the embryo, so that the whole of the latter is covered in by an amniotic roof. Nor does the process stop now: it goes on with the result that there is formed a long tunnel (Fig. 212, C, a.t) continuous in front with the amniotic cavity, ~23.e. the cavity between 468 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the body of tlie embryo and the amnion, a11d terminating behind in an opening bounded above by a concave free edge (a.e)._

An important point to realize is the relation of the amnion to the cell layers. The ﬁrst rudiment, as has been indicated, is composed of the two primary layers ectoderm and endoderm, and this applies also to the lateral prolongations baclnvards of the free edge. The whole of the amniotic roof however except these marginal parts is formed at first of solid ectoderm and of ectoderm alone (Fig.

FIG. 214. —-—Diagrammatic transverse sections through Clielonian en1ln‘_\'ns (f-.'!v-m-nz.a/.s-. A, stage with 2-3 mesoderin segments; B, 6-7 segments) i1ln.s-1.r::t_i'n«,-; tlw rela1..ion.~e of tinamnion. (Based on ﬁgures by Mitsnkuri, 1891.)

a.f, amniotic ﬂap; am, amnion: H", I'(:l.u¢lterm; rm/, vndoderrn; _/Zn, fnI.~u-_ amnion: 'nu-.<-, 1m-sods-.rm segment; N, notoehord; srI., so-ru-amniotzie junction; mm, somatoplenre: spl, sphmuhnupleme; splc, splanchnocoele.

214, A, a.f). As development goes on the mesoderm extends between ectoderm and endoderm and then splits into somatic and splanchnic layers. The result of this is that the endoderm, with its covering of splanchnie mesoderm, sinks down and no longer projects upwards on each side into the base of the amnion (Fig. 214, B). The somatic mesoderm on the other hand does continue to project into the base of the amnion just as did the endoderm previously (Fig. 214, B). The originally simple ectodermal roof of the amniotic cavity undergoes a process of splitting from its lateral margin inwards and as this split extends towards the mesial plane. the amniotic fold of VIII DEVELOPMENT OF AMNION 469

somatic mesoderm spreads with it. Except along the middle line the amniotic roof thus becomes double——-the inner roof being formed of ectoderm internally and somatic mesoderm externally, the outer roof of somatic mesoderm internally and ectoderm externally. Of these. two roofs the inner is the amnion (Fig. 214, B, am), the outer is the false amnion or serous membrane (f.a). The portion which retains its original condition of being formed of unsplit ectoderm (sa) may be. called the amniotic isthmus or the sero-amniotic connexion (Mitsukuri). During later stages of development this becomes reduced to a thin vertical partition in which form it persists throughout, except in the region of the head where it disappears entirely so that there is here a continuous coelomie space stretching from side to side between amnion and serous membrane.

’J‘he posterior tubular prolongation of the amniotic cavity becomes obliterated through part of its extent and in this way the amniotic cavity becomes completely closed.

The first-formed part of the amnion, lying in front of the head of the embryo, remains for a time proamniotie in character, '£.e. composed of ectoderm and endoderm, but eventually the mes'oderm and eoelomie. space spread in between the two primary layers and the portion ot' the amnion in question comes to resemble the rest.

As the body of the embryo becomes constricted oil’ from the yolksac the basal edge of the amnion continuous with that of the embryonic somatopleure becomes tucked inwards so that the amnion, which formed in earlier stages a mere roof, comes to form a complete envelope. The amniotic cavity is ﬁlled with secreted ﬂuid in which the body of the embryo ﬂoats.

BIRDS.-——-The process of amnion formation i11 the Birds shows conspicuous dil‘f'erenees from that which has been described for the more primitive Reptiles. Two of the chief of these dili'ere.nces seem to be associated with the fact that the amnion develops relatively later in the Bird, at a period when the head and anterior body region of the embryo project prominently above the general level of the blastederm and when the mesoderm has already split into splanchnic and somatic layers. Correlated with this fact we ﬁnd (1) that in the Bird the amniotic rudiment has to grow upwards so as to surround the projecting head and trunk, and (2) that the upgrowth is composed of somatopleure only.

The amnion may be said to originate as a kind of wall, formed of a11 upwardly projecting fold of somatopleure, which comes to surround the actual body of the developing embryo. This wall is not absolutely vertical: it is tilted, or inclined inwards, towards the middle of the embryonic body. With increasing growth it projects more and more over the body of the embryo, its free edge bounding a gradually diminishing opening, through which the body of the embryo is visible when looked down upon from above. Eventually this opening is reduced to vanishing point and the body of the embryo is completely 4'70 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

covered in by a double roof formed by the amnion and the serous membrane.

The amniotic fold does not develop with equal activity throughout its extent. Its growth is much more active anteriorly than elsewhere, with the result that the headward portion of the fold becomes extended rapidly backwards as an amniotic hood over the head and anterior end of the body of the embryo (cf. Figs. 233, 235, 236). '.l‘he last remnant of the amniotic opening is consequently situated quite near the hind end of the body.

Correlated with the later appearance of the amniotic hood——at a time when the coelomic cavities are extensively developed—-it is at no period composed throughout, from side to side, of a simple layer of unsplit ectoderm as was the case with the Chelonian. It is of interest to notice however that the sero-amniotic isthmus has not altogether disappeared, although it never has the breadth that it has in early stages in the Ohelonian.

The details of amnion formation are readily observable in the Fowl and have been fully described by Hirota (1894). The process takes place as follows: The iirst step consists in the appearance of a crescentic upgrowth of blastodcrm just in front of the head of the embryo at about the stage of 14 segments. At this period the mesoderm has spread forwards on each side but has not yet extended i11to the space immediately in front of the embryonic head (proamnion). Where the mesoderm is present it has split to form the coelome and owing to this being ﬁlled with secreted ﬂuid the somatopleure bulges up somewhat so as to be conspicuously marked oil’ from the ﬂat proamniotic area. The amniotic fold makes its appearance just about the anterior boundary of the. proanmion. As it increases in height it overlaps the head of the embryo and grows backwards over it as the amniotic hood (Fig. 233). Into the fold the mesoderm and coelomic cavities have already penetrated. Where the mesoderm from the two sides meet in the mesial plane of the hood the two portions of coelome do not open freely into one another but remain separated by a septum of mesoderm—-the mcsedermal sero-amniotic isthmus. At an early period of the backgrowth of the amniotic hood the ectoderm in the middle of its free posterior edge is seen to project headwards as a small wedge, the base of which is formed by the growing edge. As this wedge is carried backwards by the continued progress of the amniotic edge it leaves behind it a kind of trail in the form of a continuous line, or rather partition, of ectoderm connecting the ectoderm on the outer surface of the amniotic fold with that on its inner surface. This is clearly the ectodermal sero-amniotic isthmus of the Reptile persisting in a much attenuated form; the attenuation being due to the fact that the coelomic spaces have extended much nearer to the mesial plane than in the corresponding stage of amnion-formation in the Reptile.

Up till about the time when the amniotic hood has completed its backgrowth its cavity--—-the amniotic coelome—-—remains divided VIII DEVELOPMENT OF AMN ION 471

into two separate halves by a septum, which in front is purely mesodermal but throughout the rest of its extent is traversed by the ectodermal sero-amniotic isthmus. The anterior, purely mesodermal, part of the septum disappears early in the fourth day so as to make the amniotic coelome continuous from side to side, but the rest of the septum persists throughout the whole period of development although its central ectodermal portion becomes gradually reduced and by the tenth day has completely disappeared.

Towards the end of the second or early in the third day the tail of the embryo begins to project, bending ventrally and dipping downwards as it does so. As it does this the tail comes to be hidden under a projecting amniotic fold precisely as happened at the head end except that here the coelomic cavity is already completely continuous across the mesial_ plane there being no trace of a septum or sero-amniotic isthmus. The free edge of this “tail fold” of the amnion is, as was that of the “head fold,” concave only here the concavity is directed headwards. Early in the fourth day the concave edges of the head and tail folds become continued into one another at about the level of the hind limb rudiment, so that the body of the embryo is now surrounded by a continuous amniotic fold———most highly developed anteriorly where it forms the amniotic hood, less so in the caudal portion and least of all laterally. The more or less elliptical opening bounded by this fold, through which the dorsal surface of the embryo is exposed, gradually shrinks as the fold grows and eventually, during the ﬁrst half of the fourth day as a rule, it becomes obliterated and the amniotic cavity closed.

The true amnion at first closely enshcaths the head and trunk of the embryo but from about the ﬁfth day onwards watery amniotic ﬂuid is secreted into its interior so as to form an extensive water jacket in which the embryo is suspended (Fig. 215). For a considerable period the embryo is gently rocked to and fro in the ﬂuid by the slow rhythmic contractions of muscle ﬁbres which develop in the somatic mesoderm covering the amnion on its outer surface.

The development of the amnion in the Sauropsida in general is adequately illustrated by the two types which have been described. There occur variations in detail. Thus the inequality in the activity of growth between the anterior and posterior portion_s of the amniotic fold so marked as a rule may be practically absent (Chameleons), or it may reach an extreme limit, the posterior

portion of the fold being obsolete and the anterior portion continuing

its backgrowth past the tail end of the embryonic body to form an amniotic tunnel, as in the Chelonians above described (Sphenodon, Gannet———S'ula, Puﬂin—Frate'rcula).‘

ALLAN'1‘oIs.———The allantois may also be conveniently studied in the Bird. In the Fowl it makes its ﬁrst appearance as a little clear

‘ Schauinsland , 1906. 472 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

vesicle, projecting from the ventral side of the trunk near its hind

Fm. 215.-—~—Diug1-ams illustrating the arrangement of amnion, allantois, etc., in the Fowl. (After Lillie, 1908.)

A, fourth da '; B, ninth day. (1.0, amniotic cavity; alb, albumen; all, allantoic cavity; u.ll.st, allantoic stalk; am, amnion; coal, eoelome ; f.a, false amnion, or serous membrane; sa, seroamniotic isthmus; spl,splanchnop1eure; v.m, vit.elline membrane; 3/, yolk; 1, outer wall of allantois fused with serous membrane; 2, inner_wa1lofa1lantois. VIII ALLANTOIS 473

end (Figs. -239, 240), and serving for the reception of the renal secretion. 'l‘he study of sections shows that the allantois is simply a pocket of the ventral wall of the gut towards its hind end--correr sponding exactly with the bladder of an Amphibian. It is thus lined with endoderm and covered externally with splanchnic mesoderm. The allantois like the bladder of the Amphibian bulges into the splanchnocoele. As development goes on the allantois, distended with ﬂuid, increases in size, projecting on the right or upper side of the embryo till it comes in contact with the inner surface of the soniatopleure (Fig. 215, A, all), and with still further growth ﬂattens

out against the somatopleure taking a somewhat 1r_1_ushljopin-likeU

shape. In the case of an independently living airiiiiial such as an

- illlilc. 2l5A.~—-Diagram illustrating the arrangement of amnion, allantois, etc., in the Fowl. (After Lillie, 1908.)

C, twelfth day. ulb, albumen ; all, allantoie cavity; 3;, yolk.

Amphibian the allantoie outgrowth of the gut can only increase in size within the restricted space of the splanchnocoele which is already occupied by the viscera. In the Bird embryo on the other hand there are available for the growth of the allantois the wide—spreading extensions of the coelome, on the one hand between amnion and serous membrane and on the other over the surface of the yolk. The allantois accordingly spreads out all round towards the limits of this space (Fig. 215, B). As it does so it loses its rounded vesicular form, its proximal (Fig. 215, B, 2) and distal walls (Fig. 215, B, 1) approaching one another. The mesoderm covering its outer surface tends to undergo secondary fusion with that of neighbouring structures. Thus about the end of- the sixth day it fuses with the adjacent surface of the amnion. Again towards the time of hatching a similar fusion takes place with part of the yolk-sac. The most important of these fusions 474 EMBRYOLOGY OF THE LOWER VERTEBRATES 011.

however is that, which commences early in the ﬁfth day, with the inner surface of the serous membrane. ' At a comparatively early period (during the ﬁfth day) the mesoderm covering the allantois becomes vascular and as the organ Recoines flattened its proximal or inner and its distal or miter walls ecome strikingly different as regards their vascular-ity, the outer wall developing an extremely rich network of capillary blood_-ves§_<_:_s with very small meshes, while the inner wall possesses merely a gparse network together--.witl1 the large vessels of supply. This difference between the two walls of the allantois becomes conspicuous about the end of the sixth day of incubation in the common Fowl. The difference is associated with the fact that the distal wall of the allantois is destined to become the great respiratory organ, taking over this function from the vascular area of the yolk-sac by which it is performed during the early stages of development. In correlation with the more efﬁcient performance of this function the albumen, or white, as it gradually shrinks in volume and acquires greater density gizavitates down to the lower side of the egg thus bringing the mushroom-shaped allantois close up to the shell membrane on the upper side. The process is still further facilitated by the ectoderm of the serous membrane becoming reduced to a very thin—-hardly distinguishable-—-layer in the region where it is underlain by, and fused with, the allantois. The capillary network thus comes into very close relation with the shell membrane and the overlying porous

' shell, and gaseous exchange can readily take place between the blood

circulating in the network and the external atmosphere.

As development goes on the respiratory needs of the embryo become greater and greater and these are met by the allantois spreading outwards all round its periphery, so as to provide a greater and greater respiratory area. During this spreading outwards of the allantois the three main alla11t(_;ic__ves_sel§ are somewhat retarded in their growth with the result that each one causes an indentation of the growing edge of the allantois beyond which the allantois bulges on each side.

When the growing edge of allantois comes, after about nine days’ incubation, into the neighbourhood of the remaining mass of albumen, a new phenomenon appears inasmuch as the allantoic margin with its covering of serous membrane proceeds to grow onwards close under the shell membrane as a circular fold recalling the amniotic

[old and enclosing the mass of albumen (Figs. 215, B, 215A, 0). l The"

ectodermal lining of the cavity so formed sprouts out into the albumen in the form of irregular projections which become vascularized from the allantoic mesoderm and no doubt play a part in absorbing the last remains of the albumen. By about the end of the second week of incubation the shell membrane is lined throughout the whole of its extent by the highly vascular outer wall of the allantois. This remains the breathing organ until-—a day or two before hatching—the y_gu_ng_chick_’s__l)eak vm DEVELOPMENTAL ADAi>rxrioy_s 475

/‘=r":“T _ , _ _ penetrates the air_-space and'\ ulmonary breathmg\ begins. The

-«up-w

all_ant9_i_c circiilation then gradua ffiecoines sluggish and stops, and eventually by a process of autotomy the allantois is separated from the body of the embryo and is left behind as the vascular membrane seen lining the fragments of ‘shell from which a young bird has hatched. '

ENCLOSURE OF YOLK—SAC WITHIN THE EMBRYONIO BODY.--—As already indicated thF3ﬁ)1K—:sac~heco'mes eventually (about a couple of days before hatching in the case of the common Fowl) ,e_nclosed within the body-wall. The process by which this is brought about appears to be as follows (H. Virchow). With the growth of the embryo a great increase takes place in the area over which the amnion is fused with the proximal wall of the allantois (cf. Fig. 215A, C), the compound and highly muscular membrane so formed extending eventually almost completely round the yolk-sac. At its edge it is continued onwards by the somatopleure, this latter terminating round the circular area where the yolk remains exposed. The yolk-sac is thus contained in a space the wall of which is formed of the following components in sequence starting from the body of the embryo : (1) amnion, (2) amnion fused with proximal wall of allantois, (3) proximal wall of allantois and (4) somatopleure in the region of the distal pole of the yolk-sac. The proximal portion of this wall, being formed of amnion, is necessarily continuous with the body-wall of the embryo at the umbilical opening and further those parts of it formed from amnion and allantois are highly muscular and contractile. l)uring the later stages of development this wall slowly contracts and as it does so the yo1k—sac is pushed into the umbilical opening which closes after it.

_._EV QRIGIN.- QE.S|l11E.A.Ml>IION .——-As regards this question,

which has excited much controversy, the following appears to the

present writer to be the working hypothesis which ﬁts most easily the facts so far as they are known.

(1) The amnion originated as a fold of l)l€t.'s't0d6’7'?Il« round the body of the embryo (Fig. 216, A, B).

As has already been shown the amnion arises in this way in ontogeny in the Reptilia which are generally recognized as being the most primitive Amniotes. The same holds for the Birds and for some of the Mammals.

The Mammalia as a group are admittedly descended from ancestors in which the egg was large and meroblastic as it is in the Reptilia. This is indicated, apart from other convincing evidence, by the fact that they still exhibit in ontogeny a well-d_eve_loped though yolkless

_Jt follows then that it iswiiiadniissilile to regard facts

derived from the study of certain mammals in which the mode of amnion formation during ontogeny is of a‘ different, even though apparently simpler, type as constituting important evidence in regard to the phylogenetic origin of the amnion, as has been done in particular by Hubrecht (1895). vm DEVELOPMENTAL ADAPTATIONS 475

’‘v’?.‘’‘__'_‘ .5 .-.-.4-..»—.~-..~. u-...—oc-no-‘Q

_ _»_M ,_ A _~_ V__ . penetrates the air-s ace and(g11lgngn,ary ,b(r_e,a_t_hi(ng) her ms. The

allantoic c1rculatioii”t en gradua y ec6i:11ss"lsmggisii*a;’iiH”st'bps, and

eventually._by“a process of autotomy the allantois is separated from the body of the embryo and is left behind as the vascular‘ membrane seen lining the fragments of ‘shell from which a young bird has hatched. ‘

ENCLOSURE or YOLK-SAC WITHIN THE EMBRYONIC BoDY.—~—As already indi.cated t1i?yo1k?sae"b“eeo‘nies eventually (about a couple of days before hatching in the case of the common Fowl) _e,ncl_gserl within__thye_”_(_bo‘dy;wal.l. The process by which this is brought about appears to be as follows (H. Virchow). With the growth of the embryo a great increase takes place in the area over which the amnion is fused with the proximal wall of the allantois (cf. Fig. 215A, 0), the compound and highly muscular membrane so formed extending eventually almost completely round the yolk-sac. At its edge it is continued onwards by the somatopleure, this latter terminating round the circular area where the yolk remains exposed. The yolk-sac is thus contained in a space the wall of which is formed of the following components in sequence starting from the body of the embryo : (1) amnion, (2) amnion fused with proximal wall of allantois, (3) proximal wall of allantois and (4) somatopleure in the region of the distal pole of the yolk-sac. The proximal portion of this wall, being formed of amnion, is necessarily continuous with the body-wall of the embryo at the umbilical opening and further those parts of it formed from amnion and allantois are highly muscular and contractile. During the later stages of development this wall slowly contracts and as it does so the yolk-sac is pushed into the umbilical opening which closes after it.

.,_E mQm:mJmAMmQ§mAs regards this question, which has excited much controversy, the following appears to the present writer to be the working hypothesis which ﬁts most easily the facts so far as they are known. .

('1) The amnion originated as a fold (pf blastoderm round the body of the embryo (Fig. 216, A, B).

As has already been shown the amnion arises in this way in gnatggeynypiri the Beptilia which are generally recognized as being the most primitive Amniotes. The same holds for the Birds and for some of the Mammals.

The Mammalia as a group are admittedly descended from ancestors in which the egg was large and meroblastic as it is ‘in the Reptilia. This is indicated, apart from other convincing evidence, by the fact that they still exhibit in ontogeny a well-dey_e_lgpe,,d‘_,thgugh yolkless

t follows then that it is inadmissible to regard facts

derived from the study of certain mammals in which the mode of amnion formation during ontogeny is of a‘ different, even though apparently simpler, type as constituting important evidence in regard to the phylogenetic origin of the amnion, as has been done in particular by (Hubrecht (1895). on. V11‘ ‘EV/OLUTION” OF AMNLION 477

(2) The amniotic fold consisted at ﬁrst of yolk-sac wall, the body of the embryo being forced down into the yolk-sac as it increased in size, possibly by the resistance of the rigid protective shell associated with the assumption of a terrestrial habit.

The tendency towards predominant development of the anterior portion of the amniotic fold may probably be correlated with the predominant growth and ventral llexure of the head end of the body which would cause it to dip down into the yolk-sac particularly markedly.

The delay in the appearance of mesoderm in the region of the proamnion may similarly have been originally due to the pressure of the downwardly ﬂexed head,

(3) The yolk-sac with its richly developed superﬁcial network of blood-vessels was the respiratory organ of the embryo at this early phase in the evolution of the Amniota. It follows that the portion of it nearest the shell, and therefore in the most favourable position for carrying on the breathing function, would tend to increasein area and would therefore bulge more and more over the body of the embryo (amniotic fold, Fig. 216, A, B, gt. f) so as eventually to utilize the whole of the inner surface of the shell.

(4) The egg being new terrestrial the excretory poisons produced by the activity of the already functional renal organs could no longer pass away by diffusion into the surrounding water. It would obviously be disastrous were they to accumulate in the space round the embryo and they therefore had to be retained within the body. This led to the great and precocious enlargement of the receptacle for these excretory poisons, already present in the pre-anmiote ancestor, the allantoic bladder.

(5) This precocious enlargement of the allantois in turn necessitated the early increase in size of the coelomic cavity to accommodate it.

(6) The allantoic wall-—-a part of the gut-wa1l——was naturally vascular, like the rest of the gut-wall, and with its great increase in size it would come in contact with the inner surface of the somatopleure. But as soon as it did this respiratory exchange would take place between its b1ood—-—through the substance of the somatopleure ——and the medium outside. The allantois would thus constitute a new, though at ﬁrst small, breathing organ.

(7) As the embryo grew its respiratory needs would grow also. Meanwhile of its two respiratory organs the one—~-the yolk-sac-— would be shrinking in size and therefore diminishing in efficiency while the other—the allantois———would be increasing in size as it became more and more distended. This would lead to the supplanting of the yolk-sac by the allantois as the main respiratory organ. As the allantois increased in size it would tend to extend in the position of greatest respiratory efliciency, 13.6. close under the somatopleure.

(8) With the development of the allantois and coelome the splanchnopleure would be freed from the somatopleure and the 4'78 llll\lliI'i-1r().I40(iiY OF THE LOWER VERTEBRATES OH.

upgrowth round the body of the embryo-—the amniotic fold- —would now become purely somatopleural (Fig. 216, C).

(9) As soon as the amniotic fold extended so far over the body of the embryo as to roof it in completely it would at once assume a new importance in protecting the delicate body of the embryo, enclosed wi.thin it as in a Water jacket, from the dangerous jars and shocks incidental to a terrestrial existence. In correlation with the importance of this function of the closed amnion we might expect to ﬁnd a tendency for its closure to be accelerated. As a matter of fact i.t will be found that in various mammals, including man, the amniotic cavity is closed from the beginning. ,

——-In many different groups of animals the embryonic phase of development is passed within the oviduct (uterus) of the mother. The advantages of this are obvious, for not only is the young individual sheltered to a great extent from the struggle for existence, as it is even Within an egg-shell, but it forms for the time being as it were part of the body of a complete adult individual with its full equipment for holding its own in the struggle. It is in the group Mammalia amongst Vertebrates that viviparity reaches its highest development, as the ﬁnal touch in their adaptation to a terrestrial existence, but it is of interest to notice that the phenomenon occurs, in a less highly elaborated form, here and ,.~,(,_ 21;_.---,.,-.H..._,__,.m.,“ Ur there amongst the lower Vertebrates-——Fishes, A(?(I../If/H'II..s‘Hi5llClOSl1lg‘ Amphibians, and Reptiles. t‘V"°«‘=’%"- (The ‘mi’ Thus amoncr the Elasmobranch ﬁshcsl there sious of the scale rc- _ ° . .

I_n.emt millimetres.) are numerous genera in which the early stages 1Il development are passed through in the uterus.

In such cases we ﬁnd in the ﬁrst place a well-marked tendency towards the reduction of the protective egg-envelopes which are no longer necessary. Thus there is found, as a rule, during early stages a typical set of egg-envelopes, but the horny shell is very thin and weak as compared with that of oviparous Elasmobranchs and as development goes on (embryo of 7-8 cm. in Acantluias) it becomes still

thinner, breaks up, and disappears.

A curious feature in such cases is the tendency for a group of eggs to be enclosed in a common set of envelopes instead of each egg having its own set. Thus in Acanthvlas (Fig. 217) there are commonly from two to six eggs enclosed in a common shell; in Trygonomvine two or three, in It/uinoliatus seven or eight.

The embryo within the uterus is still nourished primarily by the yolk in its yo1k_-sac. This primitive mode of nourishment has not

1 See Gudger, 1912.

IV. Vrvrmnrrr IN THE LOWER VERTEBRATES.Vin VIVIPARITY IN FISHES 479

yet been replaced by a process of Etbsorpiiloll from the uterine wall as is the ease in the Mammalia. But the uterine wall already plays a part though a minor one in providing food material for the young individual by its glandular activity. The beginnings of this are seen in the albuminous ﬂuid enclosed within the egg-shell, and it is possible that the elongated gill-ﬁlaments of the embryo play a part in absorbing nourishment from this. A further development consists in the secretion of an abundant “uterine milk ” which is drawn into the pharynx through the spiracles by precocionsly occurring movements like those of respiration and passed on into the digestive tract.

In accordance With its glandular activity the lining of the uterus frequently undergoes an increase of area by growing out into villi or trophonemata (Wood -Mason and Alcock, 1891). In the StingRays specially enlarged trophonemata may be drawn into the pharyngeal cavity of the embryo through its greatly dilated spiracles so that their secretion reaches the alimentary canal of the young ﬁsh directly (Fig. 218).

During the later stages of intrauterine development there usually comes about an intimate relationship between the surface of the yolk-sac and that of the uterine lining and in associ.ation with this there is found 3' Varying degree of Fro. 218. -—-Portion of uterus of Pterospecialization Of the uterine llI1- plataca. 7aiz:ru:ra slit open to show an ing (Ercolani, 1879; Widakowich, f"":?"3'r°.‘l":t')“. 1907). This latter may be smooth ,:,,l,p1(Ai1';,.,;|’{, 1g._;9i:)

(Squatina angelus, Notitlomus cine mas), or project into longitudinal folds so as to give increase of surface (Acantlvias oulgamls, .«S'c3/mnus lz'c}m'a), or grow out into papillae or trophonemata (Torjoeolo, Pteroplataea). ﬁnally it may develop folds which interlock with grooves on the surface of the yolk-sac, the uterine and yolk-sac surfaces being iii the most intimate contact so as to constitute physiologically a deﬁnite yolksac placenta (0'cm'chcm3as glcmcus, Mustelus laemls, etc.). _ p

Amongst the Teleostean ﬁshes viviparity occurs occasionally, in at least half-a-dozen different families; the Cyprinodontidae, Seorpaenidae and Embiotocidae furnishing the greatest number of cases. They are particularly numerous amongst the Embiotocidae and Scorpaenidae of the western coast of North America (Eigenmann, 1894). i 480 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

The eggs are retained in the ovary, either in the follicle, or in the cavity of the ovary; more rarely in the dilated oviduct or uterus. The developing embryo may depend for its nourishment upon the yolk (Scorpaenidae); it may absorb nourislnnent by the surface of the yolk—sae which grows out into villi (Anableps); or the nutritive secretion of the ovarian wall may be taken into the alimentary canal and there digested (Embiotocidae).

Among the Amphibia true viviparity is rare. A well-marked case occurs in Salmnanrlm atm (Wiedersheim, 1890). Here a large number (40-430) of eggs pass into the oviduct when breeding is about to take place but of these all except the one (in rare cases as many as four) next the eloaeal opening Simply break down forming a kind of broth which fills the oviducal cavity. The embryo nourishes itself, after it has used up its own yolk supply, by gulping down and digesting this ﬂuid, which contains not merely the yolky debris of disintegrated eggs, but also large, quantities of red blood corpuseles derived from extensive haemorrhages of the uterine wall.

Perhaps the most striking feature of the Mammalia is the ex't1‘e1ne degree of adaptation which they typically show to an intra-uterine mode of development in which the embryo leads a parasitic existence attached to the uterine lining of the mother. In accordance with this the external ectoderm of the blastocyst becomes modiﬁed to form organs of attachment which eventually, in the region of the yolk-sac and more especially in the region of the allantois, become vascularized and elaborated i11to the complex nutritive and respiratory organs named placentae. This being so, it becomes of much interest to enquire whether amongst those Amniota which are lowest in the scale of evolution——the Reptiles—— there are any foreshadowings of the type of adaptation to intra-ute.rine development found in the Mammalia. Probably numerous such cases exist but at the present time, with our extremely imperfect knowledge of Reptilian development, we are acquainted with only a-few. The most interesting of these is that of the Italian Lizard Ohalcides ttrlidactylus (Seps clmlcides). Giacominfs description of this (1891) may be said to form the foundation of what will one day probably form an important chapter in Vertebrate embryology.

The eggs, which measure about 3 mm. in diameter, are ﬁrst found in the oviducts early in May, while the first young are born towards the end of July, the period of gestation thus being between one and two months.1 The eggs become spaced out along the oviduct or uterus, so as to give it a moniliform appearance, each egg being arranged with its apical pole towards the mesometrium. At about the Iniddle'of gestation the “egg” presents the appearance shown in Fig. 219, A, the whole forming a kind of blastocyst about 7 mm. in diameter. The outer surface is formed by the ectoderm of the serous membrane. Within the serous membrane there can be seen the allantois with transparent, richly vascular, wall and the yolk 1 About sixty-ﬁve days according to Mingazzini. vm EMBRYONIC ADAPTATIONS IN REPTILES 481

szw, more opaque than the allantois and already much smaller than the latter as seen in surface view. The edges of the allantois and the mushroom-shaped yolk-sac ﬁt closely together and between them is the body of the embryo contained in the amnion. As in other Sauropsidans or Prototherian Mammals the yolk-sac lies on the embryo’s left, the allantois upon its right—-upon the side, in this case, next the mesometrium. As development proceeds the exposed area of yolk-sac becomes gradually reduced by the encroachment of the allantois. The latter however remains merely in contact with the edge of the yolk-sac and never comes to "surround it. Over the yolk-sac area there remain visible, for a long time the remains of the vitelline membrane (cf. Bird). Both allantoic and yolk-sac regions of the surface develop placental arrangements, the former being physiologically the more important of the two:

The allantoic placenta is already becoming apparent at the stage shown in Fig. 219, A, in the form of an elliptical area at the mesometrial pole which adheres to the uterine lining by means of numerous little projections which interlock with similar projections on a corresponding uterine area. As de- L up velopment goes on the egg assumes an elongated shape (Fig 219, The whole of FIG. 219.--“ Egg": of ()'[urlcides trzdactylus. the uterine lining in contact (After (‘”“’°"i“"" 1891')

with the outer surface of the A, 7 mm. in diaineter, showing yolk-sac (3/.3), allan “ n - - - . _ - tois (all), and foetal portion of allantoic placenta (pl); ego ls Provlded “nth 3' nch B, 15-16 mm. in longest.-diameter, seen from apical

O

capillary network C1033 pole,slm\\'in;.:I'«n-1:11portionufnllauitoic pla.centa(pI). beneath the uterine epithe lium and here and there insinuating itself between the epithelial cells. Over the allantoic placental area the maternal projections now form undulating ribbons attached along one edge and free along the other. On the surface of these ribbons the uterine epithelium instead of being ﬂattened as it is elsewhere is columnar and has a glandular appearance. With the r.il)ho11-like projections just mentioned there interlock the somewhat similar projections of the foetus. These are also covered with columnar epithelium close under which lies a rich capillary network. The latter is not conﬁned 482 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

to the actual placental projections for even the smooth parts of the surface over the allantoic area are provided with an extraordinarily rich network of capillaries which show an even more marked tendency than those of the uterus to penetrate into the epithelium. Over the smooth area the foetal and maternal surfaces are in intimate contact, so that the two capillary networks lie parallel and close to one another, separated only by two very thin epithelial membranes. In the region where foetal and maternal projections interlock chinks are apparent between the two in which there appear to be traces of a ﬂuid 1naterial—-—probably nutritive and secreted by the maternal epithelium which as already mentioned has in this region a glandular appearance.

The. yolk-sac placenta is less highly developed. In the region of the centre of the yolk-sac ﬂattened ridge-like projections also appear which interlock with corresponding uterine projections and become vascularized as the mesoderm spreads beneath them. Between the two surfaces is the remnant of vitelline membrane but this gradually disappears so that foetal and maternal surfaces come into intimate contact.

(7/utlcides ((}(m.gg/lus) ocellatus, another Italian lizard, is also

viviparous and in it occur similar though less marked adaptations to viviparity (Griacomini, 1906). Here in the later stages of gestation the general arrangement of the foetal envelopes resembles that in 0’. t9~'o'dwctg/lus. The allantoic region of the foetal surface is smooth and possesses a rich capillary network. It lies in immediate contact with the uterine lining, which in this region is covered with very thin ﬂattened epithelium overlying an extremely rich network of maternal capillaries.

.The portion of uterine lining in relation with the vitelline region of the foetal surface is less richly vascular, is covered with thicker epithelium of vacuolated cells with large nuclei, and is thrown into low folds which interlock with corresponding folds of the foetal surface so as to form an incipient yolk-sac placenta. The foetal epithelium of this region is thickened and in places columnar and appears to have an absorbent function. As in 0. tmldactg/lus remains of membrane are to be seen for a time between the foetal and maternal surfaces in this region.

To sum up, we ﬁnd in Chalcicles ocellatus a less advanced stage of adaptation to intra-uterine development than in C’. tmidactylus. Probably similar conditions will be found in various other viviparous Lizards as e._q. in the Australian Tmcltysaurus and T'il'iz1ua scincoides (0'ycloclus botidccerti) (Haacke, 1885).

In the Blind-worm (Angwis fm_q'il'is) and in the Viper (Vipem) and Smooth snake (C'oronella austrriaqa) viviparity also occurs but here in a still more deﬁnitely incipient form—a thin shell persisting throughout development and the foetal envelopes and uterine lining remaining practically unmodiﬁed.

Thus in the three sets of Reptiles above mentioned we see three steps in the evolution of viviparity: VIII EMBRYONIC ADAPTATIONS IN REPTILES 483

(1) the mere retention of the egg within the uterus, the shell still remaining and no intimate relations being developed between foetal and maternal tissues (Angwis, Vipera, Uoronella),

(2) the rupture at an early stage and eventual disappearance of the shell, and the coming into intimate relations of foetal and maternal tissues, both becoming highly vascular and there being an attempt at the formation of a yolk-sac placenta (C'halc7Ides ocellatus),

and (3) the development of an allantoic placenta (0. tridactylus).

LITERATURE

Agar. Proc. Zool. Soc. Lond., 1909.

Bartlett. Proc. Zool. Soc. Lond., 1896.

Bles. Trans. Roy. Soc. Edin., xli, 1905. ‘

Boulenger. Trans. Zool. Soc. Lond., xii, 1890.

Boulenger. Proc. Zool. Soc. Lond., 1895.

Brandes und Schoenichen. Abhandlungen Naturforscli. Gesell. Hallo, xxii, 1901. Brauer. Zool. Jahrb. (Anat.), xii, 1899.

Braus. SB. kgl. preuss. Akad. Wiss., 1906.

Budgett. Quart. Journ. Micr. Sci., xlii, 1899.

Dandy. Quart. Journ. Micr. Sci., xlii, 1899.

Eigenmann. Bull. U.S. Fish Commission, xii (1892), 1894.

Ercolani. Mem. Ac-cad. Bologna (Sez. Sci. Nat.), x 1879.

Espada, Jimenez de la.. An. Soc. Espafiola do Historia Natural, i, 1872. Giacomini. Monitore zoologico italiano, ii ; and Arch. ital. Biologic, xvi, 1891. Giacomini. Mem. Accad. Bologna (Scz. Sci. Nat.), Ser. 6, iii, 1906. Goeldi. Prov. Zool. Soc. Lond., 1895.

Gudger. Proc. Biol. Soc. Washington, xxv, 1912.

Giinther. Ann. Mag. Nat. Hist. (4), xvii, 1876.

Haacke. Zoo1.Anz. viii,1885.

Hirota. Journ. Coll: Sci. Tokyo, vi, 1894.

Hubrecht. Verh. Akad. Amsterdam, 2de Sect., 1895.

von Ihering. Ann. Mag. Nat. Hist. (5), xvii 1886.

Ikeda. Annot. Zool. Japonenses, i, 1897. ,

Kerr, Graham. Phil. Trans. Roy. Soc., B, cxcii, 1900.

Lebrun. La Cellule, vii, 1891.

Lillie. The Development of the Chick. New York, 1908.

Mitsukuri. Journ. Coll. Sci. Tokyo, iv 1891.

Plate. Vcrh. Deutsch. Zool. Ges., Kiel: 1897.

Schauinsland. Hertwig1sbHa.n(lbuch der Entwicklungslehre, I. Jena, 1906. Siedlecki. Biol, Centra latt, xxix, 1909.

Virchow, H. Inteiinat. Beitragti zur wiss. Medizin, I, 1891.

Weinland. Arch. . Anat. u. ’ iys. 185-1.

Wetzel. Arch. Entw. Mcch., xxvi,,1908.

Widakowich. Zeitschr. wiss. Zool., lxxxviii, 1907.

Wiedersheim. Arch. mikr. Anat., xxxvi, 1890.

Wintrebert. C. R. Soc. Biol. Paris, lxxii, lxxiii, 1912.

Wood-Mason and Alcock. Proc. Roy. Soc. Lond., xlix, 1891. CHAPTER IX

SOME OF THE GENERAL CONSIDERATIONS RELATING TO THE EMBRYOLOGY OF THE VERTEBRATA

IN the course of the preceding chapters many of the general principles of vertebrate embryology will have made themselves apparent: the

present chapter will deal shortly with some others of these principles which seem to require special notice.

(1) THE ONTOGENETIC EVOLUTION or THE ZYGOTE INTO THE COMPLETELY FORMED INDIVIDUAL.—Tl1e Vertebrate commences its individual existence as a zygote— a single ce1l——in which the speciﬁc characteristics, derived from the paternal and maternal ancestors, are already present though not recognizable. That this latter statement is accurate is demonstrated by such a fact as the following. The pelagic fertilized eggs of different species of Teleostean ﬁshes show no trace of the speciﬁc features which characterize the adults. Such distinguishing features as are present and enable a specialist to identify them are mere differences in size, amount of yolk, colour of oil globule and so on, and have nothing to do with adult characteristics. Yet if a selection of such eggs are allowed to develop together under a homogeneous set of environmental conditions each is found gradually to unfold the complete array of characteristics which distinguish its own kind. As the various zygotes have developed under the same identical set of environmental conditions it follows that the differences which gradually become apparent cannot be due to the moulding inﬂuence of external conditions: they must have been already present though in invisible form in the zygote.

It follows further that the evolution of the zygote into the adult is in the main not a process of acquiring greater and greater complexity, in the sense of acquiring new properties, but rather of the loca1iza_tion—the segregation—of special peculiarities in particular portions of the individual, so that these portions assume a, speciﬁc character and become recognizable as deﬁnite tissues or organs. The peculiarities were there to begin with, but they were diffuse and therefore unrecognizable—somewhat in the same way as the various

484 CH. Ix ONTOGENETIC EVOLUTION 485

colours of the spectrum are present in ordinary “ white” light but are invisible until they are sifted out from one another by the action of a prism.

The lesson learned from the developing pelagic zygote——that in its case the full equipment of the complete individual is provided from internal sources——-is one which should ever be borne in mind. It makes it- easier to realize that in other cases, where the developing organism exists in a less homogeneous environment and where it has to fend for itself, characters impressed upon it directly by the environment, however conspicuous, are still superﬁcial as compared with the really fundamental characters already present in the zygote.

The course of ontogenetic development from the zygote stage involves two main processes, (1) increase in bulk accompanied by the assumption of a multicellular condition, and (2) differentiation of parts 73.6. the segregation, into localized portions of the living substance, of peculiarities which were in the zygote distributed without deﬁnite arrangement. The topographical differentiation of the developing embryo does not necessarily keep exact pace with the subdivision into cells. Thus in Amplviomus the egg appears to be still homogeneous throughout up to the time when it has segmented into 8 or even 16 blastomeres for even at this stage a blastomere isolated from its neighbours experimentally may go on for a time pursuing the same course of development as it would have done were it a complete zygote. In other cases, as appears to be the rule in the Frog, the ﬁrst step in segregation—-the segregation of characters belonging to the right and left halves of the body into corresponding hemispheres of the egg—-would appear to take place in the zygote stage 7I.e. before the appearance of the ﬁrst cleavage furrow.

The progressive segregation of speciﬁc characters in the various parts of the developing individual is beautifully brought out in the case of various invertebrates by the elaborate studies on “ Cell lineage,” some of which have been fully described in Vol. I.

The animal individual lives its life under a particular set of environmental conditions, constituted by the external medium—— water or air—with its other living inhabitants: the latter play an important part, it may be by such comparatively simple and direct methods as by affecting the composition of the external medium, it may be by far more complex and obscure inﬂuences due to biological inter-relationships. The individual is able to go on living because of its organization and its living activities being ﬁtted, in the most intimate manner, into the particular set of conditions which constitute its environment.

So also with the various parts of the body—organs, tissues, individual cells —of the young developing individual. Each lives amidst an environment of extreme complexity and of perfectly deﬁnite type of complexity, conditioned by the nature of the body of which it forms a. portion and by the character of the parts of the 486 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

body which are in proximity to it. Its living substance is everywhere bathed by——and no doubt intimately adapted to life in--an internal medium, watery ﬂuid laden with the products of metabolism of the living substance as a whole. The differences in function of the various organs and tissues necessarily involve diﬁ“erences in their metabolic activity and therefore differences in the chemical nature of the contributions which they make to the complexity of the internal medium as Well as differences in the substances which they

. withdraw from the internal medium for their own needs.

Physiologists recognize that changes in the constitution of the internal medium play an important part in exciting and controlling vital activity.‘ In the ordinary life of the animal important examples of such inﬂuence are aiibrded by changes in the activity of the normal function of an organ, as for instance when the pancreas secretes actively in response to the presence in the internal medium of a special substance secreted by the intestinal wall when stimulated by food material. Other examples are afibrded by changes in growthaetivity-—as of the skin in response to a change in the amount of substance secreted by the thyroid, or of the mammary gland in response to the presence of substances produced by the metabolic activity of the foetus.

There is no reason to doubt that the living cells and tissues and organs of the embryo are similarly adapted to and inﬂuenced by the constitution of the internal medium, and if this be so the inﬂuence in question must play an important part i11 development. A possible example is aﬂbrded by the experimental result that ‘the grafting of the developing optic cup of an Amphibian embryo into near proximity to ectodermal tissue (such as the pigment-layer of the retina, the wall of the brain, the olfactory epithelium, the external ectoderm of the head or trunk) is apt to induce that ectoderm to develop into a lens (Bell, 1907). Such inﬂuence upon one portion of embryonic substance by another portion in its neighbourhood may well be exercised through chemical or other changes produced by the specific metabolism of the latter in the internal Inedium in its neighbourhood.

A corollary to the consideration outlined above, which has an important bearing upon much work in experimental embryology, is that it is unwise to place reliance upon the mode of development of an organ-rudiment being normal, unless its environment is normal.

(2) CELLULARIZATION or THE ZYGOTE: CELLULAR CONTINUITY AND DIsCoNTINU1'1‘Y.—-Protoplasm being a soft semi-ﬂuid substance a particle of it, as it increases in volume during the process of growth which is associated with normal metabolism, would soon reach a mechanically unstable condition, in which retention of its characteristic form, or even cohesion, would be impossible. In nature a corrective to this is provided by the protoplasm undergoing ﬁssion. In the Protozoa the products of this ﬁssion normally break apart and

‘ For their bearing upon evolutionary change see Parker (1909). IX ONTOGENETIC EVOLUTION: CELL-DIVISION 487

lead an independent existence, while in the Metazoa the subdivision is less complete and the growing mass of living substance continues to exist as a coherent individual. The physiological advantages of subdivision of the individual body into cellular units is apparent. It renders possible the intercellular deposit of rigid skeletal materials which act as a support to the organism as a whole: it facilitates localization of function and enables blocks of units specialized for particular functions to be transferred d11ring ontogeny to the positions in which they will be most useful: it enables other units to move hither and thither, either by their own activity or by being swept along in a circulating stream of ﬂuid, to wherever they are specially needed in the course of the ordinary vital processes: and it is of enormous importance in relation to attacks upon the organism from without, whether by limiting the area of injury to comparatively small tracts of living substance or by enabling portions of the living substance specialized for defence to be mobilized and ready to concentrate at the point of attack.

Modern science impresses upon us the. importance of regarding the individual not merely as an aggregation of cells and organs, but rather as a mass of living substance imperfectly subdivided up into cells and organs: imperfectly because each cell and each organ is inextricably linked up in the living activity of the whole individual. It brings to our notice numerous tissues in which the actual living substance of the constituent cells is linked up by intercellular bridges of protoplasm: it tells us of particular cases of developing embryos where similar intercellular continuity is apparent. The question is thus raised: are we correct in our belief that actual complete separation of cells takes place as a general rule when they undergo ﬁssion during ontogeny? More especially is it really the case that the individual blastomeres of the segmenting egg become completely separated from one another: is it not rather the case that the apparently complete separation is only apparent, that the individual blastomeres remain continuous through ﬁne protoplasmic bridges: and that cases of intercellular continuity observed in the adult are merely expressions of the fact that such bridges persist throughout the whole period of development ?

That the latter is really the case has been held by various workers and supported particularly strongly by Sedgwick (1895, 1896). It will however have been gathered by the reader from Chapter I. that such a view is in the opinion of the present writer not tenable. The fact that the blastomeres of a segmenting egg tend to take a spherical form, or at least to be bounded by convex surfaces, seems by the ordinary laws of surface tension to indicate that these blastomeres are not continuous with one another. Continuity of substance between the cells of the embryo or adult is therefore when it occurs a secondary and not a primary phenomenon. At the same time the present writer’s observations lead him to agree with Sedgwick that such intercellular 488 EMBRYOLOGY OF THE LOWER VERTEBRATES 011.

continuity ‘of protoplasm is much more widely spread than is generally recognized.

(3) YOLK. —- Theoretically the most primitive type of zygote should from the beginning be able to absorb food for itself. As an actual fact however the zygote is provisioned for a shorter or longer period by the highly nutritious fat and proteid, in the form of yolk which is stored up in its cytoplasm.‘ With increasing specialization the amount of this store becomes greater and greater so as to lengthen the period during which the young individual is provisioned and freed from the necessity of working for its own living. A good example of a high degree of such specialization amongst Vertebrates is afforded by the relatively huge egg of the Ostrich.

It has of course to be borne in mind that the degree of specialization in this direction is to be estimated not merely by the absolute amount of yolk present but still more by the relative amount of yolk in proportion to protoplasm. Thus two eggs may be described as equally richly yolked although very different in size provided that the proportion of yolk to protoplasm is similar in the two cases. In correlation with this we find that a group characterized by heavily yolked eggs may evolve in the direction of producing more and more numerous, and therefore necessarily smaller, eggs. Good examples of this are seen in the Teleostean ﬁshes where the eggs may be produced in enormous numbers and of very minute size although still retaining a proportionately large supply of yolk.

In C. Rabl’s discussions of his “ Theory of the Mesoderm ” (1889) an important place is taken by repeated losses and re-acquisitions of yolk during the phylogeny of the Vertebrata. Rabl arranges Cyclostomes, Elasmobranchs, Ganoids, Amphibians, meroblastic “ Prota1nniota” and Mammals, in a linear series, and concludes that Ganoids and Amphibians have undergone a diminution of yolk and have therefore reverted to the holoblastic condition; that meroblastic Protamniota have re-acquired a large amount of yolk and have therefore reverted to the meroblastic condition; and that ﬁnally Mammals have lost their yolk and again become holoblastic. In the opinion of the writer there is no suﬁicient justification for any one of these assumptions except the last. There is, as is well known, definite evidence to show that Mammals are descended from ancestors with large and heavily yolked eggs and that the small size and practically yolkless character of their holoblastic eggs are secondary acquirements. In this case the loss of yolk has brought in its train profound changes in the early processes of development but of such there are no signs in those other cases in which llabl supposes loss of yolk to have taken place. It must also be remembered that in the Mammal there is an obvious physiological reason for the loss of yolk——namely that the food material needed during the development of the embryo is provided from the tissues of the mother.

1 For a detailed account of the development of the yolk in the egg of one of the lower Vertebrates (Proteus) see J orgensen (1910). Ix YOLK 489

On the recapitulation hypothesis the segmentation and other early stages of ontogeny represent ancestral evolutionary stages common to all Vertebrates. The differences to be observed between such stages in different members of the group are consequently not to be looked on as ancestral but rather as due to the inﬂuence of disturbing secondary factors. Of these by far the most important is the presence of the particles of yolk, this dead substance clogging and retarding the living activity of the egg protoplasm. 'l‘he extent to which it does this in any particular region of the egg is roughly proportional to its relative amount as compared with the living protoplasm in that portion of the egg. The yolk is as a rule of higher speciﬁc gravity than the protoplasm. Correlated with this it

tends to be in proportionally greater amount in the lower parts of’

the egg than in the apical part, with the result that the processes of cell division and of development generally are relatively more slowed down in these lower portions———in extreme cases brought to a full stop——-by its retarding inﬂuence. Typical examples of this are seen in the holoblastic but unequally segmenting eggs of the ordinary Amphibia. In this case it is possible by replacing experimenta‘lly the action of gravity by a more potent force (by centrifugalizing the eggs) to concentrate the yolk still more than is natural iii the lower hemi_sphere with the result that the egg is now converted into a meroblastic one (0. Hertwig) the lower hemisphere being unable to segment. On the other hand by inverting the egg and so allowing the yolk granules to settle down towards the apical pole under the inﬂuence of gravity it is possible to cause the segmentation furrows to start from the abapical pole and spread towards the apical.

The influence of yolk upon the gastrulation process will have been realized from the perusal of Chapter II.: it is well illustrated by the series Amplmlowus (Fig. 18), Petromg/zon (Fig. 23), Rama (Fig. 25), Lepirlosiren (Fig. 21), II;/pogeoplzxis (Fig. 27) and Torpedo (Fig. 28). Put in a single sentence it may be said to consist above all in the gradual subordination of the process of invagination to those of overgrowth and delamination. In the succeeding stage it makes itself apparent more particularly in the modiﬁcation of the mode of origin of the mesoderm, the outgrowth of hollow enterocoelie pouches being replaced by the delamination of a solid mass or sheet.

The storage of yolk carries with it not merely the modiﬁcations just indicated in the processes of segmentation and_ gastrulation. Its inﬂuence becomes retrospective and affects even preceding stages during the growth of the intra-ovarian egg. This is shown more especially by the precocious concentration of yolk in that portion of the egg which will later become endodermal. Thus is the telolecithal condition brought about and telolecithality itself is seen to be really a foreshadowing of a particular adaptive feature of later stages of development (p. 183).

In examining sections of later stages of Vertebrate embryos in 490 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

which the eggs are rich in yolk it is readily seen that there are conspicuous differences between different parts of the e1nbryo’s tissues in regard to the yolk contained in their cells, for example endodermal structures are frequently marked out by larger yolk granules which cause them to stain more deeply with yolk-staining dyes. The condition of the yolk in a tissue may indeed give a useful hint as to the cell layer to which it belongs and as a matter of fact evidence of this kind has played a conspicuous part in many embryological discussions. '

It is important to bear in mind however the physiological signiﬁcance of the character in question. It appears to be closely related to the metabolic processes in the tissue concerned. As a given tissue in a yolky embryo goes on with its growth and development its yolk is gradually used up, a necessary preliminary being its breaking down into ﬁne particles easily assimilable. Tissues or cells undergoing active growth and multiplication have their yolk in this ﬁne-grained condition: those which are for the time being comparatively inert retain their yolk in a coarse-grained for“m. Thus a disturbing factor is introduced which has to be carefully borne in mind when using the character of the yolk as a criterion of the morphological nature of a given cell or tissue. '

A still further disturbing factor lies in the fact that while yolk is being used up and disappearing from view in one part of the body it may be deposited in cells elsewhere-—as for example takes place in eggs during their period of growth within the ovary. Such increase in the amount of yolk however, accompanied commonly by increase in the size of the individual granules, is naturally relatively rare in comparison with the breaking down of yolk which is occurring through the general tissues of the embryo. It follows that on the whole coarsely granular yolk in a cell or tissue affords more reliable evidence as to its nature than does ﬁne-grained yolk———which may be and usually is merely a symptom of active metabolism.

(4) RECAPITULATION.——'l‘he fascination as well as the philosophical interest of the study of Vertebrate embryology rests in great part upon the recapitulation of phylogenetic evolution during the development of the individual. In the early days of evolutionary embryology this idea was accepted in an unquestioning and uncritical spirit and it was supposed that all that had to be done to obtain a11 accurate‘ and fairly complete picture of the phylogenetic history of any particular animal was simply to work out its ontogenetic development. The more extensive knowledge which we have regarding embryological phenomena to-day serves on the one hand to conﬁrm fully the truth of the general principle and on the other hand to indicate how its working is interfered with by various disturbing factors.

The main controlling factor in ontogeny is the character of the adult. This is the motive power throughout the developmental period. Just as according to N ewton’s First Law a moving body tends to continue in Ia state of uniform motion in a straight line, so in Ix YOLK : RECAPITULATION 491

ontogeny the developing individual tends to progress constantly towards the goal of adult structure. Not in this case however necessarily by the straightest and shortest path. The structure of the adult is the expression of the action of Heredity. The earlier stages are not exempt from the same inﬂuence. Each step in the development of the ancestor tends to be repeated in the development of the descendant. The descendant then during its ontogeny tends to pursue the same, it may be devious, path as the ancestor. If in the course of generations the adult structure becomes shifted onwards in a process of evolution, this merely means the adding on of a new portion at the latter end of the ontogenetic path. The earlier portions of this path, built up of similar increments representing previous steps in evolutionary progress, are repeated as before, and so the complete process of individual development forms a record or recapitulation of phylogenetic history.

It cannot be too constantly borne in mind that the factor just indicated is the supreme factor in ontogenetic development. Other factors may be superﬁcially conspicuous, may have far-reaching inﬂuence upon details, but this factor—-the tendency to repeat ancestral steps in development up to and including the final characters "of the adult—is and must always be paramount.

Modern advances in knowledge of the facts of embryology, together with the assumption of a properly critical frame of mind, have shown, however, that the picture of past evolution afforded by the phenomena of individual development is at the best but a blurred and imperfect one, and that this must necessarily be so is readily realized when we remember that a large proportion of the characters of any organism are adaptive to its special mode of life. The circumstances under which a developing organism exists are, as a rule, widely different from those under which its ancestors proceeded along the evolutionary path, and in correlation with this its adaptive features are equally distinct. As we study the development of any species of animal we do not then see before us a complete and perfect picture of its evolutionary history, but merely gain ﬂeeting, and it may be misleading, glimpses through the obscuring_ clouds of adaptive features.

A further disturbing factor is indicated by the consideration that in past evolutionary history each stage in evolution was represented by a complete functional organism, all the parts of which were necessarily at correlated stages of development so as to form a functional whole. Many modern animals however develop under conditions in which the different systems of organs are no longer forced to keep accurate step with one another, and the result is that some lag behind while others, particularly organs of great histological complexity in the adu1t——-such as the brain or the eye——are accelerated in their early development, so as to give time for the complicated histogenetic processes that have to be completed before the organ can become functional. It will be realized that this latter type of 492 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

disturbance affects the development of the individual as a whole much more than it does its component organs, the result being that embryology frequently affords a much more perfect picture of the evolution of single organs than it does of the organism as a whole.

In reference to the ontogenetic record of phylogeny an interesting question presents itself regarding the reliability or otherwise of the information derived from the study of larval forms. To what extent may a particular type of larva be taken as probably representing a corresponding phase in the evolutionary history of the group: to what extent are its features to be regarded as ancestral, to what extent as mere modern adaptations to the environmental conditions among which the particular creature now pursues its individual development? In connexion with various groups among the Invertebrata larval forms have played a conspicuous part in phylogenetic speculation-—in some cases without due discrimination in interpreting their features as ancestral—the climax perhaps being reached by the view which regards such pelagic larvae as trochospheres or naup1ii——precocious1y developed and free-swimming heads without any trunk-—as representing ancestral forms (of. Graham Kerr, 1911).

In considering whether a particular stage of development is to be taken as probably repeating an ancestral stage of the adult special attention should be directed towards its mode of life, with the object of estimating the degree to which it diverges from the probable mode of life of the ancestral stage. If its mode of life is strikingly aberrant, ag. parasitic where the normal habit of the group is free-living, or pelagic where the normal habit is not pelagic, then we must always keep in mind the possibility or probability that its most conspicuous features are mere modern adaptations and are therefore worthless as evidence of ancestral conditions.

Again it should be considered whether in the main features of its organization it agrees with animals which are admittedly allied to it.

Larvae occur in the following Vertel;rates———Amp7z/ioasus, Petromg/zon, Crossopterygians, Ganoids, many Tcleosts, Lung-ﬁshes and the majority of Amphibians. Applying such criteria as are indicated above we should rule out as probably devoid of phylogenetic signiﬁcance the larva of A7nph'i0;L'u.s’ on account of its quite aberrant “pleuronectid” asymmetry (see Vol. I. Chap. XVII.). We should again rule out the Teleostean larvae on account of their extreme diversity. In Urodele Amphibians and Dipneumonic Lung-ﬁshes on the other hand we see larvae which appear to be distinctly of a common type. And in Crossopterygian and Actinopterygian Ganoids we again ﬁnd larvae which differ from these in detail rather than in fundamental characteristics. Consequently we should incline towards the view that the type of larva in question does not depart very

widely from the common ancestral type out of which existing Vertebrates have evolved. ' IX RECAPIT ULATION 493

Again in considering whether a particular feature of structure is to be regarded as ancestral or as a modern adaptation the following questions should be asked: (1) Is the feature peculiar to one group of Vertebrates or does it occur in several groups, and (2) if it occurs in several groups do the various animals possessing the peculiarity in question undergo their larval stages in similar sets of environmental conditions?

If the particular feature occurs in several groups derived from a common ancestral form this obviously increases the probability of the feature itself being ancestral. If however the several groups show similar sets of environmental conditions during their larval stages this introduces the element of doubt whether the similar features may not after all be merely adaptations to these similar sets of conditions.

Again it is important to make out whether the particular similarity has to do merely with parts of structure in direct functional relationship to external conditions. If there be deep-seated correspondences in structure with no such direct functional relationship to external factors then this gives greatly increased probability to these correspondences being truly ancestral in their nature.

The morphologist in trying to decipher the record of evolutionary history from the data of comparative anatomy or embryology is constantly impressed by the potency of nature’s economy of living substance. An organ no longer required may be eliminated within a very short period of evolutionary time. Thus in some species of Mackerel (Scombe7') so important an organ as the air-bladder has been eliminated: in various Frogs and Toads the external gills have been eliminated from development. Thus negative embryological evidence is of peculiarly little weight in relation to phylogenetic problems.

(5) THE PROTOSTOMA HYPOTHESIS.——Tl1iS is a working hypothesis which links together and in a sense explains a number of features in the early development of Vertebrates which are otherwise extremely puzzling. The more important of these features may be summarized as follows:

In Amplmloruus as has already been shown the dorsal side is at ﬁrst occupied by the widely open gastrular mouth. Later this becomes roofed in by a backgrowth of the gastrular rim anteriorly. A similar process of backgrowth appears to take place in the gastrulation of lower Vertebrates in general. The roof of the gastrular cavity formed by this process gives rise later not merely to the dorsal wall of the alimentary canal but also to notochord and central nervous system.

I. N ow occasionally there are appearances which suggest that this archenteric roof consists really of two lateral halves which have become fused together along the sagittal plane. Thus in Protopterus the down-growing dorsal lip of the blastopore is frequently indented by a median incision. Again in Urodeles the medullary plate is 494 , EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

frequently traversed by a ﬁne superﬁcial groove wh.ich passes forwards along the median line.

II. Then there occur curious cases of abnormality in which the dorsal region of the body is actually divided into two halves by a longitudinal split in the mesial plane. Thus Oscar Hertwig found

FIG. 220. -—Ahnorxnal embryos illustrating the Protostoma theory.

A, abnormal Frog embryo seen from the dorsal side; B, transverse section through hinder third of ditto (after 0. Hertwig, 1892); C, abnorma.1'I‘ront embryo (S. famio) in dorsal view (after Kopsch, 1899); D, ahnorxnal embryo of Pike (Esox lucius) (after Lereboullet, 1863). m. f, medullary fold ; m.s, mesoderm segment; mes, mesoderm; N, notochord; 0, opening leading down into enteron; ot, otocyst; 3/,'_1nass of yolk-cells.

(1892) that by fertilizing frog’s eggs which had become over-ripe, either by retention within the oviduct or by being kept for from one to four days in a moist chamber, he obtained a certain number of abnormal embryos of the type shown in Fig. 220, A, where a large expanse of yo1k—cells is visible in dorsal View instead of being completely covered in as would be the case normally. In transverse section (Fig. 220, B) such an embryo was found to have two half IX PROTOSTOMA THEORY 495

neural rudiments and two notochords of half the usual size, widely separated by the mass of yolk or endoderm.

Similar abnormalities have been observed in Teleostean ﬁshes. E.g. Fig. 220, D shows a Pike-embryo which is normal towards its anterior and posterior ends but interrupted for some distance in the mid-dorsal line by a Wide cleft in which the yolk is visible. Again in Fig. 220, C a similar cleft" is seen to traverse the whole length of a Trout-embryo from the hind-brain region tailwards.

An important feature of such abnormal embryos of ﬁsh and amphibians is that they frequently proceed with their development, the lips of the ﬁssure closing up and the two sets of half-organs being brought together in the mesial plane, undergoing complete fusion and the individual becoming in fact entirely normal. The importance of this return to the normal on the part of such split embryos is that it indicates that the departure from the normal during the split condition is far less fundamental than would appear at ﬁrst sight.

Here then we have to do with two very remarkable phenomena. Firstly there is the abnormality itself—-—the fact that the dorsal region of the body is for a time in the form of two distinct halves. Abnormalities of such a definite type as this usually have a deﬁnite evolutionary or other meaning and it is necessary to search for such a meaning in this particular case. Secondly, there is the fact that an embryo almost completely bisected in this way is frequently able to right itself and become perfectly normal. This again suggests the question whether this power of righting itself has not some special

- evolutionary meaning.

III. In the higher meroblastic Vertebrates we have seen that there exists along the middle line of what corresponds with the archcnteric roof of Avnplzxioccus (’i.8. the region which becomes converted into the dorsal part of the body, including notochord and medullary plate) the structure known as the primitive streak. We have also seen that in the lowest Vertebrates possessing it, this primitive streak represents the line of fusion of the gastrular lips, and that we are therefore justified in attaching the same signiﬁcance to the primitive streak in those higher forms in which the actual process of fusion can no longer be observed. That this interpretation is correct is indicated by the occasional occurrence of openings in the line of the primitive streak communicating ventrally with the enteron and dorsally with the outer surface of the medullary plate, or its derivative the ﬂoor of the neural tube (pp. 51, 53). Such neurenteric communications are readily explicable by the view that they represent simply parts of the line of fusion of the gastrular lips where the actual fusion has not been completed. Here again we have a phenomenon which demands explanation-——the occurrence of what seems to be the vestige of a slit-like gastrular mouth along the middorsal line.

IV. We have another remarkable body of facts associated with the fate of the blastopore or remnant of the gastrular mouth in 496 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

various groups of the animal kingdom; Thus within the limits of the groups Annelida or Mollusca the blastopore in some forms becomes the mouth, in others the anus. N 0 one would doubt for a moment that the mouth opening is homologous throughout these groups yet in one member of the group it can be traced back to the blastopore while in another member it is the anus which can be so traced. In other forms the gastrular mouth simply vanishes away during development and in some of these cases it assumes a curious elongated slit-like form along the mid-neural line before it disappears.

It is the merit of the Protostoma theory that it—and it alone—— affords an explanation of these four very different but equally puzzling bodies of facts. It falls therefore to be accepted by the Vertebrate embryologist as one of his working hypotheses.

The Protostoma theory is simply a special development of the theory of the evolution of the coelomate Metazoa which is generally accepted by morphologists, namely that the animals in question have passed, during the remoter parts of their evolutionary history, through a Protozoan and later a Coelenterate stage. The peculiarity of the Protostoma theoryis that it includes

"FIG. 221.-—~View of neural rudiment in embryo of within the coelenterate period

{;6’i'lZ}1(ltllu$’  Sedgwiclp,  ; '8.Il(l.  a Stage correspondillg in

Cpl, 081/F871;. I1 1e Case 0 epz. osrren le - - embryo is shown as it appears when straight- maln Structural matures Wlth

ened out. the Actinians of the present

time, characterized by the

presence of an elongated slit-like mouth, dilated somewhat at each

end and surrounded by a specially concentrated portion of the ecto dermal nerve plexus. The portion of the surface on which the slit-like mouth was situated was thus the neural surface.

Sedgwick (1884) was led to the idea by his studies on the development of Peripatus. He found in the species investigated by him a stage (see Fig. 221, A) in which the gastrula-mouth formed a long slit traversing the neural surface and surrounded by the ectodermal neural rudiment. As development went on the gastrular mouth or protostoma became obliterated, except in its dilated terminal portions, by fusion of its lips. The terminal parts remained open as mouth and anus respectively. The portions of nerve rudiment between the two openings became the ventral nerve cords while the portions in front of the mouth and behind the anus gave rise respectively to the IX _ PROTOSTOMA THEORY 497

supra-oesophageal ganglia and the suprareetal commissure. According to Sedgwick this stage in the development of Pervlpatus repeats the features of an Actinozoon-like Coelenterate ancestor, not merely of Pemlpatus, and therefore of Arthropods in general, but of such other groups as Annelids, Molluscs and Vertebrates.

It will be noted that on this protostoma hypothesis an important physiological distinction has at an early period of evolution marked off the Vertebrates from the other groups mentioned. This distinction came about with the acquisition of different habits of movement. In the stem which gave rise to Annelids, Arthropods, Molluscs, movement took place with the neural surface next the substratum (as in those modern Medusae which are able to creep on a solid surface-—e.g. Cladonerna), while in the Vertebrate stem on the other hand the neural surface was directed away from the solid substratum (as in the modern Actinian when it creeps). This difference in the position of the body in relation to the substratum would naturally lead in time to the different types of dorsiventrality so apparent in the fundamental organization of the two diverging stems. It is frequently stated by critics of the protostoma hypothesis that it involves a reversal of dorsal and ventral sides during the evolution of Vertebrates from their invertebrate ancestors but it will be gathered from what has been said that this criticism rests on a misunderstanding.

It will be readily seen that the protostoma hypothesis successfully explains the four categories of puzzling facts already enumerated. The paired appearance of the gastrular roof would be a reminiscence of the fact that originally it was actually paired: the split along the back of the abnormal embryos would mean the temporary re-appearance of the ancestral split or mouth: the primitive streak would be the scar along which the lips of this ancestral mouth or protostoma underwent fusion: and the converting of blastopore now into mouth now into anus would he an imperfect reminiscence of the fact that in phylogeny it gave rise to both.

On this hypothesis the various signs of a split along the neural surface of the vertebrate embryo, whether in the form of a dorsal furrow or a primitive streak or an actual opening, areinterpretable as reminiscences of the protostoma slit which traversed the neural surface of the Actinozoon-like ancestor.‘ It is of interest to notice that in two Vertebrates at least there exist what seem to be obvious traces of neural rudiment extending round behind the anal part of the protostoma precisely as in Peripatus. In Fig. 221, B, is shown an embryo of Lepidosiren spread out in one plane, with the neural rudiment in the form of a ridge which is continuous behind the blastopore or anus. If it be reﬂected that this opening may be

1 That the primitive streak and primitive groove are closely related to the gastrula mouth was perceived by Rauber (1877) but a. clear evolutionar explanation of this relationship was ﬁrst given by the protostoma theory of Sedgu 1c (1884) and Hertwig (1892)

VOL. 11 2 K 498 EMBRYOLOGY OF THE LOWER VERTEBRATES en.

regarded as being continued forwards by a potential slit, represented eg. by the primitive streak of other forms, it will be realized how close is the resemblance to the conditions in Peripwtus. The pre-anal portions of the neural rudiment in Lepidosiren come together in the mesial plane to form the spinal cord, while the postanal portion ﬂattens out and disappears so that theanal opening comes to lie entirely behind the posterior limit of the central nervous system. It is clear that if the development of the anal opening were delayed until the neural folds had already come together it would make its appearance completely behind the central nervous rudiment and with no obvious connexion with it. This is very possibly the case in Vertebrates other than those mentioned. '

Although the anal opening of Vertebrates is thus brought into the relations with the nervous system that we should expect on the protostoma hypothesis there is no such deﬁnite evidence in the case of the mouth. It is true that in some cases the dorsal furrow has been traced to the neighbourhood of the mouth and that the mouth opening has in some cases at first the form of a sagittally placed slit, but in no ease, up to the present, has the neural rudiment been traced round in front of the mouth. This difficulty however is greatly lessened when We correlate the facts just mentioned in regard to the anal opening in Lepidcisiren with the relatively late appearance of the mouth opening of Vertebrates as discussed on p. 193. It may well be that the non-inclusion of the mouth opening within the obvious neural rudiment is due simply to the pre-oral parts of the medullary folds having already ﬂattened out and disappeared before the oral opening makes its appearance. If this is the case it carries with it the interesting consequence that the supra-oesophageal or pre-oral ganglia of I’emIpatus have disappeared in the Vertebrate and it is therefore waste of energy to discuss what parts of the brain of a Vertebrate are homologous with the supraoesophageal ganglia of Invertebrates.

This Protostoma idea, dealing as it does with extremely remote phases of the Vertebrate phylogeny, must not be looked on as a deﬁnitely proved theory, nor can it be expected ever to reach that dignity, but it is a fascinating working hypothesis which serves, and which alone serves, to link together and in a sense explain a considerable body of otherwise mysterious and apparently inexplicable facts of Vertebrate embryology.‘

(6) THE VERTEBRATE HEAD.-—-The two phyla of the animal kingdom which have reached the highest stage of evolutionary deve1opment—the Arthropoda and the Vertebrata——are alike characterized by the possession of a well-developed head. In the

1 In considering the diﬂiculties in the way of the theory afforded by cases where the gastrula becomes roofed in by a process of simple backgrowth without any trace

of protostoma. (e.g. Amphiomus), it is well to bear in mind the parallel case of the

amnion-——of which a large portion may be formed by simple backgrowtb, although the sero-amniotic isthmus and the ingrowth of mesoderm from the two sides seem to point clearly to a former formation by the meeting of two lateral folds. IX THE VERTEBRATE HEAD 499

evolution of a head we may take it that the principal factors involved are probably the following:

(1) The habit of active movement in a direction corresponding with the prolongation of the axis of the body,

(2) The concentration of organs of special sense towards the end of the body which is in front during movement,

(3) The concentration of nerve centres to form a brain in proximity to these organs of sense.

In the case of the Vertebrate the brain has reached a comparatively large size and in correlation with this the protecting skeleton has become highly developed and has lost the ﬂexibility which is characteristic of it in the trunk. Further in the Vertebrate the walls of the buccal cavity and pharynx have become highly

specialized, particularly in the matter of their skeleton, in relation‘

to the functions of ingestion and mastication of the food on the one hand, and of respiration on the other.

Each of these various factors involves structural change, not affecting merely one organ but causing modiﬁcation of the whole complex arrangements of the head-region. Thus associated with the loss of ﬂexibility we ﬁnd (1) loss of segmentation of the skeleton, (2) disappearance or great modiﬁcation of the myotomes, (3) corresponding changes in the nerves supplying these myotomes and (4) disappearance of the coelomic cavities.

The full appreciation of the importance of this feature of the Vertebrata makes it, in the present writer's opinion, impossible to doubt that the possession of a deﬁnite head is a feature that has come down from the unknown ancestral form from which the Vertebrate stock has evolved. If this be correct it follows that the relatively feeble differentiation of the head end of the body seen in Amplmioams is to be regarded as a secondary condition, correlated with the peculiar mode of life of this animal, and devoid of phylogenetic signiﬁcance.

It has already been pointed out that organs of great complexity in the adult tend to be laid down at an early stage of individual development, time being thus obtained for the development of their complex detail. It is perhaps in direct relation to this principle that the highly complex head - region of the Vertebrate, which comes to assume control over most of the activities of the individual, develops particularly early in ontogeny ——the Various developmental processes making themselves as a rule ﬁrst apparent in the head region and spreading thence tailwards along the trunk. This fact is of practical importance to the embryologist for in the case of segmentally repeated organs it enables him to ﬁnd a series of developmental stages within the body of a single embryo.

Though this precocious cephalization is a marked feature of

Vertebrate ontogeny it never goes within this phylum to the length it does amongst certain Invertebrates where the larval stage 500 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

(N auplius, Trochosphere) is practically a precociously developed and free-living head which has not yet developed a trunk.

As will have been gathered, one of the most conspicuous features of the head-region is the loss of segmentation in organs in which it was once present.

Metamerie segmentation, which ﬁrst makes its appearance in typical form in the Annelida, is probably to be associated primarily with the coelome and its lining the mesoderm. The coelome is distended with coelomic ﬂuid and the turgidity so caused gives ﬁrmness to the body. The physiological advantage of the coelomic cavity being subdivided into successive compartments is obvious. The segmentation of other organs is to be looked on as secondary to that of the mesoderm, and more especially to that of the muscles. Thus the segmented character of the nervous system of an Annelid or Arthropod is due to the ganglion-cells tending to become concentrated at the level of the masses of muscle which work the parapodia or limbs. So also the segmentation of the skeleton which permits flexure of the body is correlated directly with the segmentation of the musculature which causes that ﬁexure.

So, conversely, with the disappearance of segmentation in the head of the Vertebrate. Correlated with the loss of ﬂexibility in the brain region the myotomes which produce the flexure have disappeared, and correlated with this in turn the ensheathing skeleton has lost its segmentation and the segmentally arranged motor nerves have also gone. The process has taken place from before backwards. It has been carried to the greatest extent in front, to the least at the hinder limit of the head.

It is deﬁnitely established that the head of the Vertebrate has at least in part come into being by the modiﬁcation of what was once the anterior portion of the trunk. With the gradual evolution and increase in size of the brain—-so characteristic of the phylum Vertebrata—this organ has gradually encroached upon the spinal cord, and its protective skeleton the chondrocranium has pare} passu encroached upon the vertebral column. This is clearly indicated by the fact that included within the limits of the skull are nerves which are serially homologous with those of the trunk. Putting on one side the probability-as many would regard it— that cranial nerves III, IV, V, VI, VII, IX and X are really homologous with the spinal nerves, we ﬁnd behind the Vagus a series of spino-occipital nerves (Fiirbringer, 1897), which although included within the limits of the skull are yet undoubtedly members of the same series as the spinal nerves. The number of those is very different in the different subdivisions of the Vertebrata as may be gathered from aninspection of Fig. 222. In all probability they will be found also to show considerable variation in different individuals of the same species.

During the evolution of the head there is some reason to believe adult stage of modern Cyclostomes: it is also seen in the young Lepidosiren of stage 34 (see Fig. 154, B, p. 309).

The next phase is seen in the adults of such relatively primitive groups as the Elasmobranchs, the dipneumonic Lungﬁshes and the Amphibians, in which an occipital region has been added on to the palaeocranium.

ii jjj =...‘!....'I.2_IL_A_Il_'«’..1L._$_1L_1.JL_§_iL_9._JL_7_|i_!3_i

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FIG. 222.-—-Diagram illustrating the relations of the binder limit of the cranium in an Elasmobranch (A), Lepidosiren (B), Polypterus (C), Acipenser (D), an Amphibian (E), and a Reptile (F), as seen from the left side.

The cranial floor is indicated by the broad horizontal black band : it is demarcated from the vertebral column by the vertical band which represents the occipital limit of the cranium. Dorsal and ventral nerve roots are shown as black dots, when transitory as rings, when occurring only occasionally as dotted rings. The myotomes are indicated by rectangular outlines. The Spinooccipital myotomes are lettered according to Furbringer's system—the anterior batch (occipital) with the concluding letters_of the alphabet, the posterior batch (occipito-spinal) with the commencing letters (A, B, C’). Trunk myotomes not yet incorporated in the head are designated by numbers.

Ix THE VERTEBRATE HEAD 501 that its extension backwards has taken place by successive steps. In the most ancient recognizable stage the cranium (Palaeocranium ——Fiirbringer) extended no farther back than the vagus nerve. This phase is represented--either persistent or revertive——in the 502 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Finally the hinder limit in the other Vertebrates has been shifted still farther back—one segment (Poly/ptems), three segments (Amniota) or as many as ﬁve segments (Acipenser).

As far forwards as the hinder limit of the palaeocranium there is, as already indicated (p. 317), clear evidence that the cranial wall represents a series of neural arches which have undergone fusion. As indicated on the same page it is difficult to avoid extending this homology to the mesotic portion of cranial wall lying still farther forwards. As regards the prechordal portion of the cranium there is no deﬁnite evidence, but if we regard the trabeculae as primitively in continuity with the parachordals we have to grant the possibility of even this part of the cranium being in series with

' the portions farther back and therefore also originally vertebral in

constitution.

In conclusion it must be remembered that the series of myotomes is also continued into the head-region, and the occurrence of typical myotomes as far forwards as the premandibular or oculomotor segment (p. 210) may be taken as strong evidence that the segmentation of the mesoderm originally extended throughout the head-region including its pre-chordal portion.

(7) EMBRYOLOGY AND THE EVOLUTION or THE VERTEBRATE.—— The special charm and the chief importance of the study of embryology reside in the fact that it is one of the main branches of evolutionary science. The greater part of what is ordinarily called evolutionary research deals with the possible methods and causes of evolutionary change. '.l‘he data of Embryology on the other hand form a branch of synthetic evolutionary science which deals not with possible causes or methods but with the actual facts of evolutionary change, striving to map out the course along which this has proceeded. ln compiling the record of evolutionary progress we are dependent upon Comparative Anatomy and Palaeontology as well as Embryology, and in formulating conclusions care has to be taken that whenever possible they are based on the data of all three sciences. In cases where these data are not in agreement care must be taken to bear in mind the main disturbing factors which are liable to invalidate the conclusions in each case. In reasoning from Embryology and Comparative Anatomy the possibility that particular features are modern adaptations to existence say within a uterus or egg-shell or under any other set of conditions different from those of the ancestor has to be borne in mind. In the case of Palaeontology and Comparative Anatomy there exists the same danger of error as besets the protozoologist when he endeavours to construct a continuous life-history out of a number of isolated observations on the dead animal—the error of arranging observations in a series which is not natural, or on the other hand, if the seriation be done correctly, of reversing its direction. In Palaeontology errors of this type are peculiarly apt to arise on account of the extraordinary imperfection of our knowledge. If a- series of organisms a, b, c, d, IX EMBRYOLOGY AND EVOLUTION 503

become known from a series of geological deposits A, B, 0, D, this affords convincing evidence in most cases that the particular organisms lived at the time the particular deposits were laid down: the conclusion may also be fairly justifiable that not only did they exist but that they were abundant at the period in question. The conclusion however which is so apt to be drawn that w, b, c, d, actually made their ﬁrst appearance in the same order as the deposits A, B, 0, 1), is quite unreliable. They may have existed in smaller numbers for immense periods of time before the periods corresponding to A, B, 0’, 1), when they were really abundant, and the order of their ﬁrst appearance may have been d, c, b, a, or any other. Such a geological series is in fact in itself of little value as an index to the order of evolution. In Embryology on the other hand where the evolutionary stages occur as part of a continuous process, each dependent upon its predecessor, we appear to be safe in assuming that the record, however incomplete, is at least arranged in proper sequence.

Another principle to be borne in mind, when the attempt is being made to work out the evolutionary history of a particular group‘ is that conclusions must be based upon broad knowledge of structure as a whole. No implicit reliance must be placed upon evidence relating to one system of organs unless it is corroborated by the evidence of other organs. Failing this precaution the investigator is liable to the pitfall afforded by convergent evolution of organs of similar function. Here again the palaeontologist ﬁnds himself much hampered as compared with the embryologist, for as a rule all evidence except that of the skeletal system has passed completely beyond his ken.

EVOLUTIONARY ORIGIN or THE VERTEBRATA.——In the preceding portions of this book it has been shown that Embryology provides us with a record in at least its main outlines——ot' the evolutionary changes which the various organ-systems have undergone within the group Vertebrata. For, amongst others, the reasons stated at the foot of p. 491 the record is less clear regarding the evolutionary history of the complete individual. Even however if we had this record complete for the various types—Fish, Amphibian, Reptile, Bird—we should ﬁnd ourselves still confronted with the interesting problem of the first origin of the primitive Vertebrate type :—from whence came these lowly original Vertebrates out of which the various existing types of Vertebrate have been evolved?

This problem of the ancestry of the Vertebrata is naturally a fascinating one and it has attracted much attention and been the theme of voluminous writing. Enthusiasts have at different times endeavoured to demonstrate that the Vertebrates are descended from this phylum or from that. It is perhaps best not to take such attempts very seriously. They have served a useful purpose in arousing interest and stimulating research but they have little claim to a place in the permanent literature of Zoology. «

We are naturally unable to get any evidence bearing upon the 504 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

problem from Palaeontology. The most ancient Vertebrates of which fossil remains are known had probably already evolved to a far greater distance from the original type of Vertebrate than that which separates them from the existing Vertebrates of to-day. And the probability is that the earliest Vertebrates went on existing and evolving through long ages before they developed those complex skeletal structures which are alone adapted for preservation as fossils in the geological record. Comparative Anatomy fails us too——-for up to the present no existing type of animal has been discovered which can justiﬁably be interpreted as an unmodiﬁed survivor of the original Vertebrates.

It is Embryology alone which yields us examples of Vertebrates in the earliest stages of evolution but the data afforded by that science do not carry us beyond the formulation of a few very broad and general conclusions regarding the prevertebrate phases in the evolutionary history of the phylum.

(1) The fact that Vertebrates, like other Metazoa, commence their existence as a unicellular zygote appears to justify us in postulating a unicellular tie. a Protozoan ancestral stage.

(2) The fact that there occurs in the admittedly more primitive Vertebrates a gastrula stage appears to justify us in postulating a diploblastic or Uoelenterate ancestral stage.

(3) The facts which are united together in the Protostoma hypothesis suggest that the coelenterate ancestor evolved along lines somewhat similar to those of the modern Sea—anemones with their elongated slit-like protostoina dilated at each end and surrounded by a concentration of the ectodermal nerve-plexus.

(4) The facts that the coelome was probably originally segmented (as indicated by Amphtowus), that the excretory organs are in the form of nephridial tubes, that the vascular system consists fundamentally of longitudinal vessels on opposite sides of the alimentary canal connected together by vascular arches, the blood passing tailwards in the vessel on the neural side of the alimentary canal—— suggest that there intervened between the coelenterate phase and the vertebrate phase a stage which possessed many features in common with those animals which are grouped together to-day in the phylum Annelida. We may suppose that this annelid-like creature became evolved from an Anemone in which the body had become drawn out, as in the genus Herrpolitha or one of the brain corals, and which had become actively motile. In the two diverging stems which gave rise to Annelids and to Vertebrates respectively we may take it that a difference existed in the normal position of the body—-—the former progressing with their neural, the latter with their abneural, surface underneath. It is conceivable that this difference may have been associated with the difference between a creeping mode of life in which the chief sensory impressions were related to the solid substratum and a swimming mode of life in which they rather came from above. Ix GERM LAYER THEORY 505

ADDENDUM TO CHAPTER IX.——More than once in the course of this volume reference has been made to the “ Theory of Germinal Layers” or the “Germ Layer Theory.” This theory, which has played a great part in the development of embryological science in the past and still dominates to a great extent embryological research, had its foundations in observations made by these pioneers of embryological science — Wolff, Pander, von Baer and Remak. Wolff (1768) observed that the alimentary canal in the Bird embryo is developed out of a thin membrane or leaf (“ Blatt”) and inferred that the other organs go through a similar stage. Pander (1817) gave the name “ blastoderm ” to the ﬁrst membrane-like stage of the embryo as a whole, saw how this became differentiated into the three layers-——outer, middle and inner——and traced out the development from _these of the main organ-systems. Von Baer (1828) carried on and elaborated l’ander’s work, recognized that the middle layer was double, and that it was secondary to the two primary layers: the outer and the inner. He also extended his observations to forms other than the Fowl and laid the foundations of Comparative Embryology. Remak (1855) ﬁnally worked out the germ-layers in terms of the Cell-theory, traced the origin of the coelome to a split in the middle layer, and worked out more precisely the relations of the layers to the deﬁnitive organ-systems.

One of the most important steps in the development of the Germ Layer Theory was made by Huxley (1859) who as a result of his researches upon the Medusae recognized the two primary cell-layers in these animals (named by Allman “ eetoderm ” and “ endoderm ”) and suggested the comparison of them with the two primary layers of the Vertebrate embryo.

Embryology, like Morphology in general, ﬁrst became a real living science as a result of Darwin’s demonstration of the fact of evolution. In the Omlgivt Qf Species (1859) the principle of recapitulation is already admitted. “Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals.” The idea was further elaborated by Fritz Muller (1864).

Kowalevsky (1871, etc.) and other embryologists had demonstrated the wide-spread occurrence among the Invertebrates of an early stage of development more or less cup-shaped in form and consisting only of the two primary cell-layers, and the important advance was made synchronously by Lankester and Haeckel of perceiving in this two-layered stage a repetition of a common ancestral form.

Lankester (1873) recognized amongst the Metazoa two distinct grades of complexity of structure so far as their cell-layers were concerned—the diploblastic grade (represented by the Coelenterate) consisting of the two primary layers, and the triploblastic grade with an interposed middle layer. Further he recognized that each Metazoon—whatever its deﬁnitive condition—passes in the course 506 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

of development through a diploblastic stage which he termed the planula. Such a planula. stage he regarded as a repetition of a common ancestral stage of evolution.

Haeckel (1872) about the same time as Lankester also developed the idea that the diploblastic stage of ontogeny was to be interpreted as the repetition of an ancestral form: Haeckel called this ancestral form Gastraea. The main difference between Haeckel’s view and Lankester’s was that the former regarded the endoderm as having arisen by a process of invagination——-as it actually does arise in ontogeny in the great majority of cases—-—while Lankester regarded it as having arisen by a process of delamination from the outer la er.

y As regards the middle germ-layer ideas remained somewhat vague until Agassiz (1864) showed that in the Starﬁsh the mesoderm arose in the form of an outgrowth of the archenteric wall. The same was found to be the case in various other Invertebrates, and in 1877 Kowalevsky showed how in Amp}:/iowus the mesoderm was during an early stage in the form of archenteric pockets. In the same year Lapkester developed the generalization that the coelome is to be regarded as uniformly enterocoelic in origin and comparable with the diverticula of the archenteric lining seen in Coelenterata.

The separation of such mesodernial cells as are in their early stages free and amoeboid under the common name mesenchyme was ﬁrst made by O. and R. Hertwig (1882).

The later developments of the theory of the mesoderm involved in the Protostoma theory have already been alluded to earlier in this volume and the same applies to what the author regards as the chief qualiﬁcation of the germ-layer theory indicated by modern work, namely that the boundary between two layers where they are continued into one another must be regarded not as a sharply marked line but as a more or less broad debatable zone.

LITERATURE

Agassiz. Contributions to the Natural History of the United States of America, v. Boston, 1864. [Vol. v printed as vol. v, pt. 1, of Mem. Mus. Comp. Zoology H8l'Va.1‘d.] _

Baer. Uber die Entwickclungsgeschichte der Thiere. Beobachtung und Reﬂexion, i. Konigsbcrg, 1828.

Bell. Arch. Entwick. Mechanik, xxiii, 1907.

Piirbringer. Gegenbaurs Festschrift. Leipzig, 1897.

Haeckel. Die Kalkschwamme. Berlin, 1872.

Hertwig, 0. Arch. mikr. Anat., xxxix, 1892.

Hertwig, 0. and R. J enaische Zeitschrift, xv, 1882.

Jérgensen. R. Hertwigs Festschrift. Jena, 1910.

Kerr, Graham. Proc. Roy. Phys. Soc. Edin., xviii, 1912.

Kopsch. Internat. Monatsschr. Anat. u. Phys., xvi, 1899.

Kowalevsky. Mém. Acad. Sci. St-Pétersbourg, (7), xvi, 1871.

Kowalevsky. Arch. mikr. Anat., xiii, 1877.

Lankester. Ann. Mag. Nat. Hist., (4), xi, 1873.

Lankoster. Quart. Journ. Micr. Sci., xvii, 1877.

Lereboullet. Ann. Sci. Nat., 4, Zool., xx, 1863. IX GERM LAYER THEORY 507

Mﬁller. Fiir Darwin. Leipzig, 1864. ‘ Pander. Beitrzige zur Entwickelungsgeschichte des Hiihnchens im Eye. Wiirzburg, 1817.

Parker. American Natltralist, xliii, 1909.

Rabl. Morph. Ja.hrb., xv, 1889.

Rauber. Priniitivstreifcn und Neurula. der Wi1'bc1thie1'e. Leipzig, 1877.

Remak. Uxltersuchuugen ulwr die Entwicke-lung der \Virbe1thiere. Berlin, 1855. Sedgwick. Q,11a.rt. Journ. Mic-r. Sui., xxiv, 1884.

Sedgwick. Quzwt. Jmxrn. Micr. Sci., xxxvii and xxxviii, 1895 and 1896.

Wolff. Nov. Conmwnt. Acad. Sci. St-I’éte1'sl>o1u'g. xii, 1768. CHAPTER X

THE PRACTICAL STUDY OF THE EMBRYOLOGY OF THE COMMON FOWL

FOR gaining practical experience in the study of embryology there is’ no type of material so convenient as that of the early stages in the development of the Common Fowl. Freshly laid eggs can be obtained practically anywhere and to obtain the various stages of development all that is necessary 1 is to keep the eggs at a suitable temperature (about 38° C.) either under a sitting hen, or in one of the incubators which can be purchased, or even in a simple water-jacketed even such as can be made by any tinsmith. If an incubator be purchased it will be provided with a proper heat regulator for use with electricity, gas or oil, while with the most primitive water-bath it is possible to arrange a lamp so as to give a temperature sufliciently constant as to carry the eggs through at least the ﬁrst few days of incubation—the most important period for purposes of study. Bird embryos—apart from their use in learning practical embryology-— provide admirable material for giving practice in the ordinary methods of section-cutting which are in such constant use in Zoology, Anatomy, Physiology, and Pathology. This chapter will then be devoted to giving an account of the development of the Fowl with directions as to the technique involved in its practical study.

In the description which follows the developmental phenomena will be described in their natural sequence but on account of the practical difficulties involved in the extraction and preservation of blastoderms of the ﬁrst day of incubation it will be found best, in actual laboratory work, after studying the new-laid egg and its envelopes, to proceed to the stage of about 42 hours’ incubation and gain some practice in the manipulation of it before attempting the earlier stages. In the following technical instructions the sequence is followed which has been found to be in practice most convenient for beginners.

TECHNICAL DIRECTIONS 2

I. NEW-LAID EGG.——Fil1 a glass vessel about 4% inches in diameter and 2 inches in depth With normal salt solution [Water

1 Provided the eggs have been fertilized. 9 The reader is assumed to have an elementary knowledge of the ordinary methods of cutting sections. See, however, the Appendix.’

508 CH.X PRACTICAL EMBRYOLOGY OF THE FOWL 509

100 c.c., common salt '7 5 gramme] heated to a temperature of about 40° C. Submerge the egg upon its side in the salt solution and remove the side of the shell which is uppermost by cutting with a pair of strong scissors and then lifting off the isolated piece of shell with blunt forceps. Take care to keep the point of the scissors or forceps close to the inner surface of the °shell so as to avoid risk of injury to the true egg or “ yolk.”

II. EGG AFTER 42 HOURS’ INCUBATION. —— Open the egg as before. On removing the piece of shell the blastodcrm will be seen as a circular whitish area on the upper side of the yolk. Excise the blastodcrm by making a series of rapid cuts with the large scissors through the vitelline membrane a short distance external to the boundary of the blastodcrm. Should the yolk happen to be tilted round so that the blastodcrm is not uppermost but rather at one side make the ﬁrst cut below the blastodcrm so that the elasticity of the vitelline membrane will tend to pull it upwards when the cut is made. Otherwise the blastodcrm may be lost by its being pulled downwards.

Having isolated the circle of vitelline membrane, with its adherent blastoderm, slide it off the yolk by pulling gently on one side with the forceps. Remove the remains of the egg from the dish so as to keep the salt solution clean. Take hold of the circle of vitelline membrane at one edge with the forceps and wave it backwards and forwards beneath the surface of the salt solution. The blastodcrm will gradually become detached. Should it not do so at once the separation should be started by freeing it from the vitelline membrane with a scalpel at one edge. Notice the difference in appearance between the vitelline membrane and the blastodcrm which has been detached from it. If the blastodcrm is yellow from adherent yolk this should be washed oil’ either by waving the blastodcrm backwards and forwards in the salt solution or by gently directing jets of salt solution 011 the yolky surface of the submerged blastodcrm by a wide-mouthed pipette.

The blastodcrm should. now be brought near the surface of the salt solution and a watch-glass slipped under it by which it may be lifted from the larger vessel. The blastodcrm is so delicate that it must be kept submerged in the ﬂuid: no attempt must be made to lift it above the surface by forceps.

A microscope coverslip slightly larger than the blastodcrm should now be submerged in the watch-glass and the blastodcrm ﬂoated over it dorsal side above. The dorsal or upper side of the blastodcrm can easily be identiﬁed from the fact that the edges of the blastodcrm tend to curl upwards. Having ﬂoated the blastodcrm over the coverslip the latter should be gently raised to the surface of the ﬂuid with a pair of large forceps. Take care to keep the coverslip absolutely horizontal and lift it out of the ﬂuid very carefully so that the blastodcrm is stranded on its upper surface, the lower surface of the blastodcrm being in contact with the coverslip. The superﬂuous salt 510 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

solution should be drawn away with blotting-paper so as to bring the blastoderm into close contact with the glass; take great care that the blotting-paper does not actually touch the blastoderm as in that event it will be apt to stick to it. Now take the coverslip between the ﬁnger and thumb and with the aid of a pipette place a very small drop of corrosive sublimate solution (or other ﬁxing ﬂuid) upon the centre of the blastoderm. This will cause the blastoderm to adhere to the coverslip. Now invert the eoverslip and drop it on to the surface of some fixing ﬂuid in a watch-glass.

The blastoderm is then passed through the various operations of staining, dehydrating and clearing, preparatory to mounting whole oi‘ conversion into a series of sections as the case may be. The advantage of having the blastoderm adherent to a coverslip is that it makes it easier to handle and above all it keeps it from becoming wrinkled or folded. The blastoderm if ﬁxed in corrosive sublimate can usually easily be detached from the coverslip at the stage of clearing if it has not already become free at some preceding stage. Should it adhere obstinately it should be placed i11 acidulated alcohol for an hobr or more.

The examination of the blastoderm should be carried out as follows:

1. Study the blastoderm and embryo as a whole under a, preferably binocular, dissecting microscope while it is submerged in the ﬁxing ﬂuid. As the ﬁxing ﬂuid penetrates the embryo the various details in its structure come into view. Continue the examination of the surface relief in the alcohol which is used for getting rid of the excess of the ﬁxing agent. After examining from the dorsal side invert the blastoderm and examine from below.

2. Repeat the examination of the embryo as a whole as a transparent object after staining and clearing. If the individual embryo is to be cut into sections a careful drawing should be made at this stage, the outline being preferably drawn by means of the camera lucida.

3. Study serial sections cut transversely to the axis of the embryonic body.

[Sagittal and horizontal sections will also be useful for study after the transverse oncs.]

III. EARLY SECOND-DAY BLAs'ro1nmM.—-—The same method is used as for the 42-hour stage but special care must be taken on account of the more fragile character of the blastoderm. In all probability the blastoderm will remain adherent to the vitelline membrane in spite of repeated shaking and the process of detachment will have to be started by gently easing up the edge of the blastoderm on the side next the forceps in which the edge of the circle of vitelline membrane is held.

To get rid of adherent yolk the circle of vitelline membrane should be laid on the bottom of the dish of salt solution, blastoderm uppermost. A pipette with a wide mouth should be held vertically X TECHN ICAL DIRECTIONS 51]

a few millimetres above the blastoderm and the india-rubber bulb squeezed rhythmically so as to wash away the particles of yolk by very gentle currents of salt solution. When the blastoderm is lifted out of the solution stranded upon the coverslip it is very apt to become folded. When this happens, on account of the fragility of the blastoderm, no attempt should be made to stretch it out by the use of needles or forceps. The folds should rather be straightened out by a current of salt solution allowed to ﬂow out from the oriﬁce of a pipette held vertically just over the centre of the blastoderm.

IV. EARLY BLASTODERMS.-—Open the egg as before. Let the albumen run off until the vitelline membrane over the blastoderm is exposed. Raise the egg until the blastoderm touches the surface of the salt solution and then bring a wide-mouthed pipette of Flem1ning’s solution, held vertically, into such a position that its tip just touches the ﬁlm of ﬂuid over the blastoderm. Let the solution ﬂow down slowly on to the vitelline membrane covering the blastoderm. If there is any albumen overlying the blastoderm this should be carefully stripped elf as it coagulatcs. A small piece of black bristle should be stuck into the vitelline membrane on each side to mark the line joining the chalazae so as to facilitate the orientation of the blastoderm for section-cutting. The ﬁxing ﬂuid should be allowed to act for several minutes and then a circle of vitelline membrane may be excised with the blastoderm adhering. to it. Floatv out the circle of vitelline membrane on a coverslip with the blastoderm above and submerge in a watch-glass of ﬁxing ﬂuid. If the circle of blastoderm adheres to the coverslip so much the better: it may be separated in the clearing agent.

Instead of a pipette as above indicated being used for the ﬁxing ﬂuid a small rim of cardboard, e.g. the rim of a small pill-box lid, may be placed on the surface of the yolk, raised up slightly out of the salt solution, so as to enclose the blastoderm and then the little tank so formed may be ﬁlled with Flemming’s solution which will gradually diffuse downwards. Minchin recommends a triangular instead of a circular rim for facilitating subsequent orientation.

For ﬁne work it is preferable to embed the whole yolk in celloidin and then after the celloidin has been hardened to c11t out the portion in the region of the upper pole for sectioning. This method consumes however much more time than does the paraffin method.

V. THIRD-DAY EGG.——A. Open the egg as before.

B. Study the embryo and blastoderm while still alive and in situ. A large outline drawing should he made. The details of the body of the embryo will be seen better later but the arrangement of the blood-vessels can best be studied now while the circulation is still active. As a rule they can be seen distinctly through the vitelline membrane but if not the latter should be carefully stripped off. A Greenough binocular microscope with its lowest power objectives is a useful accessory for examining the blood-vessels.

C. Excise the embryo with the surrounding portion of blasto512 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

derm, ﬂoat it on a slide, cover with coverslip supported by wax feet at the corners and examine as a transparent object, comparing the various features with those shown in Figs. 235 and 236.

D. Excise a second embryo with its surrounding blastoderm. Float it on to a coverslip, embryo above, and submerge it in a watchglass of ﬁxing ﬂuid. Watch it carefully under the lens or preferably low-power binocular as the tissues gradually become opaque. The amnion will be seen particularly clearly during this process. A drawing should be made of the embryo enclosed in its amnion as an opaque object.

E. Carefully strip off the amnion with a pair of needles 1 and study the conﬁguration of the head end of the embryo.

F. Stain and mount the embryo.

G. Prepare series of sections (at) transverse to trunk region, (1)) horizontal through trunk region and therefore approximately sagittal in the region of the head which is lying over on its left side.

VI. THE FOURTH DAY.-On placing the egg in the salt solution the broad end will tilt up more decidedly than before owing to the increase in size of the air space. Care should therefore be taken to make the ﬁrst perforation of the shell close to the broad end so as to allow the air to escape. Care must also be taken not to injure the vascular area as the whole blastoderm is now much closer to the shell than it was in earlier stages. As soon as the egg has been opened a careful drawing should be made while the embryo is still alive and in situ. The main features of the vascular system in particular should be carefully worked out at this stage. If the circulation becomes sluggish through cooling a little warm salt solution should be added but care must be taken not to bring about a great and sudden rise of temperature as in this case the greatly accelerated heart-beat is apt to cause rupture of a vessel.

The body of the embryo, allantois, ete., are covered over by the thin transparent serous membrane or false amnion as becomes apparent if the attempt is made to push a blunt needle down into the space round the allantois. This membrane should either be cut thrpugh with a pair of ﬁne scissors, just outside the boundary of the allantois, or carefully stripped off with ﬁne forceps. When this has been done it is possible to shift the body of the embryo into such a position that it with its blood-vessels can be observed in side view. Until this has been done it is impossible to get a proper view of the body of a well-developed embryo of this age owing to its dipping down out of sight into the yolk-sac.

The embryo should now be excised by cutting round outside the boundary of the vascular area and ﬂoated into a watch-glass of clean warm salt solution. The embryo may now be studied as a transparent object on the stage of the dissecting microscope. It is better

1 Bearing in mind that steel needles must not be allowed to touch corrosive sublimate solutions. Picric acid solutions are convenient ﬁxing agents to use for D

and E. X TECHNICAL DIRECTIONS 513

however in the ﬁrst attempt to proceed at once to ﬁx the embryo. An essential preliminary is to remove the true amnion which closely ensheaths the body of the embryo. In doing this it is best to commence at the region between the heart and the tip of the head where a couple of ﬁne needles may be used to tear the amnion. Its anterior portion may then be seized with fine forceps and pulled backwards over the embryo’s head. The operation is simpliﬁed by carrying it out immediately after submerging the embryo in fixing ﬂuid as the action of the fluid makes the amnion slightly opaque and therefore more easily visible. - If however corrosive sublimate be the ﬁxing ﬂuid ﬁne splinters of coverslip should be used for dissecting off the amnion unless this is done prior to immersing in the ﬁxing ﬂuid. The embryo should again be carefully studied during the process of ﬁxation, many details becoming particularly distinct before the creature becomes completely opaque. Finally the embryo should be studied, preferably with the binocular, as an opaque object, and then prepared either for section cutting or for mounting whole.

VII. SIX l)AYs.——Open freely into the air-space. Carefully tear away part of its inner wall so as to expose part of the vascular area, great care being taken 11ot to injure the latter. Notice the direction in which the vessels of the vascular area converge: this will indicate the direction in which the embryo is to be. found. Work towards the embryo, picking off the shell piece by piece, using blunt forceps. Frequently the escape of the air from the air-space allows the vascular area to sink down and leave a wide space between it and the shell membrane. In other cases however it remains in close contact with the shell membrane and in this event the greatest care must be taken not to injure the vascular area as by doing so the very ﬂuid yolk is allowed to escape and the salt solution rendered so opaque that observation of the embryo in situ is made almost impossible.

Notice that the allantois has increased much in size, that it has become richly vascular and that it is spreading outwards in a mushroom-like manner underneath the serous membrane. It has already spread so far as to cover the embryo nearly completely.

It is best new to remove the shell entirely and to examine its contents as they lie submerged in the warm salt solution (as shown in Fig. 242).

With ﬁne sharp scissors cut through the serous membrane just outside the limit of the allantois, commencing on the dorsal side of the embryo where the allantois is not yet closely applied to the yolksac. It is easy to do this owing to the coelomic cavity having spread outwards well beyond the limits of the allantois. The allantois being new no longer flattened out, by its continuity with the serous membrane all round, its vesicular character becomes apparent, as well as the difference in character of the vascular network on its proximal and distal walls. The relations of the vascular allantoic stalk to the vascular yolk-stalk should be noted: also the fact that the amnion is

VOL. II 2 L 514 EMBRYOLOGY 01*‘ THE LOWER VERTEBRATES on.

now widely separated from the embryonic body by secreted amniotic ﬂuid. If the embryo is a well-advanced one towards the end of the sixth day the amnion, which is now muscular, may exhibit periods of muscular contraction during which the embryo is rocked to and fro in the amniotic ﬂuid. These movements must be distinguished from the occasional contractions of the muscles of the embryonic body which also occur about this time though they are much less conspicuous.

After a careful study of the living embryo with the allantois and yolk-sac hanging from its ventral side it may be excised along with a circle of vascular area, ﬂoated into a watch-glass and examined alive with a lens or binocular, and then treated with ﬁxing ﬂuid such as Bouin’s solution. The latter brings out the surface modelling which should be carefully studied especially in the region of the gill clefts.

Dissect off the amnion, add more ﬁxing ﬂuid and after superficial ﬁxation renew the llouin’s solution. It is a good plan to suspend the embryo by the yolk-sac so that the weight of the head causes the neck to become somewhat straightened. After the embryo is sufficiently ﬁxed the neck may be cut through and the lower surface of the head studied for the relations of the olfactory rudiments and mouth.

Sagittal sections through the head are particularly instructive at this stage.

VIII. SEGMEN'l‘A'l‘ION.——T0 obtain segmentation stages hens which are regular layers should be chosen. In such cases the egg is laid at a slightly later time on consecutive days. As a rule egg-laying is conﬁned to the forenoon and early afternoon and when an egg is due after the end of this period it is retained within the oviduct and not laid until next day. The retention of an egg in this way inhibits the process of the ovulation so that a new egg is not shed from the ovary until the preceding one has been laid.

HISTORY or THE Eco UP TD run TIME or LAYING:--The egg arises as a single cell of the left ovary 1 which grows to a relatively enormous size as yolk is deposited in its cytoplasm. The yolk is of a characteristic yellow colour but in particular tracts the disintegration of its granules into ﬁner particles gives it a white colour. Of this white yolk a mass occupying the centre of the egg is continuous through a narrow isthmus with a tract lying immediately beneath the germinal disc (“ Nucleus of Pander ”) and this latter is prolonged as a thin superﬁcial layer over the surface of the egg. Between the superﬁcial layer and the central mass are a number of thin concentric layers of white yolk.

‘ The right ovary and oviduct which are present in early stages undergo atrophy, never becoming functional. This is probably to be regarded as an ada tive arrangement which has been developed in Vertebrates with large eggs to avoi the dangers which would be involved in the synchronous passage of a pair of eggs of great size, more especially if contained in a rigid shell, into the narrow terminal portion of the passage to the exterior. ' x ‘EGG or COMMON FOWL 515

As the egg increases in size it bulges out beyond the surface of the ovary, becoming eventually dependent from the ovary by a thin stalk at the end of which it is enclosed within the distended follicle. The wall of this is richly vascular except on the side away from the stalk where an elongated patch—the “stigma ”--marks the position in which the follicle-wall will rupture to set the egg free.

When this process (ovulation) is about to take place the thin membranous lips of the oviducal funnel become active, apply themselves to the follicle containing the ripe egg and grip it tightly. The follicle then ruptures and the egg is as it were swallowed by the oviducal funnel. Within the funnel fertilization takes place provided that spermatozoa are present.‘

The egg proceeds now to travel slowly down the oviduct, propelled onwards by the peristaltic contraction of the oviducal wall, the entire passage occupying about 22 hours. As it does so the albumen is deposited on its surface by the secretory activity of the oviducal epithelium. The first to be deposited is rather denser than that formed subsequently. It forms a sheath immediately outside the vitelline membrane and extending in tapering spindle-like fashion for some distance up and down F'<=- 323

U nincubated egg of the Fowl. the oviducal cavity; the two a.s, air-space; alb, albumen; ch, clialaza; s.m,

- , 3 _ ‘ shell membrane. In the centre-—at the apical pole-prolongatlons a’r(" ﬂu” chalazae is seen the germinal disc with the white “Nucleus of

  Puiulv-1"‘showingllwough it‘.

The envelope of (‘louse albumen enclosing the egg is not merely propelled onwards; it also undergoes a clockwise rotation about the axis along which it is travelling, caused probably by the cilia present on the oviducal epithelium.’ Owing to the prolongations of the albumen in front and in rear of the egg not undergoing this rotation the chalazae become twisted upon themselves in opposite directions.

Layer after layer of albumen (Fig. 223, alb) is deposited round the egg and chalazae until the full size is reached. The character of the secretion then changes and the shell membrane (Fig. 223, 3.-m) is formed. Finally in the dilatedhinder part of the oviduct (“uterus”) the secretion is in the form of a thick white ﬂuid which, deposited on the surface of the shell membrane, gradually takes the form of the hard and rigid shell perpetuating the characteristically “oval” form impressed upon the egg envelopes during the passage down the oviduct. In composition the egg-shell consists of calcium salts infiltrating a slight organic basis of keratin-like material. Structur 1 The spermatozoa remain alive and active within the oviduct for a period of about three weeks. 516 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. X

ally the greater part of its thickness consists of calcareous trabeculae forming a ﬁne sponge work. The inner surface of the shell is rough, projecting into minute conical papillae, while the outer surface is covered by a smooth apparently structureless layer perforated by numerous ﬁne pores.

SEGMEN'1‘ATION.——If the egg has been fertilized it proceeds with its development as it slowly travels down the oviduct. The process of segmentation is accomplished during this period and consequently the obtaining segmentation stages involves the sacriﬁce of the parent hens. Owing to the difficulties in the way of obtaining a complete series our knowledge remained for long fragmentary but recently (1910) a number of stages have been described and ﬁgured by Patterson which give a fairly complete picture of the process (Fig. 224). From these data we may take it that the early phases of segmentation are based on the normal plan where a meridional furrow appears traversing, or passing close to, the centre of the germinal disc ’i.6. the apical pole of the egg, and is followed by a second meridional furrow perpendicular to the first. In the third phase there is occasionally a regular set of four vertical furrows but more usually the process now becomes irregular (Fig. 224, C). In the next phase also there may be a fairly regular development of latitudinal furrows demarcating a group of about eight cells round the apical pole but typically there is no such regularity. The initial furrows, which make their appearance as above indicated, gradually extend. They eat their way downwards into the thickness of the germinal disc,-never however cutting completely through it. They also extend outwards towards the edge of the disc which however again they never quite reach. The apparent segments into which the germinal disc is mapped out by the early furrows are therefore not really isolated from one another ——there being still continuity between the segments on the one hand peripherally and on the other on the lower side of the disc next the yolk.

Complete blastomeres are ﬁrst marked off when, about the time the latitudinal furrows appear, division planes make their appearance parallel to the surface, cutting off the small segments in the centre from the underlying deep layer of the germinal disc.

The later stages of segmentation are quite irregular. Division planes make their appearance in all directions by which the germinal disc becomes completely divided up into small segments except on its lower surface and round its edge where there remains a syncytial mass in which the nuclei divide without their division being followed by any protoplasmic segmentation. It is to be noted that the process of segmentation throughout goes on more actively towards the centre of the disc, more slowly towards its margin, so that the blastoderm comes to be composed of smaller cells towards the centre and larger towards the periphery.

The result of the segmentation process is that the original FIG. 2'24.———Views of the blastoderm of the F0\vl'.~: egg during segmentation. (Afher Pa.1..tei-son, 1910.)

A, 3 hours after fertilization ; IL 5}, In s. ; U, 4 In-:~'. ; J), 4-5 hrs. ; E, about .-3 hrs. ; l , .5111-s. ;

(L 7 }n'.~:. : H, 8 hrs. . 518 _EMBRYOLOGY OF THE LOWER VERTEBRATES en.

germinal disc comes to be represented by a lenticular blastoderm lying at the apical pole of the egg and corresponding to the mass of micromeres of such a holoblastic egg as that of Lepidostren. The superﬁcial layer of cells become fitted closely together and form a deﬁnite epithelium——which is destined to become the ectoderm. The cells of the lower layers on the other hand are rounded with chinks between them representing the segmentation cavity. The lowest of all have the appearance of being incompletely cut oil from what is ordinarily termed the white yolk lying below them but which is really a syncytial layer" full of ﬁne granules of yolk and with scattered nuclei.

Apparently a few accessory sperm nuclei are usually present in the fertilized eggs and faint traces of abortive segmentation may be visible round them (of. Elasmohranch, Fig. 8, B *, p. 14).

At the time of laying the blastoderm forms a small whitish disc covering the apical pole ot' the egg. Sections show it to consist of an upper layer of ectoderm and of a lower layer consisting of numerous rounded micromeres lying about in the ﬂuid of the segmentation cavity. These micromeres become larger towards the lower face of the blastoderm and they are more crowded together round the periphery. _

It must not be supposed that all newly-laid eggs show exactly the same degree of development. As a matter of fact great variation occurs, one of the chief variable factors probably being the length of

time occupied in the passage down the oviduct. Where this time is,

longer, as e.g. towards the end of the laying season, the stage of development of the egg when laid is more advanced.

THE FIRST DAY or l"NcUeA'_r1oN.——After the egg has been laid the lowering of the temperature leads to such a slowing of its vital processes that development appears to come to a standstill. If kept at a'low temperature it retains its vitality for‘ a considerable period but makes no appreciable advance in development. If the temperature be raised by incubation the developmental processes are at once accelerated and comparatively rapid changes come about. The blastoderm increases in size, its margin spreading outwards, and at the same time there comes about a distinct difference in appearance between its central and marginal parts-the central portion assuming a dark transparent appearance (pellucid area) which contrasts strongly with the whiter “opaque area” surrounding it. - The examination of sections at once explains this difference in appearance: the more opaque appearance peripherally is seen to be due to the lower layer cells being there closely crowded together.

An important change soon comes over the lower layer cells, in as much as those next to the yolk, in the region underlying the pellucid area, lose their rounded shape, become somewhat flattened and adhere together edge to edge to form a continuous membrane-—the (secondary) endoderm. This appears first beneath the posterior poi tion of the pellucid area; it gradually extends x FOWL—--FIRST DAY’ 1 519

FIG. 225.-~111ustrating three stages of the blastoderm of the Fowl during the second half of the first day of incubation.

a..o, opaque area; mp, pellucid area; lap, head process; mes, boundary of sheet of mesoderin; m.f, medulla:-y fold ; p.g, primitive groove ; 12.3, primitive streak.

forwards and outwards, and eventually is continuous all round with the thickened marginal part of the b1astoderm.1

1 This thickening of the posterior edge of the blastoderm presents in sagittal section a. striking resemblance to a. gastrular lip growing back over the yolk and Patterson (1907) believes that an actual process of involution-—a reminiscence of gastrnlation by inva ination--takes place, It must not be forgotten that any explanation of such 0 scure developmental “phenomena. in Birds must, to be reliable,520 EMBRYOLOG-Y ()l<‘ '_l‘Hln‘ LUWEJ-t VEl~‘tTEBRATES an.

A gradual change takes plzme in the shape of the pellucid area which, up till now circular, a.ssuim,-s an oval or pear shape (Fig. 225, B)——the long axis perpendicular to the long axis of the eggshell, and the narrow end being next the observer when the broad end of the egg-shell is to the left. This narrow end may be called posterior from its relations to the rudiment. of the embryo which appears later. Together with the gradual change in the shape of the pellucid area there takes place the development of the primitive. streak. This makes its appearance usually during the first half of the ﬁrst day of incubation, as a linear opacity stretching forwards along the long axi.s of the pellucid area in its posterior third. As the ﬁrst day of incubation goes“ on the primitive streak becomes more and more distinct. A longitudinal groove develops along its middle———the primitive groove-—while on each side of this it forms a ridge, the primitive fold.

If a number of eggs be examined du.ring the ﬁrst day of incubation

 ,_m.g»,    n
  rgzasa.

FIG. 2‘26.—-Transverse section through primitive streak of the Fowl.

eat, vet-oderm ; vi-mi, 1-ndoderm; mes, mesoderm; 12.;/, primitive groove.

it will be seen that the primitive streak, as is commonly the case with vestigial organs, shows extreme variability. More especially its hinder end is commonly bent to one side or the other, or even bifurcates into two branches. At its front end one or both halves of the primitive streak swell up into a slight knob while the primitive groove becomes somewhat deeper and wider.

The primitive streak is shown by transverse section to originate from a linear tract of ectoderm along which the cells are undergoing rapid proliferation, as is indicated by the relatively numerous mitotic nuclei. The cells budded off by the ectoderm are aggregated together in a compact mass along the course of the primitive streak while on each side they become loosened out and wander away into the space between ectoderm and endoderm to take part in forming the sheet of mesoderm.

1-;_.:-: L .——r-—.—:-—

~——-:jw—j— -f

rest on a firm basis of knowledge of Reptilian development. At the present time however our knowledge of the exact relationship of these clevelopniental stages of Birds to the corresponding stages of Reptiles is not in the present writer’s opinion adequate to form a trustworthy basis for their interpretation. FOVVL ———FIRS'l‘ DAY 521

H

For a short distance in- the region of its front end the mass of cells forming the primitive streak is continuous not only with the ectoderm but with the endoderm as well: the primitive streak of this region may be deﬁned as a tract along which there is cellular continuity between the ectoderm and the endoderm.

During the latter half of the first day what is known as the “ Head process ” makes its appearance (Fig. 225, B, lap). In a view of the whole blastoderm this has the appear‘-aiice of being a somewhat less distinct prolongation forwards of the primitive streal<——-in front of the knob which marks its apparent front end. 'l‘he study of transverse sections shows that the so-called head process is exactly similar in structure to the primitive streak immediately behind it, except that it is separated fr cm the overlying ectoderm by a distinct split and that there are no primitive folds or primitive groove over it. On its lower side there is perfect continuity with the endoderm—-— as is the case with the anterior part of the obvious primitive streak into which it is continued.

During the same period of incubation there appears the first sign of the surface relief of the body of the embryo in the form of what is known as the head fold (Fig. 227, A, h. f). This is formed by the blastoderm bulging upwards and forwards, forming a projection bounded in front by a steep face crescentic in shape as seen from above, the two horns of the crescent directed backwards. The projection increases in prominence: its front edge soon comes to overhang, the blastoderm becoming tucked underneath it both in front and at the sides, the two horns of the crescent which the fold formed at its first appearance gradually extending farther and farther backwards. The projection is destined to give rise to the head end of the embryo and there are certain important details to be noticed about its structure which can be made out best by the study of sagittal sections.

The region of the blastoderm where the head fold develops is composed of the two primary layers, ectoderm and endoderm, the mesoderm not yet having spread into it. It follows that the head rudiment has a double wall, its outer sheath of ectoderm enclosing an inner wall, quite similar in shape, composed of endoderm. It will be understood that this inner wall of endoderm is continued at its hind end into the ﬂattened layer of endoderm which lies on the surface of the yolk. In other words the endoderm within the head rudiment may be described as forming a very short wide tube, blind anteriorly but opening behind into the yolk. This endodermal tube is the rudiment of the front part of the endodermal lining of the alimentary canal of the adult and is termed the foregut.

Soon after the commencement of the formation of the head fold the ectoderm of the medullary plate becomes raised up into a longitudinal ridge (Fig. 227, A, m. f) upon each side of the median line. Between the two ridges is a groove——the medullary groove: the ridges themselves are the medullary folds: the two medullary folds F10. 227.—Fow1 embryos at about the end of the first day of incubation seen by reﬂectcd light.

5-6 segments. cr..p, pellucid area‘ aw, vascular area; f.g, foregut; h.j, head; m.f, medulla:-y fold; m..c, nnesoder-n1

A, 3 mesoderm segments; B, no segments yet demamcated; 0, segments ; p.a, proamniun ; p.g, primitive groove; 11.3, pximitive st.reaLk. CH. X FOWL--FIRST DAY 523

are continuous anteriorly. The two medullary folds gradually extend backwards and at the same time they become more prominent and arch over towards one another until at about the end of the ﬁrst day they meet. It is to be _noticed (h‘ig.‘227, B) that the ﬁrst meeting of the medullary folds is some little distance back from their anterior end, in about the position in which the division between mesencephalon and rhomhencephalon will develop later. Towards their anterior end the folds remain less prominent than they are farther back with the result that the meeting of the two folds is here con— siderably delayed.

During these later hours of the ﬁrst day important advances are taking place in the development of the mesoderm. In the ﬁrst place it is to be noted that the anterior limit of this layer is gradually extending forwards, encroaching more and more upon the proanmion ——the part of the blastoderm in front of the head fold which is still two layered. In the second place the mesoderm becomes considerably thickened and more compact in the region near the median 1ine——— adjacent to the head process or notochord. This thickened portion of the mesoderm becomes divided by transverse splits into a series of blocks——thc mesoderm segments——lying one behind the other (Fig. 227, A and C, m.s). The first pair of splits to make their appearance are placed obliquely, sloping outwards and backwards: they mark the hind boundary of the first or most anterior segment. A little later a pair of similar splits develop a little farther back forming the hinder limit of the second segment, and so on, segment after segment becoming separated oil’ from the still continuous mesoderm lying farther back.

While this portion of the mesoderm is becoming segmented it is at the same time becoming sharply marked off by its greater thickness from the lateral mesoderm lying farther out from the axis. Towards the end of the ﬁrst day at further important development takes place in the mesoderm in as much as isolated splits appear in it parallel to its surface and these gradually spread and finally become continuous so as to divide the mesoderm into the outer somatic layer next the ectoderm and the inner splanchnic next the endoderm. The cavity which has made its appearance between somatic and splanchnic layers of mesoderm-is the coelome. The portion lying within the myotome, which soon becomes ﬁlled up by immigrant cells derived from its wall, is the myocoele (Fig. 228, me). The portion lying farther out, in the lateral mesoderm, is the splanchnocoele (splc). The two layers lying external to this cavity——the somatic mesoderm and the ectoderm —constitute the somatopleure or body-wall: the corresponding layers lying internal to the cavity-—the splanchnic mesoderm and the endoderm——-constitute the splanchnopleure or gut-wall.

While the changes above described have been taking place the blastoderm has constantly been increasing in area and by the end of the ﬁrst day it forms a cap covering an extent of about 90° at the upper pole of the egg. In the opaque area—--the part of the blaste524 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

derm lying outside the boundary of the pellucid area---there are present the same layers of cells as in the pellucid area-—-the ectoderm, which extends. farthest peripherally, the endoderm which passes into «W thick ¥°1kSYI1cWe1layer vermherallv <2ermina1 wall. and the

 s A v

ﬂu “L * n’

L1 1

Flu. 2‘28.—-'I‘1‘ansVerse section through the bocl y of a Fowl embryo about the end of the tiist day of incubation. c. act, cctoderm; end, endodurm; me, myocoele; my, myotomu (lll6S0(lCI‘ll1 segment); N, notochord;

'n.'r, neural rudiment; som, somatopleure; spl, splanclnioplenre; splc, splzmclmocoele.

mesoderm the outer part of which is still unpenetrated by the coelomic split. The part of the opaque area where inesoderm is present assumes a very characteristic mottled appearance (Fig. 227, 0, (M2) caused by the rudiments of blood-vessels and blood: hence the name vascular area which is given to this part of the blaste derm. When the embryo has reached the stage with about seven

inesoderni segments the secretion of ﬂuid (plasma) commences within the blood islands.

13313 Saconn DAY or INCUBATIQN:

opened during the second day of incubation is seen in Fig. 229. The blastoderm has increased considerably in size and now covers about 110". The pellucid area has assumed a somewhat ﬁddle-like shape.

On examining the excised blastederm about the commencement of this day it is seen that the formation of the head fold has progressed considerably and the head rudiment projects more conspicuously above the general level of the blastoderm. Within the head rudiment the foregut can be seen and it is noticeable that it stretches farther back than does the outer wall of the head rudiment. . In other words the head fold of the endoderm has spread farther back

FIG. ‘2‘29.~—-Egg of the Fowl about the middle of the second day of incubation.

a..o, circular opaque area. the dark pellucid area, with the rudiment of the embryonic body lying along its axis.

In the centre is

-——The cg;-,ne1~a1 appearance of an seoegggi X F OWL-—SECON D DAY 525

than that of the ectoderm. This is brought out clearly by a sagittal section such as that shown in Fig. 230. Such a section a.lso brings out the fact that while the greater part of the portion of blastoderm tucked in beneath the head of the embryo is two-layered (proamnion), there being no mesoderm present, this does not apply to the farthest back part of the fold. Here, in the wide space between cctoderm and endoderm, mesoderm has penetrated which will give rise to the pericardiac wall and the heart. The medullary folds have met over a ' considerable extent but still remain separate at their extreme front ends as well as over the whole extent which will later form the spinal cord. Here they bound a deep neural groove. Towards their posterior ends the two medullary folds diverge to pass on either side of a lance—shaped area (rhomboidal sinus) which they enclose by converging towards one another behind it. Along the centre of the

Flu. 230.»-Diagrairiiiiatic sagittal section through anterior end of Fowl embryo with 15 segiiieiits.

am, rudiment of amnion; hr, brain ; er!-, ectmle-rm ; um.-, endocai-dinm; end, endoderm of yolk-sac; _/15;, l'm'v_~,r11t2 h._/‘, p0n'l'-i'I‘l')1' limit or lH.':ul rum of uctoderm; mo, myocardinm; N, notochord; pa, ect.-o«‘lm-In of pm.-nnninn ; -vile, split!|(7lI1I()(1()Ble.

ﬂoor of the rhomboidal sinus the primitive streak is still_ visible separated by a knob—li.l<e elevation from the part of the primitive streak which lies farther back.

The mottled appearance characteristic of the vascular area is now seen to be continued inwards, though much more faint, across the pellucid area to the body of the embryo.

An embryo with about ten segments is shown in Fig. 231. The pellucid area is still somewhat ﬁddle-shaped with the body of the embryo lying along its axis. Apart from the increase in number of the mesoderm segments the most conspicuous advances in development are in the central nervous system. The medullary folds have met and fused together to enclose the neural tube except towards their hind ends where they still bound the rhoinboidal sinus on each side. The forebrain region is greatly dilated, its projection on each side being the optic rudiment (am). It will be noticed that a slight notch in its wall in the mesial plane anteriorly

indicates that at this point the two neural folds have even yet not 526 EM]-3[{.'Y(ie)l'.O(‘:i-Y 01*‘ THE LOWER VERTEBRATES (:11.

completely fused. Posteriorly the neural folds seem to be continuous with the lips of the primitive groove. A faint continuation forwards of the primitive groove may be seen in the ﬂoor of the rhomboidal s1nus.

Flu. 231.~-Bla.stml¢_-1'In with l"m\'l .wJl"1J]..‘.‘| will: :n.bm|t l() or ll 1m__e.~'(_)«lt.'-.1'n1 .«-;_;'111u11l-:4.

am, Vtl.s‘(:lll1ll':lI‘o-.1 ; _/'._:/, fur-e;;I11.: la, Imiul : m.‘/1, ma-«lnll:n'_\' rule! ; m..~:. rm-:-.mlerIn .\'I'_4_1_'lll6l|t-2 u./', upliv l'lllllllH_'lll.I /W. ]rmunminn.

Important mesoderrn features are to he noticed. The mottled appearance of the vascular area produced by the rudiments of bloodvessels developing in the splauchnic mesoderin is conspicuous. The formerly isolated vascular rudiments (white in the ﬁgure) are now becoming joined up to form a network and the network can be traced —less distinct and on a smaller scale—-across the pellucid area. At x . 'i_«‘oWI._.sil«:(":(_)N1) DAY 527

its anterior and inner corner the network is continuous with a short and wide vessel which slopes obliquely forwards and inwards and disappears beneath the hind end of the foregut (shown more clearly in Fig. 232, 72.22). This vessel is the rudiment of the vitelline vein, which drains the blood from the vascular area towards the heart. Another conspicuous vessel rudiment is the terminal sinus-a marginal vessel which bounds the vascular area externally. In front of the head of the embryo is a somewhat rectangular area of the blastoderm distinguished by its being very transparent (Fig. 232, pa). This is the proamnion—- -its transparency being due to the fact that

FIG. 232.~-—--llmul oi" l“u\vl «-mhr_\'0 of smm-. Hiilgif as that s'hu\\'n in Fig. 231, more highly inugnilieil and Ht-‘vll by tnuus-initlwl liglit.. f.g, foregut; 1!, heart: h.._/',himlerlimil ofhcml fold ()fvC.l'(_)¢ll'l‘lll; inf, infumlibnlnm; m..s, mesoderm

segments; N, notochord; 0.7-, optic rmliuu-nt: ,.u, ]nn:unniuIl ; splc, patent portion of splanchnocoele containing coelomic ﬂuid; -mi. vitellim-. \ win.

the mesoderm has not yet spread into this region of the blastoderm. On each side of the head of the embryo the surface of the blastoderm bulges upwards into a dome-like swelling (Fig. 232, splc). This is due to a precocious splitting of the mesoderm in this region to form a large coelomic space. The bulging appearance is produced by the coelomic space being tensely ﬁlled with fluid. The raising up of this region of somatopleure is preliminary to the formation of the head fold of the amnion.

By turni.ng over the excised blastoderm and examining it from below or by staining and then examining it in dorsal view by transmitted light (Fig. 232) it will be seen that between the two coelomic spaces there lines a A-sha_pe<.l structure. The two diverging limbs of 528 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the A posteriorly are the vitelline veins already alluded to (cw), While the median portion (H)——a straight tube passing forwards beneath the foregut-—is the rudiment of the heart and ventral aorta. It will be noticed that the two vitelline veins when traced backwards from the heart are seen to ﬁt round the tunnel-like opening of the foregut. In the forcbrain region is seen the downwardly projecting pocket of its ﬂoor—the infundibulum (Fig. 232, v}nf)———and extending -back from this in the middle line the notochord (IV). On each side of this posteriorly are seen the mesoderm segments (m.s).

In a slightly more advanced embryo with about ﬁfteen mesoderm segments the tucking in of the blastoderm under the head has proceeded considerably further. The neural tube has become closed in entirely except for the slit-like remnant of the rhomboidal sinus posteriorly. The optic rudiments projecting prominently from the forebrain on each side and beginning to be narrowed slightly at their base give the brain a conspicuous T-shape. The wall of the brain in its posterior region shows a series of puckerings one behind the other marking it off into a series of what used to be called brain “ vesicles.” Of these the anterior one, the largest and most distinct, is destined to become the niesencephalon while those behind it enter into the formation of the rhombencephalon. The latter are often interpreted as vestiges of a once present segmentation of the brain, but are regarded by the author of this volume as being adequately accounted for by the active growth of the l.rain within its conﬁned space, aided possibly by the varying consistency of the mesenchyme outside it (see p. 101).

On each side of the head region posteriorly, just in front of the ﬁrst.obvious mesoderm segment, the rudiment of the otocyst has made its appearance as a cup-like depression of the ectoderm.

The heart, growing in length more rapidly than the neighbouring parts of the body, has been forced into its characteristic bulging outward on the right side. The ﬁrst traces of haemoglobin are making their appearance in the posterior portion of the vitelline network.

An important new feature becomes visible about this stage in the form of a whitish line on the bulging roof of the splanchnocoele on each side. The lines in front curve in towards one another, meeting in front of the proamnion and sweeping back in a wide curve on each side. This line is the first rudiment of the amniotic fold. As the fold becomes more and more prominent it bends backwards and inwards, arching over the head region, and towards the end of the second day (Fig. 233) forming the anmiotic hood which ensheaths the head portion of the embryo.

Many of the important details in the structure of the second day blastoderm can only be made out by the study of series of transverse sections. In studying the stage new under consideration it is advisable to begin with a section taken from about the middle of x ‘ FOWL—~——SECOND DAY 529

the total length of the embryo such as that represented in Fig. 234A. The blastoderm some little distance away from the median line of the embryo is seen to consist of the usual two double 1ayers——-the somatop1eure(som) composed of ectoderm and somatic mesoderm and the splanchnopleure (spl) composed of splanchnic mesoderm and endo FIG. 233.-—Blastoderm and embryo Fowl with 18 mesoderm segments.

u,.e, hackgrowing edge of amniotic hood; asp, pellucid area; um, vascular area; 1!, heart; of, 0t0(‘,_yst,; sa, sero-amniotic connexion.

derm. In immediate contact with the lower surface of the endoderm in the complete egg there would be the yolk. I'n the splanchnic mesoderm overlying the endoderm are seen the blood-vessels of the vascular area. When traced inwards towards the mesial plane the two layers of mesoderm are seen to come together to formthe narrow protovertebral stalk or nephrotome which joins up the lateral mesoderm to

VOL. 11 . A 2M 530 ']+3MBRY()L()GY or THE Lowm: \’l~}1t5lTﬁl+}J.:itATES en.

the mesodei-in segment. Immediately above the nephrotome, between it and the cctoderm, is seen the rudiment of the arehinephric du.ct-——a rod of cells which is gradually extending tailwards. i In the centre of the section is the neural tube (s.c) with its thick walls and the solid notochordal rudiment (N) -lying immediately

FIG. 234A.——Transverse section through the middle of El .*i('(‘.(')nIl~(l:l}-‘ Fowl embryo ' (15 segments).

4, p.-iiretl dorsal aorta; a..n.d, ﬂl‘Clllllf‘])lll‘l('f duct; eat, t-(‘totlvrni : end, u-n«l0tl(‘,1‘ln ; my, myotome; N, notoeliord; s.c, spinal cord ; .~'u‘m, somatnpleure; spl, splam-hnopla.-m'e; .s-pin, splanclmoeoele.

below it. The blood-vessel (A) on each side between nephrotome

and endoderm is the dorsal aorta which is at this stage double. Working back towards the tail end of the embryo it is seen that

subsequent sections show less and less advancedstages of development

FIG. 2343.-Transverse section through a second-day Fowl embryo jllst lwliiml the binder limit of the l'(')1‘e;,5l1t.

A, dorsal aorta; and, endoderm; -‘my, myotome; N, not.oeho1.-d; .-.r_.-, spinal em-cl ; min, somatoploure; Sp], splanchnopleure; .~.-plc, splanehum-oele; r, \"('_5.‘€."~‘.(!l.\' ()f\'asc||I:1[‘;1rea,

in concordance with the fact that development proceeds from the head end tailwards. Thus the neural tube opens out by the slit—like rhomboidal sinus; the archinephric duct disappears; the notechord passes back into the undilferentiated tissue of the primitive streak. On the other hand the examination of sections farther forward towards the head region brings into view various important further developments. Such a section as that shown in Fig. 23413 illustrates x Fo_wL.....sEooNn DAY 531

clearly an early stage in the folding off of the foregut from the cavity of the yolk-sac-——a fold of splanchnopleure growing inwards on each side below what will become the foregut. The large vessels seen in the splanchnopleure external to the fold just mentioned are tributaries of the vitelline veins, and a few sections farther forwards they would be found to be united together to form the main vitelline vein on each side.

As the series of sections is traced forwards the two folds of the splanchnopleure are seen to approach one another and ﬁnally to meet and undergo fusion, so that there now exists a foregut cavity shut off (as seen in transverse ._section) from the yolk-sac, the walls of the two structures being still connected by a median vertical partition formed by the fusion of the endoderm from

FIG. 2340.~—Transverse S(‘.(‘.t-iml of a .~u-cowl-day Fowl embryo pa-1..<siiig through the I'udiment of the ll(‘iU.'l.

A, dorsal aorta : «l.nu°, 1l()l'H:ll Im-snmmliuiii 1 rm". l‘lIIl()l'L'l.l'|lllllll : crud, cnclorlc-rm : jig/, foregut; ma, myocardium; s.c, spimil cord; so.m, smiiatic nic.-‘ode’-rin; sp.m. spluncliiiiu ino.<o(lo'1'1ii; split, splanclb nocoele; v.m«_-, \'t'-ntl‘£ll mcsocardium.

the two sides. A little farther forward this partition disappears from ‘the section and the foregut as seen in section (l*‘.ig'. 2340) is quite isolated from the endoderm of the yolk-sac wall. The vitelline veins have also fused to form the tubular heart. It is seen that the splanchnic inesoderni ensheaths the endothelial wall of the heart (em) on each side and that where it -does so it is somewhat thickened (me) as compared with the same layer in the region overlying the yolk“-sac. This localized thickening of the splanchnic inesoderm is destined to give rise to the entire thickness of the heart wall except the lining endothelium. It is seen to be continuous with the extracardiac portions of the splanchnic mesoderm by the dorsal (d.mc) and ventral Inesocardium (mnc).

Traced forwards through the series of sections the heart is seen to narrow in calibre as-it tapers off into the ventral aorta. Towards its front end the latter gives off a large branch on each side which 532 EMBRYOL()(_}YOF THE LOWER VER’1‘EBPA'l.‘ES (:11.

passes outwards and upwards round the foregut to become continuous with the dorsal aorta. These two hoop-like vessels which connect up ventral and dorsal aortae are the ﬁrst pair of aortic arches.

Still further forward the region of the forebrain and optic rudiments is reached (Fig. 2341.)).

Owing to the folding oil‘ of the head rudiment the section of the head itself a.pp1*a.1‘s cmnpletely detached from the blastoderm and the latter is begiiining to form a depression which will later become more marked and in which the head will lie. In the blastoderm it

s will be noticed how away on each side it shows the normal four layers

of cells——-ectoderm, somatic mesoderm, splanchnic inesoderm, endoderm—While on the other hand in the region 1i11_de1-lying‘ the head of the embryo it is only two layered the mesoderm being here absent.

the Optic rudiments.

cot, ec.-tmlm-m : rm/, mulmlu-I-in ; /:H'S, nw.~:mu-liyiiie; n.r, optic. rudiment ; ,:u, pi-oanmion ; splr, n‘];l.'u|('l|ll0C0!..'lI'; Hull, roof ofthalamencephalon.

This two-layered region of blastoderm is the proamnion before alluded to.

The head itself is occupied almost entirely by the brain rudiinent ———the thalanieneephalon in the centre (tltal) continued outwards on each side as the optic rudiment (oxr). For the most part the external ectoderm is closely apposed to thesurface of the brain but dorsally the former is commencing to recede from the latter, the space between the two being occupied by mesenehy me (mes).

THE THIRD DAY or .lNCUI£A'rION.—During the later hours of the second and earlier hours of the third day of incubation there take place a number of important changes which render this period perhaps the most i.nterest'ing of all to the morphologist. For the student who is training llllll.S(..‘.ll" practically in the technique of embryological observation tliem is no ﬁner material than that aﬁ"orded by liinl embryos of about this :.1.g<: for l_o:1.1‘ning one of the

"most important parts of that technique namely the interpretation

of serial sections. OWL--——SEC()N 1) ANI.) '.l.‘H’[R1) DAYb 533

It is mlvisable to make {L (;:.L1‘(3l"111 sbmly of the ana.t;omy of an embryo of about; the Htztgia shnxvn 111 I919‘. 235 01‘ Ii-.’.f_’.(-3,1

Flu. 255.3. 'l'hir«l—«l:ty l*'u\\'l t‘|lliH'_\'u with thv \'u..~"u:1l1:n|':u"u.

u..r, o.-«L-"v ul' :1nminII‘. P5. I-_\'I'*: .‘.’, ln~:H'l; wt, u1uc_\'.w-t; .-.u. >'I'l‘0-ﬂllllliutiv t'Ullll\‘_\iUl|; .~'.I, sinus ternninali. ; I.‘/', Iziil-fnlulg ran. \'il».*llim- .-n'tc_*1-y; . ;u.n'l.iun u!’ splzuurhlmlalelu-v im-'0lnt.eil to form :1

rm-ass Inmul Llw he-:ul of the vInln'yn.

‘ It is custonmr_y In mmml. lI':1)1.w'\':)1'. 0 sections with the posterior or tai1\':rd surface of the section next lilv sli.-In : nulls:-ql1t_‘lltiy the ﬁgures represvnt the sectinns as seen frmn in front and Lhv sidu nl' '..:u'.h Iig1u'v tmv.-n-«is H11‘ ri;;l11-lnnici side ml‘ the pzige c<2z't'e.~‘pn|u«ls tn the M't."hu.n«l side at" the crnlrryu. 534 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

On opening the egg it is at once seen that the blastoderm has increased considerably in size, the outer limit of the opaque area having spread downwards as far as about the equator of the egg. The vascular area has also increased considerably and is still bounded by the conspicuous terminal sinus which anteriorly turns inwards and passes back parallel to the corresponding part of the sinus of the other side to open into the vitelhne vein close to its inner end. Of these two veins which run parallel to the long axis of the embryo the right is reduced in size and eventually disappears.

The yolk has assumed a more ﬂuid consistency; the proportion of white yolk has increased; the albumen has shrunk considerably in volume, and the air space has increased correspondingly.

The free edge of the amniotic hood (Fig. 235, a.e) has grown back so as to ensheath all the head and anterior trunk region of the embryo. It follows that when examined in sritw. the front part of the body is seen through two layers of somatopleure. Of these the outer—the serous n1en1brane———forms a kind of roof which passes outwards all round into the general blastoderm. The inner—the true aInnion——c1osely invests the head end of the embryo and is visible in proﬁle as a sharp line immediately outside the outline of the head itself. Anteriorly the amnion very often seems to be prolonged into a sharp peak (Fig. 235, s.a): this is the sero-amniotic connexion.

The free edge of the amniotic fold, somewhat arch-like in outline, may die away posteriorly (Fig. 235) or it may be already continued into the lateral and caudal parts of the fold (Fig. 236)——but even if present these are still low and inconspicuous as compared with the headward part of the fold.

As regards the body of the embryo it is seen that the folding off of this from the yolk is proceeding rapidly. The head and anterior part of the trunk project freely and, correlated with this and with the ventral ﬂexure of the head region, the latter has come to lie_ over on one side, usually the left, so that it is seen in proﬁle when the blastoderm is looked down upon from above. At the extreme hind end the tail region is also seen to be in process of becoming marked off from the blastoderm by a tail fold (Fig. 235, tj) of the same nature as the head fold. Similarly the trunk region between the regions of head and tail fold is becoming demarcated from the blastoderm outside it by a lateral fold (Fig. 236).

The body of the embryo has increased considerably in length and this growth in length is particularly active towards the dorsal. side of the embryo where there is greater freedom from the clogging effect of the yolk. The result of this difference in rate of growth between dorsal and ventral sides is that those parts of the embryo which are detached from the general blastoderm assume a strong ﬂexure towards the ventral side. This is particularly pronounced in the head region, the head being completely bent upon itself so that x I FOWL--THIRD DAY 535

the front end of the brain is reversed in position, what was its ventral side having come to be dorsal.

The mesoderm segments have inc1'ea*‘ed in number there being

FIG. 236.—~Third-day "Fowl embryo (N o. 47) viewed as a transparent object.

cw, edge of amnion; am, amnion; E, eye; H, in-._a.rt;; m.s, mesoderm segments; ut, otocyst; s, indications of preotic mesoderm segments ('9); u,a,, vltellino artery; me, visceral cleft II; *, portion of splanchnopleure bulging downwards into the yolk, forming a recess in which lies the head of the

embryo.

'now about 25-30 metotic segments and those towards the anterior end are showing a considerable amount of dorsiventral growth. , In some embryos (Fig. 236) the ‘series of deﬁnitive mesoderm segments is continued far into the head region by what appear to D36 EMBRYOLOGY OF THE LOWER VERTEBRA'.l‘.ES CH.

be the ghostly vestiges of formerly existing segments (see pp. 210,211)

The central nervous system has made important advances in development. The brain shows a relatively large increase in size

‘as compared with the spinal cord : thalamencephalon, mesencephalon

and rhombencephalon are marked off by deﬁnite constrictions-—tl1e mesencephalon being particularly prominent at the bend of the head. The greater part of the roof of the rhombencephalon is assuming its deﬁnitive thin membranous character. The three great organs of

special sense have made their appearance. The eye (E) forms a large conspicuous cup-like structure lying at the side of the forebrain. Its rim is cleft ventrally by the choroid ﬁssure (Fig. 236). Its mouth is partially blocked by the round lens rudiment. The otocyst (at) is also conspicuous—-—a pea_r-shaped sac, its narrow end dorsal, lying at the side of the hind brain. The olfactory organ is represented by a slight dimple of thickened eetoderm near the tip of the head.

The side walls of the foregut are perforated by visceral clefts. The series of these develop from before backwards and by this stage three have commonly appeared—c1efts I, II, and III of the series.

It is perhaps the vascular system which shows the most interesting features during the third day. The heart is still in the form of a simple tube, but its active growth in length has caused a great increase in the curvature which was already pronounced about the middle of the second day. Its y-like curvature is shown in Fig. 236. At its morphologically front end the heart is continued into the .ventral aorta and this at its end gives off a series of vessels, the aortic arches, which pass up round the sides of the foregut between adjacent gill-clefts and open dorsally into the aortic root which lies just dorsal to the clefts. Like the clefts themselves the aortic arches develop in sequence from before backwards and by this stage arches I, II, and III have made their appearance (Fig. 241, A).

At its front end the aortic root can be traced for some distance into the head as the dorsal carotid artery (Fig. 241, A, d.c). Posteriorly the two aortic roots become hidden from view by the myotomes but the study of sections shows that they have here united to form the unpaired dorsal aorta. Still farther back this vessel again becomes paired and a little behind the point of bifurcation each of the branches gives off a large vitclline artery (ea) which passes outwards at right angles to the axis of the body to supply the vascular area.

Of the venous system the most conspicuous components are the great vitelline veins (Fig. 241, A, 72.7)) which, receiving numerous branches from the vascular area, pass forwards converging towards one another to form by their fusion the hind end of the heart. Examination of the vascular area shows that the branches of the vitelline arteries and of the veins accompany one another in their ramiﬁcations. In the living condition, in which all these arrangements .\' .I*‘OWL-.-----THIRD DAY 537

of the vascular system should be studied, the arteries are seen to be more deeply coloured and more conspicuous than the veins. The

. two vitelline veins by their -fusion form the hind end of the tubular

heart and on tracing this forwards a. somewhat Y-shaped vessel is seen opening into it laterally. The stalk of the Y which is very short, though showing considerable variability within its limits, is the right duct of Cuvicr (Fig. 2-11, A, d.C). The branches of the Y are the cardinal veins. Of these the posterior (p.c.'v), coming from the region of the kidneys, is only visible for a short distance, being soon hidden as it is traced backwards beneath the myotomes. The anterior cardinal vein (a.c.v_) on the other hand can be traced forwards for a long distance into the head from which it drains the blood back towards the heart. It will be noted that here in the embryonic Bird we ﬁnd exactly the same arrangement of main veins—-—duct of Cuvier, anterior cardinal and posterior cardina1—as

F In. 237A.——'l‘ransverse sections through thircl-«lay Fowl (:lnln'_\'t_). (Partly based on figures by Duval.) A, '|‘hrough the hinder part. of the trunk region.

A, dorsal aortne; um, annniotic folds ; «mi, «-ndoderm ; 'm._c/_. xnyotume ; s.«', spinal cord; sum, sonmtoplenre ; spl, s]alams-lannplenre; splc, splanchnocoelt-r.

is characteristic of-the adult condition of lowly organized ﬁsh-like Vertebrates.

For the study of such details of structure as cannot be made out in the whole embryo the most useful sections are series cut transversely to the long axis of the trunk region. These should be supplemented by series parallel to the sagittal plane in the head region.

It is well to commence the study of the transverse sections with one through the binder trunk region, about the level of the vitelline arteries. Such a section is depicted in Fig. 237A.

In comparing this section with a corresponding section through the second-day chick (Fig. 23-1A) the same general features will be recognized--—the differences being mainly differences in detail. The most conspicuous of these is caused by the development of the amniotic fold of the somatopleure which rises up on each side, arching towards ‘the median plane over the dorsal side of the embryo (am). Traced forwards through the series the amniotic folds of the two sides are seen to meet and undergo fusion so as to give rise 538 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. rte/the inner true amnion and the outer false amnion or serous inembrane: the T6fif1“e"r"'"'contin11o11s at its inner '"ed‘g‘é"'\vitl1 the somatopleure of the embryo’s body, the latter at its outer edge with that of the blastoderm. It will be readily seen that the space between true and false amnion is morphologically part of the splanchnocoele. It will also be realized that both true and false amnion being somatopleural in nature are composed of ectoderm and somatic mesoderm but that the relative position of these two layers is reversed in the amnion compared with the false amnion. Important changes have taken place in the mesoderm. The mesoderrn segment is no longer connected with the lateral mesoderm the nephrotoine having become converted into renal structures———the archinephric duct and mesonephric tubules. The relations of these will be understood by referring back to the general description of renal

FIG. 23713.—--'l‘r:msvcrs:- section just behinzl the point of union of the two vitelline veins.

A, dorsal aorta ; um, amnion ; r~, umllls .'.lrt,m"i0suS ; err, m-1.m.h-run; emf, enteron ; ,/Lu, false amnion or scrolls membrane; my/, inyotome; N, ]I(_)l.-U(:l1UI‘Il; .s-.e, spinal coral: sn, sc-re-amniotic isthmus; sum, somatopletire; cpl, splanchnoplenru; .s-pic, splmuzlinocoole; I’, \'«-nlrivh-; 42.:-, vitelline wins; y, yolk.

organs in Chapter IV. (p. 254). The inner wall of the segment has lost its epithelial character and broken up into a mass of actively proliferating mesenchyme cells. Many of these cells will wander away in amoeboid fashion and settle down round notochord and spinal cord to form the protective sheath in which eventually develops the vertebral column. Collectively these arnoeboid cells constitute the sclerotome which is therefore much more diffuse in its origin than in the lower vertebrates illustrated on p. 285.

Certain blood-vessels are visible in the section. In the splanchnic mesoderm of the yolk-sac numerous vessels of the vitelline network

are visible : over the mesonephros may usually be seen the posterior.

cardinal vein, while on each side of the mesial plane ventral to the notochord are the two dorsal aortae.

e As the series of sections is traced towards the head the most conspicuous change is the incri:-asing asymmetry due to the body of the embryo coming to lie over niore and more upon its left side. Fig. 237]} represents a section just behind the‘ posterior limit of the foregut. X FOWL --—THIRD DAY 539

The body of the embryo lying over on its left side is closely invested by the amnimi (run) while over this lies the thin roof (_/lam) constituting the serous mcnibrane. At set the two membranes are united by the sero-amniotic connexion. «In the mesoderm of the two folds of splanchnopleure which are approaching one another to floor in the alimentary canal (ant) are Hl‘l_'ll the two large vitelline veins (72.22). The ventricle and the cones are seen out longitudinally in the wide coelomic space lying to the right of the body of the embryo.

A section a little farther forward in the series has the appearance shown in Fig. 2370. The definitive gut (ent) is completely separated at this level frorn the yolk-sac, and corresponding with this the two vitelline veins, which in sections farther back lay one on each

FIG. 237C.—-T1‘£tl1S\'el‘.<U .\f(‘(_‘llHll :1. little in front of the hind «_-ml of the lic:n't..

mu, mnnion: .-l,«l0rs.-il aor-tn ; d..r, «incl us \-mu)sii.s' : «uni, ulinu-m.-u'y 4-.-uml : _1'.vun, i'.'ilso~ mnnion ; /-i.l, 8-Ilte!‘i0l' 1i\'t-1‘ 1'luli1IwI1t ; li.;’, }no.~'.l'erioi' «lit't.o ; N. nolnvlionl : I-.r.r, ])().\'lI'l'lU[' (‘:ll'lllll:ll win ; sum, somatoph-nrv; :=_n/, spl.-in<:lmople-uri: ; splc, splanclinocoele ; I-'_, \'l'lIll‘l(‘lt.‘.

side of the yoll<—sta1l<, are now completely fused into a large median vessel, the ductus venosus (dxv), which is simply the backward prolongation of the heart. The posterior liver rudiment, a blindly ending pocket of the ‘gut-wall projecting forwards ventral to the ductus venosus, is seen in the section ﬁgured (Z222), although its communication with the gut-wall is no longer visible, lying as it does several sections farther back. At this level however a second pocketlike outgrowth of the gut-wall has made its appearance (l7§.1). This is the anterior liver rudiment. It will be noticed that it lies dorsal to the ductus venosus. In the coelomic space ventral to the ductus venosus and liver rudiments, and quite isolated, is the rounded section through the ventricular region of the heart (V).

In the sections studied so far the body-wall of the embryo is widely Open on its Ventral side—the opciiiiig l)elllf_,"lml.1I.l(l€(l by the recurved edge along which the soniatopleure ol’ the body is continuous 540 EMBRYOLOGY OF THE LOWER VEil;{.'_l‘-EBHATES OH.

with the non-embryonic region of the somatopleure forming the amnion. As however the _foli_iing oil‘ of the embryo progresses the edge alluded to grows inwards and the opening bounded by it becomes reduced in size. It will be gathered readily from Fig. 237D that through the opening in question the splanchnocoele, included within the deﬁnitive body of the embryo, is continuous with that part of the coelome which lies outside (extra-embryonic coclome). In the section ﬁgured the heart is seen to be cut through in two places. Reference to the figure of the whole embryo (p. 535) will show that the piece of heart which lies towards the leit side of the embryo (at) is the atrium, while that on the e1nbryo’s right (0') is the ventricle or conus. In the section iigiiiwl a large blood-vessel (d.0) is seen out

Flu. 2371),——'l‘r:i.nsvm-so section a short distance behind the front end of the heart.

.1, dorsal aorta ; am, amnion ; at, atrium ; l..', eonus : «(.!.', duct oi‘ (iuvier: f.«.¢.m., false amnion ; N, notm-hord; 1'-h, pharynx; .-om, sonuttopieure; .-pl, spinnohnopleure; .5-pl:-, splanchnocoele.

longitudinally in the sornatopleure. By tracing this vessel through neighbouring sections it will be found to open at its ventral end into the atrial part of the heart while dorsally it splits into the two cardinal veins-—anterior and posterior. These relations show the vessel in question to be the duct of Cuvier. The only other point calling for special mention in the section ﬁgured is that the ventral part of the pharyngeal cavity projects outwards upon either side: this dilated ventral part of the pharynx forms the rudiment of the lung.

In the region in front of the heart the dorsiventral depth of the body of the embryo becomes comparatively suddenly reduced and in the Vacant space within the amnion so provided there appears a_ new structure quite detached from the rest of the section. The structure in question is a section through the recurved tip of the head (see figure of whole embryo). In Fig. 237E this shows the thi.c.k-wallmel forobrain ( with its wide ventriE C9

3"

cc 0 -"1? S9 o C?‘ o v-:

x FOWL-——_-THIRD DAY 541

cular cavity while upon each side and ventrally’ there is seen a localized thickening (olf) of the ectoderm: this is the dimple-like 1 udi o 4'

of the sectionmthere is seen in its centre the wide pharyngeal space and on the embryo’s left side the pharyngeal wall projects out to the ectoderm as an endodermal pocket--~the rudiment of the second visceral cleft (12.0.11). Immediately ventral to the pharynx is the ventral aorta (o.A). On the left side of the embryo the aortic root (am) is seen immediately dorsal to the pharynx, while on the right side--—the section not being accurately transverse—~ ya hoop-like aortic arch (a:.a.III) is seen passing dorsalwards round the side of the pharynx from ventral aorta to aortic root. The large

FIG. 237E.-—-—Transverse section passing through the second visceral cleft and the ' olfactory rudiment.

a.a.IIl, third aortic arch: a.r:.2v, anterior cardinal vein; am, amnion; a.r, aortic root; f.am, false amnion; f.b, forebrain; h.h, hind brain; olf, olfactory rudiment; 11/l, pharynx; ‘I'.A, ventral aorta; no.1], second visceral cleft.

vessel lying dorsaland slightly external to the aortic root (a.c.v) is the anterior cardinal vein. Traced tailwards it is found. to open into the dorsal end of the duct of Cuvier. The neural tube (lab) is seen to have a thin roof and widely expanded lumen indicating that it is now passing into the region of the hind brain.

In tracing the series -of sections further forwards it will be

y organ. To return to the am part“ .

realized that the front part of the head region is, owing to its '

reflexed position, actually being traced in a morphologically tailward direction. In the section ﬁgured (Fig. 237F) the reﬂexed portion of the head is cut at the level of the eye rudiments (opt) which are seen to be in the optic cup stage with the inner or retinal layer

‘ It will be realized from an inspection of the figure of the entire embryo that the recnrved part of the head is reversed in position. Its ventral side lies therefore in the ﬁgure towards the right. 542 EMBRYOLOGY or THE Lowna VEI-LTEBRATES en.

distinctly thickened as compared with the outer or pigment layer, and with a narrow optic stalk passing to the thalamencephalon near its ﬂoor. In the mouth

of the optic cup is the lens

but this is seen better a few sections farther on in the series.

Turning to the other

Pf half of the section it is

seen that it is no longer
connected with the extraembryonic soinatopleure:
 in other words the series
    e     of sections has now passed

FIG. 2371-‘.—-—.Transverse section passing through the ' the binder limit -of the rmlunents of the eye and otocyst. hemlfold of the SOmatO_ I (l.('.I‘:{|.llt8['ll-ll"(‘ﬂl‘(illHll‘\'|.‘lll; l:.h, hind brain; .."\'t|1nl«-t.-.lu.n-«I; pleura The pharynx opt, optic cup. of. nt.u('_\ sl , 1:14, ph.'-u'_\nx, (no.1, lust \'1.~'(-er:c1 cleft; aum, ventralearotid. Passes out as 3' Pocket

on each side towards the

ectoderm-—4the rudiments of the ﬁrst pair of visceral elefts (11.0.1). The neural tuhe has become greatly increased in size forming the hind brain with its widely expanded cavity——-the -fourth ventricle. On each side is a large thick-walled sac—-—the otocyst. Examination of

Pin. ..'.‘37(:.- -—'|‘rn.nsv(-rse section passing thrnugln the (*._\'v and just in front of the otocyst.

1I_.I'. 4', il.lli;(‘]'i()!‘ (‘:ll‘(llll:ll Vein : I(..:‘, :u_n'ti(_'. run! ; 41,:-«(,«lm'.~4:nl c-:n-utitl :u't«'I'y; _I[tHI(l. g:u|;;:lin|| of I-i_L,-'l1H'1 H-,-mi,-.1 m-rm: /uh, hind hr.-Lin; lm. pit.-niI_.-n-_\' hotly; .\'_, lllrl_.H(:ll()l‘ll; pin hlmr_\'m.'; pin, ]:ine:ul nr:,r:m; Hull, 1h:'1l:uumn--‘plmlnn: I‘.v'.l. i'l]‘.~}i \i.~:---ml rl--ft.

neighbouring sections shows that it is still connected with the outer skin by a narrow neck. ,In the spongy connective tissue which forms packing between the various organs are seen a x FOWL--THIRD DAY 543

number of blood-vessels such as ventral and dorsal carotids and anterior cardinal veins. ‘

As will be gathered by sliding a straight-edge forward over the ﬁgure of the whole embryo, its edge parallel to the plane of the sections, there comes a point in the series where the sections through the reﬂexed part of the head and the rest become continuous. This happens as soon as the deep niche in the bend of the head is passed. Such a section is represented in Fig. 2370.. Comparison of this ﬁgure with the preceding one will make clear the fact that the extreme ends of the section are both “of them morphologically dorsal. The brain is cut through twice—on the right of the ﬁgure is the hind brain while on the left is the thalamencephalon distinguished by

FIG. 238.—-Diagrammatic sagittal section through third-day Fowl embryo. '.l‘he notochord and dorsal aorta are omitted Ectoderm and endoderm are indicated by cotltinllous lines, mcsoderm (except endocardium) by dots.

a, position of anus, not yet perforate; ull.,al1antois; am, zumiiong at, atrium; a.e, amniotic edge; f.a,, serous mcinbrane ; fl g, foregut; l, lung rudiment; nws, mesencephalon ; pa.g, postnnal gut; pt, pituitary involution; rh, rhombencephaion; .s~pl, splanchnopleure of yolk-sac; t, thalamencephalon; th, thyroid; V, ventricle; v.A, ventral aorta; 'v.m., remains of velar membrane; ,1/.s, ‘cavity of yolk. sac; 3:/..s-t, cavity of yolk-stalk.

the pocket-like rudiment of the pineal organ (pin). The thin optic

stalk lies outside the section, but the structure of the optic cup

otherwise is well seen.‘ The lens is in the form of a closed vesicle which has by this stage become completely nipped off from the external ectoderm. Immediately ventral to the thalamencephalon is the pituitary involution cut transversely- The section passes through the ganglia of the auditory nerve (gang) and on the embryo’s right through the nerve root connecting the ganglion with the medulla oblongata. Various blood-vessels are cut through: their names and relations with one another are most easily determined by sliding a straight-edge along the drawing of the embryo as a whole.

The study of this stage should be completed by examining series of sections parallel to the sagittal plane in the ‘head region and 544 EMBRY'0LdGY OF THE LOWER VERTEBRATES CH.

interpreting them by what has been made out from the whole embryo and the series of transverse sections. The most instructive sections are those in or close to the sagittal plane. Fig. 238 shows diagrammatically a sagittal section through the whole length of the embryo, but it will of course be understood that, owing to the head of the embryo having come to lie over on its left side while the trunk region retains its original position, a section which is sagittal in the head region will, in actual fact, be practically horizontal in the trunk. ‘

The feature that dominates the section is the cerebral ﬂexure-— the strongly marked curvature of the head region towards the ventral side. The brain is of relatively enormous size: a distinct dip in its roof marks the boundary between the thin-roofed rhombencephalon which lies behind it and the region in front of it-—the cerebrum--whichwill give rise to mesencephalon, thalamencephalon and hemispheres. .

The next instructive feature brought out by such a section is the general relation of gut to yolk-sac. The rounded head-fold of the splanchnépleure has extended far back so as to floor in the foregut (jig). The velar membrane (am) has just ruptured so that the foregut communicates in front with what will become the stomodaeum into which also opens the pituitary involution of the ectoderm (pt). The ﬂoor of the foregut dips downwards to form the rudiments of the thyroid (th) and lung (1). In a slightly more advanced embryo

the two liver rudiments would be seen also as pocket-like outgrowths of the enteric ﬂoor in the neighbourhood of the atrial end of the

cardiac tube.

The posterior end of the deﬁnitive alimentary canal is also becoming folded off from the yolk-sac though the cavity of the yolkstalk—-—the communication between the definitive alimentary canal and the cavity of the yolk-sac—-—is still very wide. The position of the future anal opening is indicated by a thick septum (Ct) composed of fused ectoderm and endoderm. Dorsal and posterior to this the enteron extends back as a blindly ending pocket—-the remains of the postanal gut (pay), while anterior to the anus the enteric ﬂoor dips downwards as the rudiment of the allantois (all). The latter is covered with a thick layer of mesoderm and bulges into a dilated portion of the splanchnocoele. Towards the front end of the embryo a still more widely dilated portion of the splanchnocoele accommodates

the cardiac tube. At its anterior (o..A) and posterior ends (at) this,

is ensheathed in the thick mesoderm on the ventral side of the foregut, while its middle portion (V) hangs free in the cavity.

Finally the amniotic fold of the soinatopleure is seen to extend almost completely over the body of the embryo, the amniotic edge (cue) bounding a comparatively small opening near the tail end. '

Having studied in some detail the features- characteristic of an individual third-day embryo it will be convenient now to give a X FOWL-—THIRD DAY 545

general sketch of the chief advances in development which take place during this day.

At the commencement of the day the body of the embryo lay ﬂat along the surface of the yolk: only at its head end was it clearly demarcated from the surrounding blastoderm and this head region owing to the commencing ventral curvature was beginning to lean over on to its left side. During the course of the third day the tucking in of the blastoderm under the deﬁnitive body proceeds apace so that the body becomes more and more completely demarcated from the part of the blastoderm forming the yolk-sac wall, and the yolk~stalk becomes correspondingly narrowed. The preponderance of growth activity on the dorsal side which leads to the ventral curvature is during the early hours of the day especially marked in the region of the mesencephalon but as the day goes on becomes very pronounced about the level of the heart and still later in the tail region. Thus the axis of the body develops strong ventral curvature especially marked at three different levels——mesencephalic, cardiac and caudal. Along with this increasing curvature the whole body of the embryo comes to lie over on its left side so that the observer looking down upon the egg from above sees the body of the embryo in proﬁle from its right side.

During the day the embryo becomes ensheathed in the amnion in the manner already described. The vitelline network of bloodvessels attains to its highest development, forming as it does the organ for respiration as well as for absorption of the food and its transport into the body of the embryo. Correlated with the lying of the embryonic body over on its left side the paired venous channels which convey the blood from the vitelline network into the heart gradually lose their symmetry, those of the right side dwindling in size while their fellows show a corresponding increase.

In the brain the main regions become established: the roof of the thalamencephalon and medulla oblongata. assume their thin membranous character while the hemispheres bulge out in front of the thalamencephalon. The central canal of the spinal cord becomes reduced to a vertical slit by the thickening of the side walls. The olfactory rudiment makes its appearance: the auditory rudiment becomes converted into the closed pear-shaped otocyst, still however connected with the ectoderm by a solid strand of cells. In the eye the lens thickening has become involuted and converted into a closed vesicle with its inner wall markedly thickened. The optic cup has been completely formed and the retinal layer differentiated from the thin and degenerate pigment layer. In the latter the ﬁrst deposition of pigment takes place during the later hours of the day.

The deﬁnitive alimentary canal is still open towards the yolk-sac over about half its extent but in addition to the foregut there becomes folded off during the course of the third day a considerable extent of hind-gut, the ventral wall of which commences to bulge out to form the rudiment of the allantois towards the close of the

voL. II 2 N 546 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

day. The hind-gut is still closed posteriorly but the foregut late in the third or during the fourth day becomes thrown into communication with the stomodaeum by rupture of the velar membrane. The pituitary rudiment makes its appearance. The four gill-pouches are formed and reach the ectoderm, the fourth in the closing hours of the day, and the first or it may be the ﬁrst two become perforate. The thyroid rudiment makes its appearance and during the latter half of the day becomes closed. The pulmonary rudiment develops and becomes constricted off from the pharynx except at its front end. About the beginning of the day the two liver rudiments appear and during its course the process of anastomosis begins between the branches which sprout out from them. During the latter half of the day the pancreatic rudiments make their appearance——ﬁrst the dorsal, then the left ventral, then the right ventral.

During the course of the day the mesoderm segments increase from about 20 to 25 up to about 40. Early in the day the Wolﬁian duct becomes tubular and in the latter half of the day it completes its backward growth and reaches the cloaca. The germinal epithelium becomes recognizable.

The skeleton remains throughout the day purely notochordal.

The heart retains its S—shape and during the latter half of the day the atrial septum begins to develop. The two dorsal acrtae begin about the commencement of the third day to undergo their fusion to form the deﬁnitive unpaired aorta. In addition to the first one or two aortic arches which are already present the third makes its appearance (Fig. 241, A, III, p. 550), then the fourth, and during the latter half of the day the sixth, while the ﬁrst becomes obliterated. As regards the venous system the most important feature is the assumption of the same general plan of the main trunks as is characteristic of Fishes.

Finally it should be noted that during this day the body of the embryo-becomes enclosed within the amnion.

It will be realized even from the bare summary that has been given that the third day of incubation of the Fowl’s egg'is morphologically the most important of all and the student will be well advised to devote a good deal of time to making a detailed study of embryos of this period. _

THE FOURTH DAY or INCUBATION.-——-By the end of the fourth day of incubation the blastoderm has spread about half-way round the yolk. The vessels of the vascular area are conspicuous, though it is to be noticed that the terminal sinus is becoming relatively less so than it was during the third day. The folding off of the body of the embryo has progressed greatly. By the extension backwards of the head fold the region of the heart has become ﬂoored in on its ventral side. Posteriorly the tail fold is deepening in a similar fashion. Between head fold and tail fold the somatopleure of the embryonic body is prolonged ventralwards into a very short and wide tube--the somatic stalk-——the wall of which is reflected dorsalwards as the true X FOWL—THIRD AND FOURTH DAYS 547

amnion. The latter is now complete and closely invests the body of the embryo. Lying loosely within the somatic stalk and of much smaller diameter is the splanchnic or yolk stalk--the continuation of the splanchnopleure in a ventral direction as it passes out into the wall of the yolk-sac. The body of the embryo has undergone a great increase in size. The growth of its tissues has been particularly active in its dorsal region and this has led to a continuation of the ﬂexure towards the ventral side which was already well marked in the third day embryo. ,

An important new feature in the fourth day embryo is provided b y the two pairs of limb rudiments each in the form of a dorsiventrally flattened ridge with rounded edge and broad base of attachment to the body. The head of the embryo at once attracts attention by its relatively enormous size. This is due to the relatively immense size of the brain and eyes. We have here to do apparently with a case of the precocious growth in size of organs which in the fully developed condition possess extreme complexity of minute structure. The main regions of the brain can be seen very distinctly: the relatively large mesencephalon with its bulging dome-like roof, the thalamencephalon with the pineal rudiment, the rapidly growing rudiments of the hemispheres, and the hind-brain with its relatively thin and membranous roof. The three main special sense organs are all conspicuous—-the olfactory organ, the eye with its choroid ﬁssure and lens, the pyriform otocyst. Arranged in a row ventral to the otocysts are the pharyngeal clefts—three or four in number. In the case of cleft I the ventral part of the cleft is becoming much narrowed by the approach of its anterior and posterior walls. The dorsal end of the cleft on the other hand remains dilated: it corresponds to the spiracle of ﬁsh-like forms.

The heart, which forms a large structure lying between the tip of the head and the region of the fore limbs, is still in the form of a coiled tube but the appearance of localized bulgings of its wall foreshadows its division into the various chambers characteristic of the

adult. Thus the curve of the tube lying posteriorly and on the right

is becoming dilated to form ‘the ventricle: the part morphologically in front of this leading towards the ventral aorta is slightly dilated to form the conus arteriosus, while the curve lying anteriorly and on the left side shows a slight bulging on each side foreshadowing the two auricles. Slight constrictions separate these various bulgings-—an atrioventricular constriction narrowing the cavity to form the auricular canal, and a less conspicuous one between ventricle and conus. '

The general arrangement of the peripheral vessels is intermediate between that of the third day (Fig. 241, A) and that of the ﬁfth day (Fig. 241, B) and need not be described in detail. Aortic arches I and II undergo in turn a gradual process of obliteration while arches IV and VI make their appearance farther back if they have not already done so. It is also during this day that arch V makes its brief appearance. 548 EMBRYOLOGY OF THE LOWEPt TVERTEBRATES CH.

The allantoic veins, which at ﬁrst are merely veins of the body-wall, during the fourth day establish their connexion with the allantois, and in the course of the day the right vein disappears.

The allantois itself forms a conspicuous new feature for towards the end of the day it begins to project distinctly from the ventral side of the embryo about the level of the hind limb.

Owing to the increasing size and complexity of the embryo the elementary student will not as a rule prepare complete series of sections later than the third day. He will however ﬁnd it proﬁtable to have transverse sections through the developingsense organs, sagittal sections through the head, and transverse sections through the posterior trunk region.

From the study of sections the following advances in development during the fourth day may be made out.

In the brain the rudiment of the paraphysis makes its appearance Fm. 239.--Fowl's egg opened at the end and the pineal outgrowth begins to

of the fifth day. The embryo enclosed sprout; out into diverticula, about;

in its amnion is sunk down in the - 1 centre of the vascular area, the allan~ the end of the day‘ rlhe Olfactory

tois projecting upwards towards the» rudiment b3C0m3S Conneclied With serous 1nembrane——a transparent mem- the buccal gavjty by a, slight,

brane through which theembryo and ' allantois are seen. The increasing groove‘ The rudunents of lagena

ﬂuidity of the yolk is shown by the and recess make their appear-.

outward bulging of the yolk-saewall ance as bulgingg of the

?J:2§‘;§1‘;’3l":i1..°2§ii.‘3§ “s::‘°.:.‘..*:.‘.::‘: ototytt watt The cavity of the

Iiowlies completelyunderneath the yolk 13113 b(.3C01T1€3 Obliterated by th9

so as ‘to be invisible in a view from grgwth of its inner wall; pigment,

“b°“"‘ becomes conspicuous in the outer

““‘°;‘éi‘£::§2.‘:‘..2é’s:‘.l':3.§2‘:é wall of the optic cup: the layer of

nerve ﬁbres in the retina becomes

recognizable: mesenchyme begins to invade the cavity of the optic

cup and about the end of the day also intrudes between the lens and the ectoderm.

The post-anal gut becomes reduced to a solid strand of cells and ﬁnally disintegrates. The yo1k—stalk becomes narrowed to a ﬁne tubular channel. The gall-bladder begins to dilate towards the'close of the day :. the dorsal pancreas begins to develop outgrowths: and the rudiments of the caeca make their appearance.

The mesoderm segments increase in number to about 50. Early in the day, if it has not done so already, the Wolfﬁan duct opens into the cloaca. The mesonephric glomeruli begin to appear and the tubules become elongated and coiled. In the posterior region of the X FOWL-—-FOURTH AND FIFTH DAYS 549

mesonephros secondary tubules make their appearance while in the anterior region a process of degeneration becomes apparent. During the second half of the day the ureter begins to sprout out from the Wolfﬁan duct and about the end of the day the rudiments of Milllerian ducts and of the metanephric units may become recognizable.

In the heart the atrial septum becomes completed about the end of the fourth day and the endothelial cushions begin to develop.

FIG. 240.—-—Chick extracted from the egg at about the middle of the fifth day of incubation.

all, allantois; C.H, cerebral hemisphere; Is‘, eye; Hy, opereulum; M, mandibular arch; pin, pineal rudiment faintly visible as slight elevation on root’ of thalzuuencephalon; Rh, thin roof of rhombencephalon; som, edge of somatopleure cut through where it becomes reﬂected back over the body of the embryo to form the amnion; Lu, roof of tnesencephalon (optic lobe); V, ventricle; 47.0,

visceral clefts Ill and IV ; y.s, yolk-sac.

FIFTH DAY.—--The progress in development during the course of the ﬁfth day is illustrated by Figs. 239-241. The albumen has so shrunk in volume as to be no longer visible in a view of the opened egg from above: the yolk has become extremely ﬂuid: the vascular area has increased considerably in size. The allantois is now a conspicuous object and the mesoderm covering its surface is beginning to develop blood-vessels. The ‘head of the embryo is, as before, of relatively very large size: the ﬂexure in the region of the mesen550 TEMBRYOLOGY‘ OF THEALOWER VERTEBRATES; CH.

cephalon is still more pronounced. The operculum (Fig. 240, Hg) is conspicuous, growing back from the hyoid arch over the posterior visceral clefts. The limb rudiments now project freely though their form is that of simple ﬂippers without any of the peculiarities of the leg or wing of the Bird. The body of the embryo is ﬂoored in onits ventral side completely but for the rounded opening (som) along

- whose lips the somatopleure is continued into the amnion and through

which emerge the narrowing yolk-stalk and the stalk of the allantois. ilThe study of the living embryo in situ shows the general plan of the blood system to be as is shown in Fig. 241, B. The heart still

FIG. 241.—Diagrarn showing the main parts of the vascular system as seen in a Fowl embryo during the third day (A) and the ﬁfth day (B).

a..a, allantoic artery; a.c.w,-, anterior cardinal vein; at, atrium; (1.17, allantoic vein; d.C, duct of

Cuvier; d.c, dorsal carotid; il.a, iliac artery; p.a, pulmonary artery; p.c.v, posterior cardinal vein; p.v.c, posterior vena cava ; v.A, ventral aorta; 1:.a, vitelline artery ; v.c, ventral carotid ; v.z~, vitelline vein ; I-VI, aortic arches. '

betrays its tubular origin though the chambers are clearly recognizable as dilatations. Three aortic arches (III, IV and VI) are distinctly visible and occasionally the ﬂeeting vestige of the penultimate arch as in the specimen represented in the diagram. In front of the aortic arches the ventral aorta is seen extending forwards as the ventral carotid (ac) :. the pulmonary artery (p.a) passes back from the sixth arch. Dorsally the aortic root extends forwards into the head as the dorsal carotid artery (d.o). A little distance behind the liver the vitelline artery (ua) leaves the dorsal aorta and farther back the allantoic artery (cm) a branch of which, the iliac artery, passes to the hind limb.

In the venous system the duct of Cuvier is seen, continuous at its dorsal ‘end with the anterior and posterior cardinal veins. ' The

‘former (a..o.q2) branches through‘ the head: the latter (19.0.72) can be X FOWL-—FIFTH AND SIXTH DAYS 551

traced dimly back into the region of the kidney. The main blood-'

stream to the heart comes from the vitelline vein (ac) and is joined

within the substance of the liver by the blood from the left allantoic

vein (am) and the posteriorvena cava (p/ac).

Ignoring the vitelline and allantoic vessels which are clearly adaptations to the peculiar conditions of the developing embryo the main plan of the blood system is seen to be clearly the same as is characteristic of Fishes.

By cutting off the head after ﬁxing and viewing it from below (Fig. 245, A) the modelling of the face can be studied. The frontenasal process ( f .72) is bounded on each side by the shallow oro-nasal groove connecting it with the buccal cavity. The ridge forming the outer boundary of the olfactory organ is demarcated from the maxillary process by a faint transverse groove passing outwards towards the eye-—the lachrymal groove. Posteriorly the stomodaeal opening is bounded by the mandibular ridge with a distinct break in the middle line between the two mandibular arches.

Of other developmental features of the ﬁfth day we may note the following. The first indications of turbinals appear on the mesial wall of the olfactory organ, and of semicircular canals in the otocyst. The optic stalk becomes solid: the rudiments of the ocular muscles become recognizable. The pituitary body begins to form outgrowths. The rudiments of thymus and bursa fabricii make their appearance: the bronchi begin to develop branches. The formation of new mesonephric tubule rudiments comes to an end and the mesonephros begins to show signs of functional activity. The atrial septum develops secondary perforations. The fourth aortic arch on the left side, and the portions of aortic root immediately behind the third arch undergo reduction. The horizontal septum of the ventral aorta begins to extend back into the conus and the anterior portions of the posterior cardinal veins begin to undergo atrophy.

SIXTH DAY.-—During the sixth day of incubation the body of the embryo increases rapidly in size and in correlation with this it dips down into the very ﬂuid yolk, pushing the splanchnopleure of the yolk-sac wall in front of it, so that it is almost hidden from view when the egg is first opened. The amnion is, now raised up from the body of the embryo by a marked accumulation of amniotic ﬂuid (Fig. 242). The allantois has increased greatly in size and in the natural condition is ﬂattened rnﬂushroomwise againetnthﬁ iIme1‘..Su1:fa0B

\.—__-—-on-n.n-—. -—a of the serous membrane. In the embryo excised as directed on p. 513

‘it will be seen that the somatopleure of the embryonic body is

completely closed in ventrally except for a small circular space round which it is reﬂected outwards in a funnel-like fashion and continued into the thin membranous amnion. Through the funnel-like opening a slender probe can be passed from the extra-embryonic coelomic space beneath the serous membrane into the portion of coelome enclosed within the body of the embryo which will become the deﬁnitive splanchnopleure or body-cavity. Through the opening i552 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

there pass out the stalks of the yolk-sac and the allantois (Fig. 246, B) each conspicuous owing to its large blood-vessels. The peripheral distribution of the vitelline and allantoic vessels shows a characteristic difference (Fig. 242)———the vitelline network (vascular area) terminating, in the now greatly reduced terminal sinus at a considerable distance from the distal pole of the yolk-sac while n the other hand the allan

      : .               ' mic networkis most richly

developed on the distal side of the allantois (p. 474)

The body of the embryo now for the first time’ begins to show indications of bird - like form, and faint traces of digits and of feather-rudiments may become apparent about the end of the day.

In the eye the rudiment of the pecten, which ﬁrst became recognizable during the fourth day, is now conspicuous as an ingrowth of . mesenchynie through the choroidal ﬁssure, bounded on each face by the inflected lips

, _ of the ﬁssure. FIG. 242.-Coiiinioii Fowl. View of contents of the

, . egg-shell extracted at the end of the sixth day of he tongue beglns 1,30 _ incubation. The serous meinbraiie has been removed Pr0.]eCt and the thyrold

- so as {to gllow the allantois to be f(iltﬁ1pl&1C(3(l1 slightllly becolneg constrictgd off in or( er 0 give a c carer view 0 e )O(y o t e embryo contained within its ainnion. from the pharynx‘ The oesophagus towards the a. ii, edge or vascular area; all), reimilns of albumin; all’, . outer wall of allaiitols; all”, inner wall of allantois; am, end of the day loses lbs amnion; *, portion of vascular area lying, in the natural cavity; the dilatation of

ppiiaigilcgn. beneath the head of the eiiibryo and free from blood- the gizzard becomes evi_

. dent; the intestine begins to grow actively in length (Fig. 246, B). The three pancreatic rudiments become continuous with one another. ,

The ‘muscles of the body begin to exhibit contractility, the trunk

occasionally showing twitches of ventral ﬂexure. The ureter develops outgrowths to form the primary collecting tubes of the metanephros about the beginning.of the sixth or the end of the ﬁfth day and the terminal part of the duct of the opisthonephros may become incorporated in the cloaca so. as to give the ureter its independent opening. About this time the ﬁrst indications of sexual differentiation become recognizable, the genital strands beginning to show signs of degeiiera-T tion in the female. X FOW'L-—-—SIX’_l‘l--I T() EIGHTH DAYS p 553

The main portions of the skeleton become laid down in prochondral tissue and, towards the end of the day, in cartilage.

The heart begins to assume its deﬁnitive external form; the ventricular septum develops and the conus septum begins to do so. The fourth aortic arch becomes obliterated on the left side.

SEVENTH DAY (Figs. 243 and 244).-——The mushroolm -shaped allantois is spreadin r activel all round beneath t 1e serous membrane. The amnijon is begilining to show waves of contraction passing along its wall. The brain and eyes and consequently the head as a whole are of relatively enormous size. In sections the roof of the fourth ventricle is found to be developing irregular folds in which the vessels of the choroid plexus will appear. All three turbinal rudiments are present in the nose. The crop is beginning to expand. The visceral clefts are all closed. The glands of the stomach are beginning to make their appearance as rudiments. The cavity of the enteron disappears for some distance forwards from the point of Omugln the a'11antmS' The Flu. 243.——Fowl's egg opmuul during the seventh day. Mllllerlan ducts 1113)’ Show The body of the chick is semi dimly through the

incipient asymmetry, The, highly vascular allantois. 'l‘ho \(.'.\‘.\'(.‘lf\‘ of the

. ° ° ' allantois can he tlistitlgllisllc-<l lmm lhn.~'(* of the notochorq 18 beglnnlng to vascular area by their turning back at the edge of be consbrlcted by the Var 179'‘ the allantois while those of the vascular area pass

brace, The ﬁrst traces of onwards uninterruptedly. The highly ﬂuid charQssiﬁcation are Inaking their {‘l.Ct(‘.I: of the yolk‘ is .>‘hm\'l1 the _\'w«.»ll<-sac. wall

. . iulgmg outwards oi oi the broken shell at. the appearance, especially in the point m,.,k.,.1 «_ skeleton of the limbs. ,,,,_ .,,,,,,,toi,,_

The septum of the conus arteriosus is complete and the muscular coat extends into it from each side: the pocket-valves are becoming excavated. The fourth aortic arch on the left side has disappeared while the portion of aortic root between arches III and IV on the right side, and behind arch III on the left side, are becoming ulvlitica-1':1.tf.ed.

EIGHTH l)AY.—'_l.‘hc inoveinents of the amnion now reach their highest degree of activity. The fronto-nasal process (Fig. 245, C) is growing out to form the. pointed beak while the lower jaw is taking a siniilar pointed form, the two mandilmlar arches being new con tinua-.<l into um-. :uml<her ventrall,\' \\'il;.hout a l>1'e.ak. 'l‘he rudiments‘

of l'eathers are beginning to inake themselves apparent. In the brain the cerebellum is becoming folded on itself so as to bulge outwards, The oro-nasal grooves are covered in to form the ‘554 EMBRYOLOGY. OF THE LOWER VERTEBRATES CH.

tubular communication between nose’ and mouth. The lachrymal groove is no longer visible: the lachrymal glands are developing as solid ingrowths of ectoderm. The pituitary body now forms a rounded mass of branched glandular‘ tubes lying between the trabeculae and communicating with the buccal cavity by a narrow tubul_ar duct opening immediately over the glottis. The air-sac rudiments make their appearance on the surface of the lung (Fig. 246, C, a.s).

The mesonephric tubules have been growing actively up till now: the metanephric units are making their appearance: theMiillerian duct reaches the cloaca if it has not already done so although no actual communication is established until about six months after hatching.

Ossiﬁcation becomes conspicuous in the limb-bones and the investing bones of the head. The keel of the sternum forms an ossiﬁcation distinct from the two lateral rudiments of the body of the sternum.

The terminal sinus of the vascular area has disappeared. The septum of the conus is now completely

traversed by muscle so that both aortic and pulmonary cavi. ties are completely ensheathed by muscle. The splitting apart of the two vessels is inaugurated by the appearance of a longitudinal incision along the line of attachment of the septum. ‘

FIG. 244.—-—Chick extracted from egg during seventh day showing operculum (op).

As regards the further progress of development the following approximate times maybe mentioned.

About the ninth day the oesophagus gradually becomes patent again. On the tenth day the arterial arches have practically assumed the deﬁnitive condition and the metapodial skeleton is ossiﬁed. x rowL—.;LA'rER ‘DEVELOPMENT 555

Up to about the eleventh day. the contractions of the amnion remain very active, but thereafter they gradually become more gentle until during the closing days of incubation they stop. The mesonephros also attains to its maximum activity and there commences the process of degeneration which will continue till the time of hatching: tubules have developed throughout the length -of the metanephros. ‘

By the twelfth day the duct of the pituitary body has become reduced to a solid cellular strand: the exact time at which this happens is very variable; it may be as early as the sixth or seventh day. The lachrymal duct, which originated as a _solid ingrowth of ectoderm along the line of the lachrymal groove, now-becomes tubular. About the twelfth or thirteenth day the cavity reappears over the greater part of the rectum except just at the hinder limit of the occluded portion immediately in front of the allantois. Here the cavity remains blocked till nearly the time of hatching.

Flo. 2515.-—-View of head of Fowl embryo as seen from below. (After l‘)uval, 1889.)

A, five days; B, six days; C, eight days. ﬁn, fronto-nasal process; mac, maxillary process; olf, olfac-V tory opening; o.'n., oro-nasal groove ; sp,hyomandihu1ar cleft; V, ventricle; I, II, visceral arches.

About_the thirteenth day the cartilaginous skeleton is complete and the rudiments of claws begin to develop.

About \the ﬁfteenth day the Eustachian valve develops in the heart.

By the sixteenth day the albumen has all gone -and the yolk-sac wall becomes completed ventrally.

About the nineteenth day the yolk-sac becomes enclosed within the body-walland the partition between mesenteron and proctodaeum breaks down so that the alimentary canal communicates with the exterior. a .

About the twentieth day the umbilicus closes. The violent struggles of the young bird cause its beak to penetrate the air-space: its lungs are ﬁlled with air: its further struggles cause its beak to break the shell and it emerges, leaving behind the broken shell lined with the cast-off allantois and serous membrane.

Correlated with the, process of hatching important changes take place in the circulation? the gap in the atrial septum (foramen 556 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

ovale) becomes closed so thatthe blood arriving in the right auricle can only reach the left auricle by the circuitous route through the

Fro. 246.-—Dissections from the right side showing the general arrangement of the viscera of 9. Fowl embryo at the end of the ﬁfth (A), sixth (B), and eighth (0) days of incuba,tion_ (After Dnval, 1889.)

a.s, abdominal air-sac; all, allantois; c.a; conus arteriosus; wee, caecum; gi, gizzard; li, liver; mp, mesonephros; rr.a, right auricle; r.l, right lung; V, ventricle; y.d, yolk-stalk; y.s, yolk-sac.

right ventricle and pulmonary circulation, and the allantoic vein, duct of Botallus, and ductus venosus in the liver become obliterated. X . EMBRYOLOGY OF COMMON FOWL 557

LITERATURE

Duval. Atlas d'Embryologie. Paris, 1889.

Poster and Balfour. The Elements of Embryology. Second Edition, edited by A. Sedgwick and W. Heape. London, 1883.

Koibol und Abraham. Keibels Normentafeln zur Entwicklungsgeschichte der Wirbeltiere, II. Jena, 1900.

Lillie. The Development of the Chick. New York, 1908.

Marshall. Vertebrate Embryology. London, 1893.

Patterson. Biol. Bulletin, xiii, 1907.

Patterson. Journ. Morpli.. xxi, 1910.

The most complete account of the development of the Fowl is that by Lillie. It, and Duval’s Atlas if a copy can be obtained, for it is unfortunately out of print, should form part of the equipment of every embryological laboratory. CHAPTER XI

HINTS REGARDING THE PRACTICAL STUDY OF THE EMBRYOLOGY OF THE VARIOUS TYPES OF LOWER VERTEBRATES

AMPHIOXUS.-—The interest and importance of Amphioxus to the student of Vertebrate morphology are due to the fact of its position near the base of the Vertebrate phylum. It is true that in its adult structure Amphvlomus is intensely specialized in correlation with its burrowing habit. Further, it is necessary to recognize that a burrowing like a pelagic mode of life, in which the environmental conditions are comparatively uniform, is likely to lead to a kind of ﬁxing of the organization which will be fatal to its adaptability to new sets of conditions and consequently to its capacity for evolving along new lines. We must therefore regard it as improbable that the Vertebrata passed through an ancestral condition of specialization for a burrowing habit and the specialized features of the later stages of the life history of Amphioxus cease on that account to have a phylogenetic interest. The main interest to the Vertebrate morphologist lies therefore in the earlier stages before the specialization of the adult has developed——in such features as segmentation, gastrulation and the origin of the main systems of organs. And the interest of these stages is heightened by the fact that food yolk-— that potent disturbing factor—is present to a far smaller extent in the egg of Ampiaxioxus than in that of any other of the lower Vertebrates.

Unfortunately the known localities in which fresh ernbryological material of Amphioxus can be obtained in abundance are still few, and in most laboratories recourse must be had to preserved material purchased from supply stations such as the Naples aquarium.

The best locality so far known for obtaining developmental stages of Amphiomus is the pantano or shallow lagoon at Faro near Messina. Here the spawning takes place each evening, when conditions are favourable, during the summer months from April to July. The eggs pass to the exterior through the atriopore. If in a dish on board a boat the eggs are liable by its movements. to become distributed through the water and they are then apt to become drawn by the inspiratory current in amongst the buccal cirri. When the

558 CH. xx PRACTICAL HINTS 559

Amphioams becomes inconvenienced by such entangled eggs amongst the cirri it is able suddenly to reverse the respiratory current so as to clear them away, and in this Way there is produced a misleading appearance as if the eggs were being laid through the mouth. The ﬁrst meiotic division has been completed before oviposition while the second is in the spindle stage at this period. Fertilization probably takes place immediately, spermatozoa being disseminated through the water.

It is best (Cerfontaine, 1906-7) to bring the adults into the laboratory and wait until they spawn which operation may be considerably delayed. To a dish of pure ‘sea-water is added a little sea-water containing sperm then the eggs, collected with a pipette as soon as extruded, are added.

Batches of eggs are ﬁxed periodically, preferably in strong Flemming’s solution or Hermann’s solution. After dehydration they are placed in a mixture of 2 parts clove oil and 1 part collodion in which they may be kept indeﬁnitely. For examination whole the egg or embryo is placed on a slide or|coverslip in a drop of the clove-oil-collodion. After the specimen has been arranged in .the desired position by means of needles a drop of chloroform is applied in order to cause the collodion to solidify. The whole is then cleared with cedar oil and mounted in canada balsam. For the preparation of sections the procedure is similar, only in this case the slide or coverslip should be coated with paraffin as a preliminary to allow the collodion block to become detached, and the latter should be embedded in parafﬁn. ,

PETROMYZON.-The various species of Lamprey make their way up streams to suitable gravelly spots for spawning in the spring or early summer (April, May, in the northern hemisphere). Material

for emhryological study is best got by “stripping ” the ripe males and females 7}.e. by passing the hand back along the body with gentle

pressure so as to force out the eggs or sperm. The gametes from the male and female are collected separately in two small dishes: they are then mixed together, stirred gently with a feather, and water added. This “ dry ” method gives a smaller proportion of unfertilized eggs than when the eggs are received from the ﬁsh directly into water (Herfort, 1901). As ﬁxing agent the ordinary corrosive sublimate and acetic acid is quite satisfactory.

MYxINOIDs.——-The only Myxinoid eggs that have been obtained in any numbers are those of Bdellnstoma which are dredged near Monterey, California, on shelly and gravelly bottom at a mean depth of about 12 fathoms (Bashford Dean, 1899). Much still remains to be done in working out the details of their development but it is clear that this is of a highly peculiar and specialized type.

ELASMOBRANCHI1.-—The eggs are fertilized in the upper part of the oviduct. They may traverse the oviduct comparatively rapidly and be laid as in Birds at an early stage of development [0’h'£mae'ra, Scylliidae, Castration, Rain] or they may remain in the oviduct for a prolonged 560 EMBRYOLOGY or THE LOWER VERTEBRATES

FIG. 247.—-Blastoderm of Torpedo with meulnllary folds x 18). (After Ziegler, 1892.)

A, stage four (b'c.'unmon, 1911); B, stage six; 0, stage ten. The rounded projection near the anterior edge of the blastoder-m is the bulging I‘00f of the so-glm-IlL:1l.inlI (::|\'il.,\-'. In (I the bl()0«‘l-islamls form :1. row of (‘HlI.\‘pl(.'llIJll.*~‘. I-h-\'.'tt-inns of the :«'11rl':-we of the l)l:lh'lA)ll(‘I‘lll p:u':alls-I to its 4*Il;_{4-,

r 14: :

CH.

period and the young born in an

advanced stage [Notidam/us, Mus—'

talus, Galeus, Uarcharias, Zygaena, Lamna, Alopias, Uetorlzxinus, Acumtlmlas, Scymnus, Squatina, Torpedo, Trygonidae,Myliobatidae]. Amongst the viviparous Elasmobranchs preserved developmental stages of Torpedo (Fig. 247) may be obtained from Naples, and of Acanthias from various marine laboratories.

Amongst the oviparous forms certain species of Skate (Rania) are used as food-ﬁshes and their eggs can frequently be obtained in quantity at trawling centres. In such cases arrangements can be made with local ﬁsh-dealers to send on by post the “ skate-purses ” taken from the oviducts when the ﬁsh are cut up.‘ The eggs of the different species differ in size and in the characters of the shellshape, colour, degree of translucency (Williamson, 1913). Of the European species B. batvls is the most convenient species to use; the normal period of spawning is from December to April but the retarding effect of the low temperature i.s so great that December eggs are practically overtaken in their development by the April eggs. The complete period of development is roughly 20 months, most of the eggs hatching about August.

The eggs should be posted in damp seaweed. On arrival the soft sticky marginal zone of the shell, which separates off except at

one end and serves to anchor the.

egg to the sea-bottom, is removed, and the date is marked in ink with a wooden style upon the flat portion of shell between the two horns.

3 —u-: t ‘

1 Jamieson observed out of many thousa.°nds of eggs only one case of the inclusion

of two eggs within a common shell. XI PRACTICAL HINTS—ELASMOBRANCHII 561

For hatching boxes it is convenient to take ordinary ﬁsh boxes freely perforated with anger holes, provided with a cross partition in the centre, and pitched inside and out to discourage the growth of seaweeds. The hatching boxes are moored aﬂoat in pure sea-water within a breakwater or other shelter. About 20 eggs are placed in each compartment.

On alternate days the boxes are drawn a few times backwards and forwards through the water to dislodge any sediment that may have accumulated. Once a week they are hauled’out of the water and each egg-shell tested by rubbing the ﬁnger over its surface. If a slippery mucus-like layer has developed on its surface the egg is useless and should be got rid of.

When the egg has reached the desired period of development it is removed from the Water, placed in a horizontal position with the more strongly convex side below and opened by carefully removing the greater part of the less convex side of the shell. The isolated piece of shell must be lifted off very carefully as the albumen is very adhesive and the vitelline membrane extremely delicate.

In the early stages the embryo is almost invisible in the fresh state so the egg, still held carefully in a horizontal position, is gently submerged in ﬁxing ﬂuid. The blastoderm then comes into view and after a short time may be excised and ﬂoated into a watch-glass to complete ﬁxation and the subsequent processes.

In later stages (Fig. 248) where the body of the embryo is constricted off from the yolk—sac, it is narcotized by submersion in sea-water containing alcohol and then the yolk-stalk is ligatured with thread and the embryo excised for further treatment.

Embryological material of the Sharks is to be preferred to that of the Skates or Rays on account of their less specialized character but unfortunately it is more diﬂicult to obtain in quantity. Small sharks of the genus Scyllelum and allied genera occur commonly round the shores of the various continents and their eggs may be found attached to seaweed at extreme low tides.

' On the British coasts a well-known spawning ground for Sag/llium canicula exists at Careg Dion about 2% miles from Beaumaris on the Anglesea side of the Menai Straits in between 3 and 4 fathoms of water and in spots not exposed to strong tidal currents} The eggs are deposited usually in the morning, the shorter stouter pair of ﬁlaments which issue ﬁrst from the cloacal opening being trailed about /amongst tufts of the seaweed Halidrys siliquosa until they become entangled when the ﬁsh swims round so as to wind the elastic ﬁlaments ﬁrmly amongst the seaweed. The eggs can only be obtained at very low and specially favourable spring tides and as White ﬁnds at one time embryos of all stages of development it would appear that oviposition is not limited to any deﬁnite season. ~ Sag/llwlum not infrequently deposits its eggs in aquaria and at the

1 For the details in regard to this locality I‘ have to thank Professor Philip J. White of Bangor.

\roi.. 11 2 o 562 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Berlin Aquarium it has been observed that pairs of eggs were deposited at intervals of about ten days. The methods of technique mentioned in connexion with the Skate are also applicable to the eggs of Scyllium.

It should not be forgotten that, as mentioned earlier in this

FIG. 248.—Raia batis, embryos.

at, atrial portion of heart; E, eye; c, conus ; f.g, foregut; H, heart; l, lens; Zi, liver; ot, otocyst; pin, pineal organ ; rh, thin roof of fourth ventricle; v.c.I, etc., visceral clefts; y.s, yolk-stalk; V, VII, VIII, cranial nerves.

volume, one of the greatest desiderata in Vertebrate embryology is an oviparous shark with eggs of small size. ‘

TELEOSTOMI.-—-The most archaic and therefore the morphologically most important surviving member of this group is.Polypterus and strenuous efforts have been made to obtain ‘developmental material. Harrington lost his life on an expedition to the Nile with this object. Budgett made two expeditions to the Gambia, one to. Nigeria and XI PRACTICAL I-IINTS—-FISHES 563

the Nile, and a fourth to the Niger Delta. with the same object in view. The three ﬁrst expeditions were fruitless but on the fourth he was fortunate enough to obtain ripe males and females and to accomplish fertilization of a number of eggs. Unhappily Budgett did not live to work out this precious material, falling a victim to blackwater fever soon after his return to England. The Budgett material has been investigated (Graham Kerr, 1907) but further material is urgently needed to work out much of the detail.

On the Gambia and on the Upper Nile Budgett found females with eggs in the oviducts during July and August; in the Niger Delta during August and September. During these periods he found that at any one time only a small proportion of males had active motile spermatozoa in their urinogenital sinuses so that it looks as if the actual breeding season of each individual male were very short. The fertilizations which were successful were effected with teasedup testis, the tubules being much distended and the sperm clear instead of opaque as it frequently is. In some cases Budgett found that eggs from the splanchnocoele gave a larger percentage of successes than those from the oviduct. .

The fertilized eggs adhered strongly to the bottom of the dish and this supports the statements made by the natives that in nature the eggs are attached to sticks and stems ol' plants under the water.

Nothing is known regarding the development of the other surviving Orossopterygian-—Oalamichthys.

Of the Actinopterygian ganoids, whose haunts are more accessible

and less unhealthy than those of I’ol_2/pterus, the development has’

been worked out more or less completely in the case of each of the main types———the Sturgeon (Acipenser), the Garpike (Lepidosteus), and the Bowﬁn or Dogﬁsh (Amia). .

At the large ﬁshery stations such as those on the Elbe or Delaware Rivers ripe Sturgeons are caught during a brief season on their way into the river to spawn. The eggs and spermatozoa may be obtained by “ stripping ” the ﬁsh 73.6. by ﬁrm pressure passed backwards along the sides of the body, or by opening the ﬁsh. The eggs are immediately placed in a dish and a little of the sperm mixed with a small volume of Water is poured over the eggs, the whole being stirred gently for about ten minutes. They are then distributed in a single layer over the bottom of a submerged shallow tray made with coarse mosquito netting to which the eggs adhere ﬁrmly within twenty minutes. ‘The trays are then placed in wooden hatching boxes with gauze ends and moored in the river so that they are traversed by a constant current. The dark-coloured somewhat tadpole-like larvae hatch out in from three to six days.

Lepidosteus (Dean, 1895) breeds at Black Lake, N .Y., normally between the middle of May and the middle of June, the eggs being fertilized at the moment of spawning and being distributed over the bottom in shallow water, adhering ﬁrmly to stones and other solid 564 EMBRYOLOGY OF THE LOWER VERTEBRATES C1--I.

objects. For laboratory purposes it is best to employ artiﬁcial fertilization as in the caseof the Sturgeon. T Amia (Dean, 1896) spawns at Black Lake during the latter half of April or May. The eggs are deposited on a compact site over which the vegetation is pressed aside so as to form a clear space with about a foot of water over it. The eggs, fertilized at the moment of laying, adhere to roots or other portions of the water—plants. The rate of development as in other cases varies greatly with the

F10. 249.—Stages in the development of Symbmnchus. (After Taylor-, 1914.) our, optic rudiment; .I’.F, pectoral nu rudiment.

temperature and from four days to fourteen have been observed to elapse between the deposition of the eggs and their hatching.‘

Of Teleostei (Figs. 249 and 250) by far the most convenient for systematic laboratory work are the Salmon (Salmo salar) and the Trout (S. famlo), eggs of which can be obtained in quantity from the various hatcheries. The eggs obtained by “stripping” are fertilized artiﬁcially and may then be sent by post packed in damp moss. Small hatching boxes suitable for laboratory use can also be purchased?

The eggs and larvae of marine Teleosts are often obtained in great

1 Excellent developmental material of Lepidosteus and Anita may be obtained from the Woods Hole Laboratory or from Mr. J. C. Stephenson, Washington University,

St. Louis. 9 E.g. from the Snlway Fisheries Co., Dumfries, Scotland. but these are not so con XI PRACTICAL HINTS——-—I<‘ISHES 565

numbers in the tow-net

venient for investigation on account of their reduced size. As there is little doubt that the 'l‘eleostei have been evolved out of ancestral forms with large eggs investigations are particularly desirable on those teleosts, mostly freshwater forms inhabiting warm climates, in which the large size of the egg has been retained. There is an important ﬁeld for investigation in the embryology of tropical freshwater ﬁshes. Of individual families the Siluridae, Characinidae and Gymnotidae call especially for investigation.

DIPNOI.-— The Lungﬁshes form a group of much importance to the Vertebrate morphologist on account of, on the one hand, their great antiquity and the retention of many archaic features in their organization and, on the other hand, of the -fact that they present to us foreshadowings of various features which become prominent characteristics in the tetrapoda or terrestrial animals. A knowledge of their embryology consequently became one of the great desiderata of Vertebrate Embryology. The ﬁrst

- Fm. 250.——-Bla.stmlei‘1ns anal mubryos of Trout dlscovered of the three (Salmo fa/riu). (After Kopscll, 1898.)

surviving representatives of the gI‘0llp—-L6}9?«d0- y, oxpo.~ae(l su1'fa.ee of yolk.

Iv), eye; at, otocyst; p.f, pect.0r:ll Mn; -rh, rhombeneephalou; 566 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

s7?rem—remained unknown so far as its development was concerned until 1896 when Graham Kerr succeeded in obtaining abundant embryological material in the Gran Chaco of South America.

The developmental stages of Protopterus, the next representative of the group to become known to science, were ﬁrst obtained on the Gambia River by Budgett who had taken part in the Lepidosiren expedition a few years earlier. Ceratodus, the last of the surviving genera to become known in the adult condition, was the ﬁrst to be made known embryologically by Caldwell and Semon as already mentioned (p. 435).

The Lung-ﬁshes like other animals living under similar conditions breed at the commencement of the rainy season (Protoptems, Gambia, August; Lepidosiren, Ohaco, November but incidence of rainy season irregular and may be de1ayed—till e.g. J une-—or omitted altogether; Oeratodus, September to December). In the case of Oemtodus the eggs are scattered loosely about amongst the water plants, while in Protoptems and Lepidosiren they are deposited in a special burrow at the bottom of the swamp where they are guarded by the male parent.

¢ Dipnoans live well in captivity and there is little doubt that it will be found easy to induce them to breed by using similar methods to those described under the heading Amphibia. It is particularly desirable that this should be done in the case of Lepidosiren on account of the large size of its histological elements which make it a peculiarly suitable type for the investigation of various problems of histogenesis.

The eggs of Dipnoi, especially of Lepidosiren, are of large size and this makes it especially advisable to use celloidin in addition to paraffin methods of embedding. When paraﬂin is used it is necessary to remove the egg envelope by slitting it up with ﬁne scissors, care being taken to keep the point of the scissors close to the envelope so as to avoid injury to the surface of the egg.

Corrosive sublimate and acetic acid is a good stock ﬁxing agent. For stages before hatching 107 formalin is convenient.

AMPHIBIA.—-The most easi y obtained embryological material is that of the common Frogs of the genus Roma the masses of spawn of which are familiar objects in pools during the early weeks of spring in temperate climates. The exact time differs with climate and also with species, some species such as R. esculenta in Europe and R. catesbiana in North America lagging several weeks behind the others. The spawn, fertilized as deposited in the early morning, may conveniently be kept during its development in earthenware pans. The water should be left stagnant and unchanged during the period prior to hatching as under these circumstances the spawn is less liable to be attacked by fungus but the hatched larvae should be at once transferred to clean water.

Investigations are greatly needed on the embryology of Anura outside the genus Rana (of. Figs. 251, 252, 253 and 254). The different genera and species differ greatly in the size of the egg XI DIPNOI, AMPHIBIA 567

and its richness in yolk and there is no group of Vertebrates which oﬁ"ers anything like the same facilities for studying the inﬂuence of yolk upon the course of development. Further it will be only after greatly extended studies on different species that we shall be in a position to have a really com- prehensive idea of typical Anuran development.

Many tropical species of Frogs and Toads fire to be Flu. ._')l.—-String of eggs of unknown Frog from the Gambia. Obtalned ahve from Individual variations in the rate of developnu.-nt are indicated animal dealers and hy the varying size of the yolk-plug.

in these it may be taken as a general rule that breeding takes place at-the commencement of the rainy season, or in other words when environmental conditions become favourable after a prolonged period during which they have been unfavourable. By bear ing this principle in mind such tropical T i amphibians may usually be induced to breed in captivity. Bles in his excellent account of the life-history of Xenopus (1905) describes a method which will be found to be of general use. The pair of animals were kept in a Budgett tropical aquarium consisting of a glass bell-jar 20 inches in diameter dipping into a galvanized iron water-tank heated by a small Bunsen burner and oxygenated by plants of Vallisnemla. During summer the temperature of the water in the belljar was kept at about 25° C. The water was not changed. The frogs were fed daily with small earthworms or thin strips of raw calf’s liver until they would eat no more. In December the temperature was allowed to fall to 15°-16° during 1*‘1G-252--EI§ibI_'y0 of 1’hz/llmIwd'u~sja the day and as low as 5"-8° during the "”P"”f;::l”“l’8 "““‘°”°d °“t 1" night. As the temperature rose With the °;lﬁI1:ucca'1wity_M bmmpm_ onset of spring the frogs became more N’ mes, n1c.s'odcrnlsnigincnts. 4, actives Wilkmg “P out of the lethargic condition induced by the winter's cold. Breeding was induced by simulating the natural conditions of the rainy season. The temperature was raised to about 22° 0. Each morning and evening about two gallons of the water was drawn off, allowed to cool for twelve hours and then returned to the aquarium in the form of a fountain of spray from the upturned 568 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

end of a glass siphon drawn out to a fine point so as to produce the effect of a shower of rain. Within a week or two breeding took place.

The chief diﬁiculty in the way of cutting sections of _Frog’s eggs is due to the presence of the jelly-like envelope. This may be got rid of by prolonged soaking, six months or more, in -5% formalin (Ogushi, 1908), or by ﬁxing in Zenker’s ﬂuid and leaving the efgs 111 this ﬂuid renewing it after 2 to 3 days and continuing the treatment

FIG. 253.-—Stages in the development of ]’l:.y/llunwdusa /1.3/po¢:h¢mal7"£alz's. E, eye; e.g, external gill; op,opercu1um; oz, otocyst.

for 8 to 14 days or longer, shaking gently so as to remove the envelopes (Kallius, 1908). '

For cutting sections paraﬂin is commonly used but it should be supplemented by celloidin e.g. the clove-oil method mentioned under Ampimloxus.

In the Urodeles the eggs are commonly laid singly in water and attached to water plants (Triton) or other solid objects such as logs or stones (Proteus, Necturus). In Oryptabranchus and Amphiuma they form a beaded string, adjacent envelopes being connected together by a narrow isthmus.

Fertilization is rarely external (0'ryptob'ranchus——Smith, 1912). In the Newts the female takes up a spermatophore into the cloaca. xx ' PRACTICAL HINTS-——AMPHIBIA 569

Such internal fertilization leads up to the condition in the Salamanders where fertilization takes place in the upper part of the oviduct and the developing embryo is retained for a less or more prolonged period within the body of the parent. In Salamcmdm mooculosa larvae about an inch in length are born in May resulting from fertilization during the preceding summer. '

As in the Anura wide differences exist in the richness of yolk and consequent size of the egg—the latter varying from under 2 mm. in the Newts to 6 mm. (Necturus) or 7 mm. in diameter (Org/ptobranchus japom'cus): so that here again though not to the same extent as in

F10. 254.-—Tadpo1e of unknown Frog from Tropical Africa.

A, side view; B, ventral view. inc, huccal cavity; c.o, ('(‘.lllf‘.I|l.r organ ; rz, anus; E, eye; e.g, external gill; u/,/', olfactory organ; up, operculum.

the Anura there is an excellent ﬁeld for investiga c tion into the inﬂuence of yolk upon developmental processes. The eggs of Urodeles are commonly collected under natural conditions and kept in earthenware dishes. Or the adults just about to breed may be brought into the laboratory and allowed to deposit their eggs in a suitable aquarium;

The Urodela form one of the relatively primitive groups of Vertebrates and their embryology‘ deserves much greater attention than it has hitherto received. Most of the older literature deals with special details i.n the development of the Newts but comprehensive monographs, including “normal plates” on the development of such genera as Proteus, Siren and Amphiuma are much wanted. A general account of the development of the American species of Uryptobranohus has been given by Smith (1912), while the Japanese species has been dealt with by Ishikawa (1918), De Bussy (1915) and Dan. de Lange, Jr. (1916). Of Necturus normal plates with accompanying tables have been worked out by Eycleshymer and Wilson (1910).

The Gymnophiona—-—though an aberrant group of Amphibians highly specialized for a burrowing existence—are of much embryological interest and have provided the material for work of great morphological importance, such as that of Brauer upon the excretory organs. A general account of the development of Icltthyophis 570 EMBRYOLOGY or THE LOWER VERTEBRATES on.

will be found in Sarasin (1887-90) and of Hypogeophis in Brauer (1897).

The eggs, fertilized internally, are normally deposited in the soil and the embryologist has, as a rule, to depend upon such scanty material as can be obtained by digging in the damp soil of localities where Grymnophiona are abundant. Ty/phlonectes in South America and _De7~mophz's in West Africa are viviparous.

Of the group in general it may be said that a comprehensive monograph on the development of each genus beyond Ichthyoplmls and Hypogeoph/is is a great desidcratum.

As standard ﬁxing agents for Amphibia corrosive sublimate and acetic acid, and for the later larval stages strong F lemming’s solution, may be used. For the early stages (segmentation and gastrulation) quite good results are obtainable from eggs that have been preserved alive in 10,°/O formalin: in this case it is well to treat the egg before dehydration for an hour or two with corrosive sublimate solution as without this precaution the formalin-preserved eggs are diﬂicult to stain well. When any other ﬁxing agent than formalin is used it is necessary, as a preliminary, to remove the egg envelopes. In the case of the larger eggs of the Urodela and Gymnophiona this can be accomplished with the aid of ﬁne scissors and forceps.

REPTILIA. — For gaining practical knowledge of Reptilian development the student will ﬁnd the group Chelonia most convenient as it is possible to obtain 1 excellently preserved series of developmental stages of Terrapins (Ohrysemg/s) and Snapping Turtles (0helg/dm). In particular localities especially in warm climates he may have opportunities of obtaining the eggs of Lizards, Snakes or Crocodilians. In all cases the same technique may be used as in the

case of the Fowl. ' AVEs.—The Birds, although showing conspicuous differences in

external appearance and in minute details of structure, form a very compact evolutionary group and there is little likelihood of important differences in principle existing in their development. Interesting differences in detail however are to be found—such as the presence or absence of neurenteric canals. Groups which there is any reason to suspect of being particularly archaic——such as Divers, Grrebes, Penguins-—-are worthy of careful scrutiny for possible persistence of Reptilian features.

LITERATURE

B103. Trans. Roy. Soc. Edin., xli, 1905.

Brauer. Zool. Jahrbiicher (Anat.), x, 1897. do Bussy (do Lange), L. P. Eerste ontwikkelingsstadién van Megalobatrachcpos

Mamimus, Schlegel. Amsterdam, 1905.

Cerfontaine. Arch. de Biologie, xxii, 1906.

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

1 E.g. from Mr. J. C. Stephenson,‘Washington University, St. Louis,'or The-Marine Biological Laboratory, Wood's Hole. XI - LITERATURE 571

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

Dean, Bashford. Kupifers Festschrift. J ena, 1899.

Eerfort. Arch. mikr. Anat., lvii, 1901.

Iahikawa. Mitt. Deutsch. Gesell. Natur- und Viilkerkunde Ostasiens, xi, 2, 1908. Kalliua. Anat. Anz., xxxiii, 1908.

Kerr, Graham. The Work of John Samuel Budgott. Cambridge, 1907.

Kopsch. Arch. mikr. Ana.t., Ii, 1898.

do Lange, Dan., Jr. Onderzoek. Z061. Lab. Groningon, iv, 1916.

Ogushi. Anat. Anz., xxxiji, 1908.

Sarasin, P. and H. Ergebnisse nnturwissenschaftlicher Forschungen auf Ceylon, ii.

Wiesbaden, 1887-90.

Soammon. Keibels Normentafeln, xii. Jena, 1911.

Smith. Journ. Morph., xxiii, 1912.

Taylor. Quart. Journ. Micr. Sci., lix,\1914.

Williamson. Fisheries, Scotland, Sci. Imwst., 1912, i. 1913.

Ziegler, H. E. and F. Arch. mikr. Ana.t., xxxix, 1892.

APPENDIX

THE GENERAL METHODS OF EMBRYOLOGICAL RESEARCH

EMBRYOLOGY is one of the youngest of the sciences and it offers a wide ﬁeld for fascinating and important research. Regarded as a branch of morphology its main object is to gain information concerning the lines along which the structure of existing groups of animals has evolved. In the phylum Vertebrata there is an immense amount of work still to be done and it is important that the would-be researcher should be guided by certain general principles as to the technique of the subject, otherwise he is apt to achieve no more than the addition of relatively unimportant details to the vast accumulation of details which during the past few decades has tended to hide away general principles and incidentally to smother interest in the subject.

The incompetent or inexperienced investigator frequently betrays-himself by his choice of subject: he chooses a problem of relatively minor interest when there lie ready at his hand others which are of real importance, or he chooses a subject really important but of such difficulty that the probabilities are heavily against the feasibility of its solution under existing conditions. The beginner then should see that he has the aid of some competent adviser before he decides upon his line of research.

Having chosen his particular problem he has next to decide regarding the particular animals upon which his research is to be carried out. The earlier workers were guided mainly by the accessibility of the material. Fowls and Rabbits -—-of which embryos were easily obtained and easily investigated—--provided the material for the great pioneers of vertebrate embryology and the embryology of to-day suffers much from the difﬁculty of getting rid of general ideas founded on such narrow bases. N ow that embryology has taken its place as a branch of evolutionary science we recognize the importance of basing our general ideas upon the phenomena of development as displayed by the more primitive existing groups. In attempting any important problem of vertebrate morphology, evidence must be got from Elasmobranchs, Crossopterygians, Lung-ﬁshes, Urodeles, before we can feel completely conﬁdent as to general principles: in other words we must go to groups which are admittedly archaic. Apart from directly adaptive features an animal which is archaic in its adult structure may be expected to show primitive features in its development. Naturally we should not look for this in cases where development takes place under peculiar conditions, for these necessarily involve adaptive modiﬁcation. A pitfall into which investigators frequently stumble is that, starting from

573 574 EMBRYOLOGY- OF THE LOWER VERTEBRATES' APP.

some particular group—-say Ampluloxus, or the Mammalia——with whose structure they'happen to be thoroughly familiar, they assume its general organization to be primitive. As a matter of fact it may be assumed with considerable probability that every existing vertebrate is to a certain extent a mixture of primitive features and specialized. It is only by careful comparative study that it can be decided which features are probably primitive and it is quite certain that these will not be found all within one group. Consequently speculations based upon the intensive study of one particular group are to be distrusted, though there is always less ground for distrust if the group is one which is recognized for reasons other than embryological, as being on the whole archaic. ‘ When minute histological details are concerned another qualiﬁcation

which should be possessed by the animal chosen for investigation is large size of its cell units.

The material should be abundant. Not only should there be a continuous series of stages but there should be numerous specimens of each stage. There is no such thing as an absolutely normal individual: the conception “normal” is an abstraction based upon the observation of numerous individuals. Only by observing numerous individuals can we therefore arrive at a knowledge of normal development. Work carried out on a? few specimens may of course provide isolated observations of much interest and value but it is inadequate to serve as a basis for general conclusions.

In all descriptive embryology it is necessary to have some method of specifying the stage of development of individual embryos. Unfortunately there has been a great lack of uniformity as to the particular method of doing this. One of the most frequently used is that of specifying the period of time during which development has been going on as for example a “chick embryo of 40 hours’ incubation.” This method is quite unsatisfactory, owing to the fact that the actual stage of development of any individual embryo is a function of other factors in addition to mere time, such as temperature and individual idiosyncrasy. Thus in many tropical freshwater animals a statement of the age of the embryo is practically worthless unless accompanied by a record of the temperature, and even then there remains the unknown element of individual peculiarity such as is for example illustrated by Fig. 251 where a number of sister eggs of a Frog are seen to have “lost step” with one another to a marked extent even at a comparatively early stage of development. In other words eggs or embryos of the same age are liable to vary greatly in their degree of development, and a statement of their age is not adequate as a precise indication of the stage of development. The want of precision varies in different cases: it is less for example in a Eutherian mammal where development takes place at a fairly definite temperature than it is in a Fish or Amphibian inhabiting a tropical pool or swamp where the temperature is liable to great variation. ,

It is necessary then in referring to particular stages of development to deﬁne them by structural features. Here however a new difficulty presents itself in the fact that the relative rate of development of different organsystems is not the same in different individuals. It follows that if a. number of individuals be grouped together as being at the same stage of development as judged by a particular organ A it Wlll be found that other APP. METHODS OF EMBRYOLOGICAL RESEARCH 575

organs B, C, etc. are not exactly at the same stage of development—some are less developed some more in the various individuals. Still for practical purposes this is a useful way of indicating roughly the stage of development. For example early stages in the development of Vertebrates may be deﬁned by giving the number of mesoderm segments which have developed——these being fairly conspicuous structures and deﬁnable by a number. A much better system, however, is to use numbered stages deﬁned by the general external form——the ﬁrst structural feature met with in the examination of an embryo. Keibel has published “normal plates” of the development of various Vertebrate types in which standard stages in development are deﬁned by accurate ﬁgures. Unfortunately some of the normal plates are incomplete as regards the earlier stages during segmentation and gastrulation, but wherever the plates extend over the whole period of development they should be made use of by the working embryologist as his standard stages. Where no normal plates exist the embryologist should m_ake_it his ﬁrst business to construct one by carefully working over the external features of development and deﬁning by careful drawing and description a series of stages which he judges to be roughly equidistant. The embryology of any animal is an account of the observable changes which take place in its structure from the zygote stage up to the adult. Logically the investigation of its embryology should proceed similarly from zygote to adult but in actual practice it is better to work in the opposite direction—-—to commence by getting a clear idea of the adult organization and then to work back from the known to the unknown of earlier stages. An embryological investigation should commence with a careful study of the entire embryos or larvae at the various stages. Each stage should be examined ﬁrst alive by transmitted and reﬂected light, careful note being taken of any movements due to muscular contraction, ciliary action etc. Particular attention should be paid to the arrangement of the blood-vessels, the time of commencement of heart movements, of circulation of the blood and of the appearance of haemoglobin in the corpuscles. The appearance of chromatophores should be noted: the seat of their ﬁrst appearance and their reactions-——whether by changes of form, movement of pigment granules in their protoplasm, or by actual migration-—in

response to changes in direction or intensity of light. During this phase_

of the work constant use should be made of the binocular microscope and rough sketches should be made.

Embryos of each stage should be submitted to the action of various ﬁxing agents and it is important to watch the embryo during the process of ﬁxing, for the fluid as it gradually penetrates the tissues often makes special structures stand out distinctly for a short space of time——to disappear again with further penetration. The fully ﬁxed embryo should be subjected to further careful scrutiny by reﬂected light under the Greenough binocular. To detect small inequalities of the surface it will be found necessary to arrange the lighting carefully. The light from an mcandescent gas-mantle may be concentrated by a large condenser and caused to illuminate the embryonic surface in a tangential direction. It is often well to cover the specimen with a little house of opaque cardboard or metal resting on the stage of the microscope and possessing two apertures one in its roof through which the observation is made and one at the side through which light is admitted. The embryo must of course be 576 EMBRYOLOGY OF THE LOWER VERTEBRATES APP.

completely submerged in ﬂuid and is preferably contained in a round glass dish with a layer of pitch or black wax on the bottom in which, if necessary, small excavations can be made in which the embryo can rest securely in the desired position. The glass vessel should be rotated slowly during the observations so as to allow of the incidence of the light from different directions. It is important to observe a number, preferably a considerable number, of embryos of the same stage, as owing to individual variation particular features may be much more distinct in some than in others.

A number of thoroughly typical specimens of each stage should be picked out for further investigation and these should be carefully drawn under the camera lucida, a piece of millimeter scale being placed by the side of the embryo and drawn at the same time so as to form a reliable record as to dimensions.

' At this stage the normal plates should be constructed if not already in existence and the embryos classiﬁed in accordance with them,

For the study of internal structure the great method is that of cutting the embryo into serial sections‘ but a much older method, that of dissection, should by no means be ignored. Careful dissections made under the Greenough binocular are often extraordinarily instructive. It is advisable to experiment with embryos ﬁxed according to various methods as diffcrrent methods give diﬂ‘erent degrees of consistency, opacity etc. Van Beneden and N eyt’s ﬂuid will be found in many cases to give very good results.

In section - cutting a fetish to beware of is excessive thinness of sections. The expert section cutter is liable to become so interested in his feats in accomplishing the preparation of sections of an extraordinary degree of thinness that he is apt to forget that the criterion of good sections is not simply their degree of tenuity but the relation which their thickness bears to the size of the cell-elements of the particular embryo. Thus while in some cases it is of advantage to have sections so thin as 1 [L2 or even '5 ii, in other cases, such as segmentation and gastrulation stages of some of the large heavily-yolked holoblastic eggs, the sections should reach as much as 80 p. or 100 ,u in thickness.

Before an embryo is cut into sections its soft protoplasm has to be supported by inﬁltration with some suitable embedding mass. For this purpose the two substances used at the present time are paraﬁin of high melting-point and celloidin. Of these the ﬁrst is used frequently alone but the student should realize from the beginning that if he is to obtain reliable results, especially yvhere yolk is present in the embryonic tissues, he must use both methods and control and check the results obtained from one by those obtained from the other.

The process of inﬁltrating the embryo with paraﬂin is usually carried out in a hot-water oven heated by oil, gas or electricity and kept at a temperature just above the melting-point of the parafﬁn by a thermostat. The melted paraﬂin may be contained in small copper pans preferably plated inside with silver or nickel. An essential preliminary is a very thorough dehydration followed by a very thorough soaking in the clearing agent. To get the best results it is well to take the embryo through

1 A useful guide for beginners is.Sect7}on- Cutting by P. Jemieson in preparation. For those who already possess an elementary knowledge of the subject an .xcellent work of reference is Bolles Lee's Miicrotomicfs V ads-nwcum. .

9 1 p.=n1n millimeter. APP. METHODS or EMBRYOLOGICAL RESEARCH‘ 577

three changes each of 90% alcohol, absolute alcohol, and xylol or other clearing ﬂuid. The actual process of inﬁltration with paraffin should last for the minimum time (which will have to be determined by experiment 1) and be carried out at the minimum temperature.

It may be remembered that the complicated and bulky water-bath with its thermostat is in no way necessary for the embedding process. A very simple apparatus which is perfectly eﬂicient consists of a small metal trough (copper, or tinplate) resting upon a metal table kept heated at one end by a small ﬂame. By sliding the trough lengthwise along the table a position can be found such that the entire thickness of paraﬂin is ﬂuid at the end next the ﬂame and solid towards the other end. Between these two points stretches an inclined plane of solid paraffin upon the surface of which the embryo rests without any risk of the temperature rising appreciably above melting-point. A simple embedding trough of the kind indicated is of great use in the ﬁeld as there is no method of storing and transporting embryos so free from danger of accident or of histological deterioration as having them embedded in solid paraffin.

'I‘o get a block of parafiin in good condition for section~cutting the embryo should be transferred to a bath of fresh parafﬁu as soon as it is inﬁltrated. With certain clearing agents, e.g. cedar oil, it is well to give two or three changes of paraffin. 'l‘he vessel containing the embryo in"a considerable volume of paraflin should now he ﬂoated on cold water so as to give a homogeneoustranslucent block of solid parafﬁn. On no account should the vessel be actually submerged in the cold water for in this event the contraction of the inner paraffin as it cools within the already rigid outer layers will lead to the formation of cavities into which the water penetrates.

For the actual process of section-cutting it is necessary to use a mechanical microtome. The Cambridge Rocking microtome is one of the most convenient for ordinary enibryological work while the ReinholdGiltay ‘microtome is a most excellent instrument both as regards accuracy and rapidity of working. '

The paraﬂin block containing the embryo is trimmed down so as to be rectangular in section and is then ﬁxed by, the interposition of a hot spatula to the paraﬁined surface of the microtoine carrier in such a position as may be necessary to give the required direction of sections.

Where the object is a “diflicult” one, e.g. containing much yolk, it is advisable to have it surrounded by a paraffin block of considerable size. A considerable mass of paraffin above the specimen makes it out better, while a considerable mass to the side causes successive sections, with their long edges, to adhere better together and form a continuous ribbon. The embryo should be near one of the lower corners of the block to facilitate exact orientation.

For thorough investigation of the structure of embryos it is advisable to have specimens cut into sections in the three sets of planes-—transverse, psagittal or longitudinal vertical, and coronal or longitudinal horizontal. To obtain these it is tiecessary to have the embryo orientated exactly on the microtome. In most cases this can be accomplished with a sufﬁcieutly close approximation to accuracy when ﬁxing the paraflin block on to the

‘ E.g. for a Chick at about the middle of the second day about 20 minutes will be found to be suﬂicient. VOL. II 2 1» ' 578 EMBRYOLOGY OF THE LOWER VERTEBRATES APP.

carrier, especially if care has been taken to trim the surfaces of the block parallel to the three chief planes of the embryo.

Where greater accuracy is needed, as in the case of very small embryos, they should be arranged in position in the melted paraffin with warm needles under the prism binocular microscope. This may be done by placing the watch-glass or other vessel on the top of a small ﬂat copper cistern full of water, provided with inlet and outﬂow tubes, and heated up by contact with the top of the water-bath or hot stage. In the bottom of the embedding vessel is placed a small plate of glass on the upper surface of which are engraved parallel lines intersecting one another at right angles. When the embryos have been accurately orientated with regard to the engraved lines a stream of cold water is allowed to run through the cistern and this causes the paraliin rapidly to solidify. When the block is quite llard the glass plate is picked off and the ridges formed by its engraved lines serve as accurate guides to the position of the embryo.

Still greater accuracy is obtainable by arranging that the melted paraffin in which the embryo is being orientated is already in its deﬁnitive position on the holder of the microtome, the parafﬁn being kept melted as long as necessary by an electric current passing through a loop of high resistance wire.‘

' For the actual cutting care must be taken that the razor (solid ground) or other knife has a very ﬁne edge which does not show irregularities when examined under the low power of the microscope. The blade should be thoroughly cleaned with pure spirit before commencing work. If very thin sections, e.g. of l [L in thickness, are required it is well to commence with sections of 5 pt, then without stopping to change to 4 p., then to 3 p., then to 2 p, then to l p.——cutting a continuous ribbon throughout and going ahead rapidly when the 1 /1. sections are cutting properly.

The celloidin method should be constantly used as a check on the paraffin method. Where yolk.y eggs or embryos are being cut the celloidin method gives the only trustworthy sections as by it the yolk granules are held in position and prevented from sticking on the edge of the knife, ploughing through the tissues and destroying much of the ﬁne

detail, as is always liable to happen if paraffin alone is used under such ‘

circumstances.

In cases where there is no need for specially thin sections (say under 25 pi.) a convenient method is that in which the celloidin block is hardened

by exposure to: chloroform vapour and then cleared by immersion in cedar-wood oil.

The block of celloidin is usually ﬁxed to a block of wood which is

gripped by the holder of the microtome. Care should be taken that such wooden blocks are baked for several days so as to ensure their being

absolutely dry. Otherwise moisture will diffuse out and produce a milky opacity in the celloidin which ought to be absolutely clear and transparent. Sometimes it will be found that the block becomes too hard and will

not cut properly, its edges frilling or breaking. This is sometimes due to the presence of a trace of chloroform in the cedar oil used for clearing.

When this is the case the cut surface of the block should have perfectly pure cedar oil applied to it with a brush just before each section is ‘cut.

‘ A special apparatus for this purpose is made by the Cambridge Scientific Instrument Company. APP. METHODS OF EMBRYOLOGICAL RESEARCH 579

To obtain thinner sections it is necessary to embed the celloidin block containing the object in paraﬁin. This may be done simply by transferring the block saturated with cedar oil to melted paraffin. A better method is to use a solution of celloidin in clove oil of about the consistency of treacle. The object, thoroughly permeated by this and surrounded by a small quantity of the celloidin, is hardened and cleared in chloroform. The block is then carefully trinnned with one face accurately parallel to the plane of the required sections. It is now immersed in melted paraiiin for a minimum time (ten minutes suilices for a small object). After cutting and mounting the sections the slide is immersed in xylol ,until the paraﬁin is dissolved out, then in absolute alcohol, then in a mixture of equal parts of absolute alcohol and ether until the celloidin is removed. The slide is now taken down through the series of alcohols and the sections stained and mounted in the ordinary way.

The arriving at a clear idea of the structure of an embryo from the study of a series of sections involves fitting the successive sections together into a continuous whole. To a great extent this reconstruction of the whole from the successive sections can be done mentally but where complicated structures are being investigated, some aid- is either absolutely necessary or at least desirable for the sake of accuracy. The preseht writer finds the most reliable as well as the most convenient of such aids in the method of reconstruction by means of glass plates.‘ Successive sections are drawn with a hard (9 H) lead pencil by means of a camera lucida upon ﬁnely ground sheets of glass such as is used for photographic focusing screens and then the successive drawings are fitted together, a ﬂuid of as nearly as possible the refractive index of the glass being interposed between them so that the ground surfaces disappear and the heap of plates appears as a clear block with the structures drawn running through it and appearing as a kind of solid model.

The following details may be noted. Sections are cut to a standard thickness of 10 ,u (z'.e. T55 mm.): the glass plates are 1 mm. thick: the drawings are made at a magniﬁcation of 100 diameters. But it will be found in practice that much use can be made of the method even if these three dimensions are not so exactly correlated. The outlines made with pencil of the particular organ that is being studied are ﬁlled in with water colour. Vermilion is the most generally useful colour for it retains its opacity and light-reﬂecting properties to an unusually high degree when submerged in ﬂuid of high refractive index. When the plates are dry N o. 1 is laid, ground side up, on a ﬂat surface——_preferably a glass stage with a_ mirror beneath so that light may be reﬂeeted up through it—a few drops of the ﬂuid used, e.g. clove oil or cedar oil or a mixture of fennel oil (two parts) and cedar oil (one part) as recommended by Budgett 2 are placed by a pipette on the centre of the ground surface and then plate N o. 2 is lowered gently into position and fitted into its place over plate N 0. 1. The outlines of the drawings should be made to coincide exactly, and the two plates should be pressed firmly into contact care being taken to avoid interposed air bubbles which act as elastic cushions and prevent the upper plate from settling down into contact with the other. Successive

1 Quart. Jowm. Micr. Sea, xlv, 1902. 2 Trans. Zool. Soc. Landon, xvi, Pt. 7. 1902. 580 EMBRYOLOGY OF THE LOWER VERTEBRATES APP.

plates are ﬁtted on in a similar manner until the particular organ stands out like a solid model in the mass of plates. _

The same set of drawings may be used for different organs : the clove oil is removed by treating with strong spirit, and the water colour by holding under the tap, and then, after drying, a new organ can be coloured in. By colouring merely the cavity of an organ the relations of the cavity can be displayed as by an injection. When ﬁnally done with the drawings are removed by scrubbing with “ Monkey brand” soap.

By this method, after a little practice, reconstructions can he made with great rapidity and accuracy.

Though less accurate and much more tedious the older method of reconstructing with plates of wax is useful for building up a permanent model. Its use is also indicated where only a single specimen is available. Instead of wax plasticine may be used 1 which allows of a kind of dissection being made, in as much as particular parts of the model may be bent out of the way to display structures which would otherwise be hidden.

1n investigating the development of the skeleton the cartilage is often found to pass by imperceptible gradations into unmodiﬁed mesenchyme. The absence of sharply deﬁned surfaces in such cases makes the reconstruction method unreliable and it is advisable to supplement it by subjecting the embryo to treatment with a speciﬁc stain which picks out the cartilage while leaving the other tissues uncoloured so that the cleared and transparent specimen may be studied as a whole under the binocular microscope.

An excellent stain for this purpose is v. Wijhe’s Methylene Blue.” The embryo is ﬁxed preferably in '5% watery solution of corrosive sublimate, with 10% formalin added just before use, and preserved in alcohol. When about to be stained it should be treated for a day or two with alcohol containing :1-% hydrochloric a.cid——care being taken_ to renew this so long as it develops any yellowness due to traces of iodine. The stain consists of a solution of 1% methylene blue in 70% alcohol to which 1% hydrochloric acid has been added some time before use. The embryo is stained for a week and is then treated with 70% alcohol containing {/0 hydrochloric acid and renewed several times the ﬁrst day and thereafter once daily until no more colour comes away. The embryo is now dehydrated, cleared gradually in xylol, passed through stronger and stronger olutions of canada balsam in xylol, and preserved eventually in balsam so thick as to be solid at ordinary temperatures though liquid at 60° C.

An excellent method of cleaning small cartilaginous skeletons is to

place. them amongst Frog tadpoles which remove the muscle etc. from the surface of the cartilage by means of their oral combs.

In regard to the general principles of embryological research it need hardly be said that, as in other branches of science, accuracy of observation occupies the ﬁrst place. And yet, curiously, accuracy may become a fault. In those branches of science which are more effectively under the control of mathematics it is well recognized that in any type of investigation there is a limit of- probable error of observation-—due to instrumental or sensory imperfections or to disturbing factors of one kind or another— ‘ Harmer, Pterobranclyia of Seiboga Expedition, 1905. 9 Proceedings Akad. Wetensch. Amsterdam, J une 1902. APP. METHODS ‘OF EMBRYOLOGICAL RESEARCH 581

beyond which it is mere waste of time to push observation. In all biological observation the limit of probable error is particularly high yet this fact is peculiarly apt to be ignored and it is no unusual thing to ﬁnd dimensions or other numerical data stated to three or four places of decimals when anything beyond the ﬁrst place is worthless for the reason indicated. _

To secure accuracy of observation not merely training and experience in the art of observing is needed but also a proper psychological outlook: the observer must be able to take a completely detached point of view and must ever be on the watch to guard against some particular hypothesis or preconceived idea causing actual error instead of fulﬁlling its proper function of keeping the powers of observation tuned up to the highest pitch of alertness. '

The whole spirit and aim of scientiﬁc investigation is directed toward

‘the seriation of facts and the devising of general expressions or formulae _ which unite them together. In this it contrasts with the more primitive

state of mental development which observes isolated phenomena, noting the differences between them but blind to the common features which link them together. In embryology as in other departments of knowledge the able investigator sees the general principles which run through and organize the masses of detail: he interests himself in discovering the likeness which is hidden under superficial difference; he is constructive not destructive.

In this volume embryology is treated as a branch of morphology but it must be borne in mind that morphology and physiology are inseparably intertwined. The living body whether of an embryo or an adult is above all a piece of exquisite mechanism fitted to live and move and have its being, and to ignore this is to make morphology as sterile and as misleading as would he the study of machinery apart from the movements and functions of its various parts. More particularly in attempting to delineate the evolutionary past of .an organ, or set of organs, speculation must always be rigidly controlled by the reﬂexion that at each phase in evolution it nmst have been able to function.

When at length the stage is reached of putting results into form for publication the ﬁrst thing to aim at is absolute clearness of expression. It must be remembered that clearness of language and clearness of thought are closely interdependent. Sloppy obscure language means sloppy obscure thought. The greatest care should be taken in the correct and precise use of technical terms. Argumentation in regard to scientific and other matters is, when the disputants are equally well informed, due as a rule to some word or expression being used in slightly different senses. Elegant literary style, however desirable, must always be subordinate to clarity and precision of language. Indeed actual harm is sometimes done to scientiﬁc progress by the writer whose literary skill carries away not merely himself but others of uncritical and impressionable mind. Scientiﬁc problems are eventually settled not by skill in dialectic but by increase of knowledge.

As a rule the proper presentment of an embryological thesis involves

_ pictorial illustration. In this the elaborate coloured lithographs of former ’ days may conveniently be replaced to a great extent by simple line or

half-tone drawings in India ink ‘or process black which can be reproduced 582 EMBRYOLOGY OF THE LOWER VERTEBRATES APP.

photographically and inserted in the text in contiguity with the passage which they illustrate. Their function is to render more clear the statements of the author: they represent as accurately as possible phenomena as observed by the skilled and trained eye with a brain behind it. Actual photographs, which repr'(:sent merely details lying in one particular plane and as seen by the untrained photographic lens, should be avoided. Apart from the imperfections indicated they are so blurred by the ordinary processes of reproduction as to be liable to misinterpretation and in these days of skilful manipulation they are of course useless as guarantees of truth. Ahapical, 4 Abdominal pores, 2.31 . I (:(mUI7I(L.s'-brain, 92, 93 C110l1(l1'O(‘1‘ﬂ.1l1l1ll!, 313 egg-shell, 478 heart, 374 liver, 187 nmsclc-bucls, 208 spinal ,<,ranglia, 123 spines, 322 A ¢'(fpr')I.s'r_’1'———arcualia, 295 ccnicnt-nr,«.»,ans, 180 egg: 23 gill-lamellac, origin, 160 segmentation, 23, 24 Am-ochordal cartilage, 315 Actinopterygii-— — gastrulation, 46 nares, 125 pyloric oaeca, 191 segmentation, 19 Adrenal, 282 Ail‘-l)la.(lI.1(‘I‘ -cvolution of, 173 of Telcosts, 166, 167 Air-sacs, 164, 166 Allantoic vcin, 423 . Allantois, 212, 471, 476, 549 A lg/tes—— oviposition, 461

A mblystnmu ( = Si'rc(lo7L = Axolotl) —

arcualia, 296

eye, 141

gonad, 269, 272

vertebral centra, 300

vomer with teeth, 332 Am2'a— air-bladder, 168

cement-organs, 178, 181

segmentation, 24

vertebrae, 297, 339 Amnion, 465

evolutionary origin, 475, 476 Amniota-—

buccal cavity, 148

INDEX

(m-ferences to ﬁgures in Clarendon typo)

. Anmiota (van/£mml)——

external gills, physiological replacement

oﬂ 157 _ gastrulation, 47 liaemal arclics, 297 narcs, 126, 151 2-zclerotome, 205 Amniotic istlniiim, 469 Ampliil)ia— zulaptations in early dcvelopnient, 458

983’, '1. ‘-31 eiulolyiiiplnatic duct. 132 gastrulation, 238

gomul, 268

lIt‘ﬂ.lt.. 389

rm-.s-mlvrm, origin, 60 opistlioneplnros, 251 pharyngeal clcfts, 159 practical hints, 566 proncphros, 235 segmentation, 27 testicular network, 279 vertebral centra, 299

. I in 12/11 iu.2;u.s- -~

brain, 96

egg, 2, 3

gastrulation, 30, 31 gonad, 267

liver, 186 . iuesoderm, origin, 57 practical hints, 558 sclerotomc, 286, 286 segmentation, 7, 8, 9

Ampulla, 130 Ancestry of Vertebrata, 503 Anura.—— cement-organs, 80 horny jaws, 75, 76 oral combs, 75, 76 Anus, 51, 144, 192, 194, 497

Aortic arches, 394, 395, 396, 397, 398, 401

Apical, 4 Archencephalon, 85, 92 Arclienteron. 30

Archinephric duct, 221, 225, 238, 239, 240, ‘

241 Archinephros, 221

583 584 EMBRYOLOGY OF LOWER VERTEBRATES

Archipallium, 91 Arcualia, 292

Arterial system, 393 Auditory organ, 129, 130

skeleton, 343

Axial mesoderm, 63 Axon, 84

Balancers, 156, 157 Basilar plate, 310 B(lellostoma——egg-envelope, 456 segmentation, 16 tongue, 151 Bilateral segmentation, 9 Birds. See also under Fowl '- allantois, 471 amnion, 469 auditory organ, 129, 130 blood and vessels, 368 bursa fabricii, 192 carotid arteries, 404 chondrocranium, 314, 316 embryo with amnion, etc., 472, 473 endoderm, 51 'Ilight, origin, 453 heart, 367, 383, 384, 385 liver, 188 lung, 162, 163, 165 mesoderm, origin, 64 mesonephros, 253, 254 Inetanephros, 256 neonychia, 74 neural crest, 122 neurenteric canal, 53 otocyst, 129, 130 pancreas, 191 pecten, 140, 552 pelvis, 956 pleural cavity, 202 practical study, 508-557 primitive streak. 53, 519, 520 pronephros, 237 ureter, 257 venous system, 425 Blastocoele, 7 Blastoderm, 7 name, 505 Blastomere, 5 Blastopore, 31, 496 Blastula, 7 Blood, 365 islands, 368 vessels, 368 Bombz'nator—external gills, 156 lung, 169 pancreas, 191 with grafted limb, 115 Bone, 321, 332 evolution, 333 Bonesdental, 332, 334 investment, 335

Bones (conlimwd)—— substitution, 335 Brain, 85 anterior end of, 90 sagittal sections, 88, 93, 146, 147, 149 Branchial-— arch, skeleton, 318 buds, 177 Buccal cavity, 145 glands, 152 solid rudiment, 148, 149 Bw,fo—— cement-organs, 79 Bursa fabricii, 192

Calcareous bodies of Frog, 132 Oallurhynclms-— arcualia, 297 scales, 323 teeth, 330 Carotid arteries, 403 Cartilage, 292 Cell—continuity, 37, 486 Cement-organs of Lung-ﬁsh and Amphibians, '18, 79, 80, 181 of '.l‘e1eostomi, 178 Cephalization, 54 Gem led us— egg. 25 external form, 436 limb-skeleton, 350 lung or air-bladder, 169, 172 nares, 125 pancreas, 191 pronephros, 231 segmentation, 25, 26 teeth, 324, 330, 331 thymus, 177 Cerebcllunn, 89 Cerebral ﬂexure, 92 Corebrum, 85 Chalaza, 515 Ohalcidcs—-— placenta, 481 Chameleon-— air-sacs, 162. tooth-succession, 328

" Cheiropterygium, 356, 357

Olwlomla (= Ohelone)— amnion, 466, 467, 468 gastrular lip, 49 neurenteric canal, 52

Ohelydra, amnion, 466

Chiasma, optic, 87

Chondrocranium, 306, 317

Choroid, 139 ﬁssure, 135

Chromatophores, 80, 82

Chroniophile organs; 283

Ciliation of ectoderm, 70

Claw, 74 pad, 74, 75

Clefts, visceral (pharyngeal, branchial,

gill), 158 Olemmys, amnion, 466, 458 Cloaca, 192 Cochlea, 129 Coelome, 59, 197 Collecting tubes, renal, 247, 256, 257 Commissure—anterior, 87 posterior, 89 superior, 89 Connective tissue, 291 Cornea, 139 Cortex, 91 Crocodilia-blood-vessels, 401 carotid arteries, 404 heath 391 neonychia, 75 tooth-succession, 329 Crossopterygii (Pol3/pte*ms)-segmentation, 19 Org/ptob'ra'nchus— ‘ ciliation of external gills, 156 Cuticle, 70 Cyclostomatagenital pores, 246 horny teeth, 77, 78 olfactory organ, 128 pancreas, 189 ’

Delamination, 34 Demersal, 20

Dentine, 322, 325 Dermis, 69

Diaphragm of Birds, 164 Diplospondyly, 341 Dipnoi (Lung-iish)——

practical hints, 565

Disc, germinal, 4

Dorsal lip of blastopore, 32 Dorsal sac, 89

Ductus venosus, 539

Ectoderm, 30 name, 505

Eggenvelopes, 455, 515 size, 2 tooth, 326

Elasmobranchii (Sharks, Skates, etc.)-—

aortic arches, 398 archinephric duct, 240 arcualia, 295 blastoderms, 560

brain, 92, 93 chondroeranium, 312 chromophile organs, 284

external gills, physiological replace ment of, 157 ﬁn rudiments, 445 gastrulation, 45 gill-ﬁlaments, 160 heart, 366, 373, 374 interrenals, 283 lens, 138, 139

INDEX

Elasmohranchii (crmtz"nued)—~ limb-skeleton, 351 liver, 187 mesoderm of head, 208 motor nerve-trunk, 111 Mtilleriau ducts, 243, 244 muscle-buds, 207 neural crest, 122 opisthonephros, 247, 248 origin of mesoclerm, 63 otocyst, 132 ovary, 276 pancreas, 189 peritoneal funnels, 251 pharyngeal elefts, 158 practical hints, 559 pronephros, 236, 237 segmentation, 12, 14, 15, 29 sympathetic, 124 urinogenital sinus, 242 vasa efferentia, 279 venous system, 413 viviparity, 478

Electrical organs, 212

Embryonic development, 464

Enamel, 322, 325

Eudoderm, 30 name, 505

' secondary, 49

Endolymphatic duct, 130, 132

End-plate, motor, 204

Enterou, 58, 144, 146 development of openings, 193 muscular sheath, 217 temporary occlusion, 192

Epidermis, 70

Equatorial, 5

Eye, 133, 134, 136 evolutionary origin, 141 muscles, 210

Face, 555 * Feathers, 71, 72 Fin—rays, 346 Fins, median, 439 skeleton, 346 paired, 441 skeleton, 349, 353 Flask-glands, 79 Foregut, 144 solid, 148, 521 Fowl, common—blood-vessels, 550 early blastoderms, 519 egg, 514, 515 embryosﬂrst day, 522 second day, 524-532 third day, 532-546 - fourth day, 546-549 ﬁfth day, 548-551 sixth day, 551, 552 seventh day, 558, 554 sagittal sections, 525, 543

585 586 EMBRYOLOGY OF THE LOWER VERTEBRATES

Fowl (continued)-— segmentation, 516, 517 Fronto-nasal process, 147, 555

Gamete, 1 Ganoids, actinopterygian (A cipenser, Amia, Lepi¢losteus)— cement-organs, 180 gastrulation, 47 opisthonephros, 259 ovary, 277 pancreas, 189 practical hints, 568 pronephros, 234 segmentation, 23, 24

Ganoine, 326, 336

Gcastraea, 506

Gastrula, 30

Gastrulation, 30

Gecko—— enrlolymphatic duct, 132 gastrulation, 48, 50

Genital fold, 268 pores, 246 ridge, 267

G'mtcz'a—— pineal eyes, 100

Germ cavity, 16 layer theory, Historical note, 505 layers, delimitation, 150, 181 wall, 17

(terminal epithelium, 268

Gills——evolutionary history, 160, 419 external, 154, 155, 157, 450

as forerunners of linibs, 449

internal, 159

Glomerulus, 223

Glottis, 161

Gonad, 266

Gonocytes, 266

Grafted limb of Toad, 115

(I3/m.narclms—— egg, 20 external form, 432 gill-ﬁlaments, 160

Gyn1nophiona—external gills, 155, 156 gastrulation, 42, 43 pronephros, 223 segmentation, 28

Habenular commissure, 89 ganglia, 89»

Haemal arches, 292, 296 process, 297 spine, 297

Haploid, 17

Hatching, 459

Head fold, 521 general morphology, 498 process, 53, 521 segmentation of, 208, 501

Heart, general morphology, 370, 391

Hemispheres, cerebral, 86, 91, 95 Hippocaznpus, 91 Holohlastic, 7 Holonephros ( =archinepl1ros), 221 {I3/laexternal gills, 156 with eggs, 462 Hyostylic skull, 320 Hypobranchial, hypoglossal, muscles, 212, 213 Hypochord, 290 1Iypogeophi.e—chromophile organs, 284 external form, 437 gills, 155 gastrulation, 42, 43 interrenals, 284 opisthonephros, 246 pharyngeal clefts, 159 proncphros, 224-227 thymus, 177 llypophysis cerehri, 142

[Cht]L_1/up/I 133intestine in section, 186 modelling of intestine, 183 segmentation, 28 If/Il.¢'I)l!(, pineal eye, 97 I nfundibular gland, 90 lnfumlibulum, 86 Internielliate cell-mass, 253 Interrenal, 283 ‘ Intestine, 145, 183, 184, 185, 186 lnvagination, 32 Iris, 136 lsolecithal, 4

Jacobson’s organ, 128 Kidneys, evolutionary origin, 217, 221

Labial cartilages, 307 La('€rla—-— archenteron, 51 heart, 380 neural arches, 296 pancreas, 191 pineal eye, 97, 98, 99 tympanic cavity, 343 venous system, 422, 424 yolk-plug, 50 Lachrymal groove, 551 Lagena, 129 Lamina terminalis, 90 Larvae, evolutionary signiﬁcance, 492 Lateral line organs, 132 Latitudinal, 6 Lens, 133, 138, 139 Lepidos~.Iren—brain, 85, 86, 87, 88 buceal cavity, 149 cement-organs, 78, 79 chondriﬁcation of notochordal sheath, 293 chondrocranium, 308, 309, 311 INDEX

Lepz'dos£1'm (mntz'1meol)~~ compound nature of pharyngeal constrictor muscle, 217

eggs

external form, 434 external gills. 154 gastrulation, 34, 35, 36 heart, 375, 379

lung, 162, 170, 171 modelling of intestine, 184, 185 motor nerve-trunks, 106, 108, 109, 110 myotome, 201, 202, 203 neural rudiment, 496 olfactory nerve, 113

optic rudiment, 136 origin of mesoderm, 58 otocyst, 131

pancreas, 190

Pinkus’s organ, 133 pronephros, 231, 232, 233 respiratory limbs, 451 rods, 13?

sclerotome, 285 segmentation, 26

skull and inyotomes, 213 teeth, 324, 331

thymus, 177

thyroid, 175

venous system, 107, 409

Lcpi¢lo.cteu.s-bone and scales, 334cement-organs, 181 external form, 431 muscle-lnuls, 206

ovary, 277

segmentation, 24, 25

Limbs, 441

evolutionary origin, 443 skeleton, 349

varying situation, 448

Liver. 1°86 Lung, 161 Lung-ﬁsh (bopirlosirm, 1’/'otoptcrus, ('em!0¢lus)—

cement-organs, 78, 79 eloacal caecum, 242

egg, 25

eye muscles, 211 hypobranchial muscles, 212, 213 opisthonephros, 260 otocyst, 131

practical hints, 565 segmentation, 25, 26

' vasa efforentia, 279 Lymphatic system, 426

Macromere, 6~

Malpighian body, 227, 246, 260

Mandibular arch, 319 ridge, 145

Maxillary process, 127 ridge, 145

Medullary folds, groove, plate, 83 sheath, 85

587

Meridional, 5 Merohlastic, 7 Merocyte, 14, 17, 18 Mesencephalon, 87 lVIesenchyme, 66, 286, 506 Mesenteric arteries, 407 Mesenteron, 144 Mesentery, 199 Mesoderm, axial and peripheral, 63 general remarks on, 54, 506 lateral, 59 origin of, 65 segments, 59 of head, 208 Mesonephros, 221, 253, 254 Mesothelium, 67 Metanephros, 221, 256 Methods, embryologival, 573 Mieromere, 6 Micropyle, 457 Month, 498 Mulleriau duct (oviduct), 242 1VIusele-buds, 205, 206, 207,208 Muscles, 205 of median ﬁns, 205, 206 of paired fins, 206, 207 Myoblast, 201, 202, 203, 204 Myocoele, 202 Myoepithelial cells, 202, 204 Myosepta, 202 Myotomes, 198, 201, 203 Myxinoids—— _ pericardiac cavity, 200 segmentation, 18 spawning locality, 559

Nares, 125 external, temporary obliteration of, 129 internal, Anmiota, 151 Nasal process, median, 127 Nectu rlI.s- rih, 304 Neonychium, 74, 75 N eopallium, 91 Nephridia, 217 Nephrocoele, 198, 226

' N ephrostome, 222

Neplirotome, 198 N ephrotomes, secondary, 247 N erve development, general discussion, 113, 345

optic, 140 pineal, 97 Nerves as landmarks, 345 cranial, 122 sensory, 112 Nerve-trunks, motor, 102, 104, 105, 106, 108, 109, 110, 111, 204 Nervous system, 82 evolutionary origin, 1 18 Neural arches, 292, 294 crest, 121 spine, 296 tube, 83 588 EMBRYOLOGY OF THE LOWER VERTEBRATES

Neurenteric canal, 41, 51, 52, 53 Petromyzon (crmtinued)—-— Neurite, 84 thyroid, 176 Neurobiotaxis, 121 tongue, 151 Neuroblasts, 84 veins, 416 Neuroﬁbrils, 85, 112, 119 Pharynx, 153 Neuroglia, 84 I 1’hyllobates, 462 N euromasts, 132 [’Ia,:/llomedusa— Neuromery, 101 ' early stages, 567, 568 Neurone, 84 egg-laying, 459, 460 theory, 104 lens, 139 “Normal plates,” 575 Pigment, egg, 4 Notochord, 147, 289 skin, 80 Notatrema, 463 Pigments, melanin, 81 Nuclear division, synchronism of, 16, 17, 22 Pineal body, 87, 96 eye, 96, 97, 98. 99 Odontoblasts, 322 Pinkns’s organ, 133 Olﬁactory bulb, 91 Pipu, 461 organ, 125 Pituitary body, 90, 1-12 tract or peduncle, 94 Placenta, 465, 480 tubercle, 91 Placodes, nerve, 123 Ontogeny, general principles, 484 Planula, 506 Opaque area, 518 ' Plasmodesms, 111 Opercnlum, 158 Platybasic, 307 Opisthonephros, 221, 246 I’lat_2/clactg/[us---— Optic cup, 134 gastrular lip, 49 Qlerve, 140 gastrnlation, 48, 50 recess, 90 Pleural cavity, 202 thalamus, 89 Plexus, choroid, 91 ’Otocyst, 129 Pneumatic duct, 161 Ovary, 274 Poison-fangs, 326, 329 Overgrowth, 32 , Polar cartilage, 313, 315 Ovidnct (Miillerian duct), 242 . 1’oZ,I/pteru_s——— archinephric duct, 241 Palaeontology and Evolution, 502 blood-corpus-cles, 365 Imludicola, larval jaws, 76 brain, 85, 95, 146 Pancreas, 189 eement—org-ans, 178, 179 evolutionary origin, 19] dorsal aorta, 363, 364, 365 Pander, nucleus of, 514 egg, 3 Panizza, foramen of, 391 external form, 430 Parachordal cartilages, 307, gastrulation, 19, 32, 33 Paraphysis, 87 heart, 388 Parapineal body, 97 limb-skeleton, 353 Pecten, 552 lung, 169, 172, 173 Pectoral girdle, 354, 357 male genital duct, 281 Pellucid area, 518 olfactory organ, 127, 128 Pelvic girdle, 355, 358 opisthonephros, 258 Periblast, 23 oviduct, 278Pericardiac cavity, 200, 210 pancreas, 147, 189 Perricardio-peritoneal canal, 200 practical hints, 562 Periderm, 70 ‘ pronephros, 228, 229 Peripheral mesoderm, 63 sagittal sections, 146, 147 Peritoneal canal, 226 scales, 336 funnel, 226, 227, 251 sclerotome, 285 funnels, persistent, 251, 253 ' segmentation, 19 Peritonenm, 217 _ size of egg, 2 Pet’:-omyzon (Lamprey)—— ‘ venous system, 416, 418 arcualia. 294, 296, 299 Polyspermy, 17 gastrulation, 37, 88 ' Postanal gut, 145, 147, 195 horny teeth, 77 Postbranchial body, 178 ‘ liver, 187 Pouch, enterocoelic, 57 mesoderm segments of head, 21] Primary cell-layers, 30 pericardiac cavity, 200 Primitive groove, 53 pineal eyes, 99 plate, 48 practical hints, 559 streak, 51, 53, 495, -197, 519, 620 INDEX 589

1’rz'stim-u.9——heart, 366 lens, 139 motor nerve-trunk, 111 muscle-buds, 208 opisthonephros, 248, 260 origin of mesoderm, 63 ostium tnbae, 243 segmentation, 14, 16 Proamnion, 467 Proctodaeum, 144 Proncphric chamber, secondary, 231, 236 Pronephros, 221, 222-237 in adult Teleosts, 235 Protonephridium, 218 . P7'0t0pt('rus-— buccal cavity, 149 chonclrocranium, 308, 309, 311 larva, renal organs, 230 lung, 169, 190 olfactory organ, 125, 126 otocyst, 131 pancreas, 190 l’inkus’s organ, 133 Protostoma, 3] theory, 493 Protostylic skull, 320 Pmtovertebrae, 59 Protovertebral stalk, 198 Pseudobranch, 169 Pulmonary artery, 402 Pyloric caeca, 191 Pyriform lobe, 91

Radial segmentation, 9 Rain, (Skatc)——electrical organ, 213, 214, 216 embryos. 662 practical hints, 560 R(ma——— buccal cavity, 148 calcareous bodies, 132 cement-organs, 80 ciliation of ectoderm. 70 egg-envelopes, 459, 460 eye, 140, 141 gastrulation, 38, 40 heart rudiment, 361 larval teeth, 76 origin of mesoderm, 61, 62 renal organs, 222, 223 segmentation, 10. 11, 12, 13, 28 Recapitulation, 490, 505 Recess (of labyrinth), 129 Rectal caebum, 192 Renal ducts, evolutionary origin, 264 organs, general, 217, 261 Repti1ia—- , aortic arches, 896, 396, 397 archenteron, 51 egg-tooth, 326 embryological material, 570 gastrular lip, 49

Reptilia (continu,ed)—— gastrulation, 47 opisthonephros, 260 origm of mesoderm, 64 pronephros, 238 vertebral centra, 301 viviparity, 480

Retina, 135, 137

Rhacop/wrus, 461

Rhinodcrma, 462

Iflwdeus, air-bladder, 167

Rhombenccphalon, 85, 89

Rhomboidal sinus, 525

Ribs, 302

Saccule, 130 Salamanalm (Salamamh-r)—— heart, 389 opisthonephros, 262 rib, 304 tongue, 160 venous system, 421 vertebral centra, 300 viviparity, 480 Swlmn——— air-bladder, 168 egg, 20 gonad, 267 heart, 389 lepidotrichia, 347 opisthonephros, 259 ovary, 277 pronephros, 234 pseudobranch, 169 segmentation, 21, 22 Sarcoplasm, sarcolemma, ‘Z03 Sauropsida.—lung, 162 segmentation, 29 Scales, cycloid, 336 ganoid, 336 of Birds, 73 placoid, 321, 322Reptilian, 71 Schneiderian folds, 128 Sclerotic, 139 Sclerotome, 67, 198, 205, 286, 286, 538 Sag/llium-— chromophile organs, 284 elongated buccal opening, 147, 148 interrenal, 283 practical hints, 561 segmentatién, 14, 16 vertebral contra, 298 Segmentation; 5 abortive, 14, 29 cavity, 7 metameric, 500 of coelome, 197, 199 of head region, 500, 601 Semicircular canals, 129 Sero-amniotic connexion, 469 590 EMBRYOLOGY OF THE LOWER VERTEBRATES

Serous membrane, 469 Serranus, segmentation. 21

Sheath, medullary, 85, 112

of notochord, chondriﬁcation, 293

primitive, gray, 105, 110, 112 Shield, embryonic, 48

Siredmz (Amblystoma, Axolotl), lens, 139

Skin, 69 Skull, 306, 341 Some, 266 Somatopleure, 144 Somites, inesoblastic, 59 Spite?! oduu -—pincal eye, 97, 99 ribs, 305 vertebrae, ossiﬁcation, 338 -_ vertebral centra, 302 Spinal cord, 83 ganglia, 121 S'pinaa:—muscle-biuls, 207 occipital niyotonics, 209 visceral arch skeleton, 321 Spiracle, 158, 344 Spiral segmentation, 9 talve of intestine, 183, 184. 185 Splanchnic mesoderm, 144 Splanchnocoelc, 198 Splanchnopleure, 144 Spleen, 427 \ Spongioblasts, 84 Sternuin, 305 Stoinodaeuin, 144 Subclavian artery, 405 Subgerminal cavity, 49 Subnotoehordal rod, 290 Suprapericardial body, 178 Suprarenal, 282 S3/mbrancIms-external form, 444 opercnlar opening, 158 Symmetry of egg, 8, 11 Sympathetic, 124 Syncytium, yolk, 16, 22

Tail, autotomy in Lizards, 338 evolutionary origin, 453 form, 440

Technique of Embryology——— Amphibia, 566 Birds, 508 Fishes, 558 general, 573

Tectum opticum, 89

Teeth, 323 succession, 327

Telencephalon, 95

Teleostei (ordinary Fishes)-— air-bladder, 166, 167 brain, 95, 96 cement-organs, 182 egg, 20 ' falciform process, 140

Teleostei (contz'mwd)—— gastrulation, 46 lepidotrichia, 347

- male genital duet, 281 opisthonephros, 259 ovary, 276, 277 oviduct, 276, 278 pancreas, 190 practical hints, 564 pronephros, 234 segmentation, 19, 21, 22 separation ‘of yolk-sac, 464 urinary bladder, 242 viviparity, 479

Telolecithal, 4

Terminal sinus,‘ 527, 533

Testicular network, evolution, 279

'|‘estis, 274

'l‘halamenccpbalon 87

'l‘halamns, 89

'I‘hyinus, 177

Thyroid, 1'76 evolutionary origin, 176

Tongue, 150

Tovymtlo (Electric Ray)~— buccal opening, 1-17 ﬁn rudiments, 445 gastrulation, 4:5 heart, 366 inesoderin segments in head, 209 muscle-buds, 208 ostium tubae, 243 pronephros, 237 segmentation, 14, 15 vertebral centra, 299

'l‘rabeculae cranii, 307

'l‘rachea, 161

’I’r¢'tun (N ewt)—heart, origin, 360, 361 larvae, 157 opisthonephros, 252 origin of incsoderm, 62 rib, 304 tongue, 150 vertebral centra, 300

Trophonemata, 479

'l‘ropibasic, 307

Tropiclowwtus (European Grass Snakc)—origin of mesoderm, 64

Turbinals, 128

Tympanic cavity, 344 membrane, 344

Umbilical vein, 423 Ureter, 257

Urodela, segmentation, 29 Utricle, 130

Valvula cerebelli, 95 Vanellus, primitive streak, 53 Vase. etferentia, 272, 273, 279 Velar membrane, 145

L ‘Veliun transversum, 90 Vena. cam anterior (superior), 420 posterior (inferior), 410, 419

Venous system, 407

Ventricle, lateral, 91

Vertebra, components of, 339 Vertebrae, ossiﬁcation, 337 Vertebral centre, 293, 297 Vertical furrow, 6

Vipers, poison-fang, 326, 329 Visceral arches, skeleton, 318, 321

clefts, 158

Vitelline artery, 237, 550 Vitreous body, 135, 140 Viviparity, 478

INDEX

Wolﬂlan duct, 254

Yolk, 2, 488 colour, 3 -plug, 33 -sac, 464

-stalk, 186 -~4yncytium, 16, 22

Z ygote, l ' envelopes, 455

lamp or Von. 11

Przartrd by R. & R. CLARK, LIMITED, Edinburgh.

retraction into body, 475

591

Text-Book of Embryology 2-6 (1919)

Chapter VI Vascular System

Chapter VII The External Features Of The Body