The printable version is no longer supported and may have rendering errors. Please update your browser bookmarks and please use the default browser print function instead.

Embryology - 19 Apr 2024 Expand to Translate
Google Translate - select your language from the list shown below (this will open a new external page)
العربية \| català \| 中文 \| 中國傳統的 \| français \| Deutsche \| עִברִית \| हिंदी \| bahasa Indonesia \| italiano \| 日本語 \| 한국어 \| မြန်မာ \| Pilipino \| Polskie \| português \| ਪੰਜਾਬੀ ਦੇ \| Română \| русский \| Español \| Swahili \| Svensk \| ไทย \| Türkçe \| اردو \| ייִדיש \| Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations)

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

- Currently only early Draft Version of Text -

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Chapter IV The Coelomic Organs

Introduction

The mesoderm of A7np/w'oa;us consists in an early stage, as already indicated (p. 57), of a row of closed sacs arranged serially one behind the other" upon each side of the body. At this time the coelome of Amp/2/ioarus is in the extremely archaic condition of a series of metamerically arranged paired compartments ——a condition resembling that of the less modiﬁed forms of Annelids. The coelomic sacs gradually spread in a ventral direction until they meet. For a time after this happens the sacs of opposite sides of the body remain separated by a longitudinal partition the ventral mesentery. Similarly the apposed posterior and anterior walls of neighbouring sacs belonging to the same side of the body, form thin membranous septa like those of Annelids.

A highly characteristic difference from the Annelid arrangement begins to show itself a little before hatching in the ventral portion of the body, in as much as the transverse septa break down and disappear thus converting what was hitherto a chambered coelome in this region into a continuous space. There is no obvious reason why this loss of segmentation of the ventral portion of the mesoderm has come about in evolution. A general characteristic, however, of the phylum Vertebrata is the loading up of the ventral part of the endoderm with yolk and it may well have been that the loss of the mesoderm septa ventrally arose in correlation with the presence of a greater amount of yolk in the ancestral condition than exists in the present-day Amphioxus.

A further striking difference between the Vertebrate and the Annelid is expressed in the extent to which the coelomic wall gives rise to muscular tissue. In the Annelid practically the whole extent both of the, somatic layer lining the body-wall and the splanchnic layer covering the gut gives rise to muscular tissue. In Amphvlowus however, and the same holds for Vertebrates in general, the ventral portion of the somatic mesoderm, the portion which loses its segmental character — loses also its capacity for producing muscle.

On the other hand the dorsal portion of the mesoderm, which retains its segmentation, retains also, and to an accentuated degree, its muscle-forming capacity. It separates oﬂ‘ from the ventral or splanchnocoelic portion of the mesoderm in the form of a series of segmentally arranged sacs—-—the myotomes--— and the wall of these gives rise to almost the whole of the muscular system. The myotomes are at iirst, from their mode of origin, restricted to the dorsal side of the body, but as development goes on active growth of their ventral portions takes place and they extend downwards, overlapping and covering in the splanchnocoelie mesoderm right down to the midventral line. In this way a muscular body-wall is provided for the ventral region of the body in which the original muscle-producing capacity of the somatic mesoderm had disappeared.

The evolutionary origin of this curious secondary muscularization of the ventral body-wall of the Vertebrate is unexplained but the suggestion may be hazardcd that it was associated with the loss of segmentation of the ventral splanchnocoelie mesoderm, the primitive mode of movement of the Vertebrate———by waves of lateral ﬂexure—-— being only able to utilize longitudinal muscles divided into segments. We may take it that the splanchnocoelic muscular layer, as it lost its segmentation, would become less efﬁcient for purposes of movement, and that, correlated with this, its territory would then tend to be eneroached on by the still segmented, and therefore more eliicient, dorsal portion of the muscular layer until eventually it came to’ be replaced completely by it.

As a result of the developmental processes which have just been indicated the mesodcrm of Amplmioazms, which for a time consisted of a metameric series of paired sacs, is now represented by (1) the segmentally arranged myotomes and (2) the unsegmented splanchnocoelic lining. To these a third element becomes added in the form of a pocket-like outgrowth from the myotome wall close to its lower end (Fig. 144, A, sol, p. 285). This grows first towards the mesial plane and then dorsally, insinuating itself into the space between myotome on the one hand and notochord and spinal cord on the other, until it occupies practically the whole of that space right up to the mid-dorsal line. This pocket-like divertieulum is the sclerotome (p. 286 . ‘

In the typical Vertebrate a fourth derivative of the mesoderm segment is of importance: it takes the form of a connexion which persists for some time between the myotome and the splanchnocoelic mesoderm as a narrow stalk or isthmus. This-—the protovertebral stalk or nephrotome (Riickert, 1888) with its cavity the nephrocoele-—is of great importance from its relation to the nephridial organs but its existence has not up to the present been demonstrated in Amphiomus.

We will now proceed to trace out the subsequent fate of these various derivatives of the primitive mesoderm segments.

Coelomic Cavities

The only portions of the coelomic cavities which remain patent are the nephrocoeles (which will be dealt with later on) and the splanchnocoele or peritoneal cavity.

It may be taken as probable that the body -cavity of the ancestral Vertebrate was divided up into segmentally arranged compartments by transverse septa, and into a right and left half by a sagittally placed partition supporting the alimentary canal and forming the dorsal and ventral mesentery ; in other words that the general arrangement was like that of a primitive Annelid worm. This seems to be indicated by the mode of development of the mesoderm in A7n_ph'i09ms.

ln Vertebrates above Amplmloams the segmented condition of the splanchnocoele has disappeared even from development.‘ The sagittally placed mesentery on the other hand still appears in ontogeny in the form of the partition remaining between the edges of the lateral mesoderm as they approach one another on the ventral and on the dorsal sides of the alimentary canal respectively. In correlation with the great increase in length, and consequent coiling, of the alimentary canal of the Vertebrates—a condition which probably existed even in the ancestors of those gnathostomes in which the alimentary canal is new short (p. 184)-the ventral mesentery disappears at an early stage of development throughout that portion of its extent which lies on the tailward side of the liver.

The dorsal mesentery on the other hand persists throughout life, serving as a bridge to carry the complicated connexions of the gut wall with the vascular and nervous systems, although perforations may appear in it, more or less extensive in different groups of Vertebrates. The complicated foldings and frillings which the dorsal mesentery undergoes, owing to its enteric edge having to keep pace with the increase in length of the gut, are of interest mainly to specialists in the anatomy of particular groups and need not be dealt with here.

In the fishes, in which the lung performs an important hydrostatic function, that organ grows back in the substance of the dorsal mesentery, and in accordance with its tendency to assume a more and more dorsal position, the portion of mesentery lying above it may become incorporated in the dorsal wall of the splanchnocoele, the result being that the lung in the adult now lies entirely dorsal to and beyond the limits of the body-cavity (Dipnoi,2 Actinopterygii).

Apart from its primary segmentation, the splanchnocoele shows a tendency for special portions to become secondarily separated off from the main cavity. The most important case of this occurs at the hinder end of the heart where there exists on each side a broad bridge by which the duct of Cuvier passes from the somatopleure to the sinus venosus. This bridge becomes extended headwards and dorsally on each side of the oesophagus until it meets the dorsal wall of the splanchnocoele thus forming with the oesophagus a ﬂoor separating the anterior portion of the splanchnocoele into two cavities, one dorsal and one ventral, each opening posteriorly into the main splanchnocoele. Of these two cavities the dorsal becomes completely obliterated by fusion of its splanchnic (oesophageal) and somatic walls from before tailwards. The ventral one on the other hand roofed in by the oesophagus persists as the pericardiae cavity.

1 While it has to be granted that the splanchnococle of the Vertebrates represents the ventral portion of the coelome which has lost its segmentation, care must be taken not to assume that this loss of segmentation has necessarily extended dorsalwards to precisely. the same level in all Vertebrates. Like other anatomical boundaries the dorsal limit of the splanchnoeoele is doubtless fluctuating and vague. It is therefore wise not to attach too great importance to the exact position of the first rudiment of an organ which develops in one case on the dorsal and in another on the ventral side of the boundary between segmented and unsegmented mesoderm such as for example the gonad (p. 270).

3 Ct‘. Graham Kerr, 1910.

The communication of this posteriorly with the main splanchnocoele is obstructed in the middle by the llattened headward surface. of the liver which is embedded in the distended ventral mescntery, while laterally the communication is for a time open. As development goes on however the opening on each side becomes obliterated by an ingrowth from the somatopleure which spreads downwards from the bridge of tissue containing the duct of Cuvier and the free edge of which meets and fuses with the mesoderm covering the headward surface of the liver. The pericardiae cavity comes in this way to be bounded posteriorly by a complete wall of tissue a large part of which consists simply of the mesodermal sheath of the liver.

As the body of the embryo increases in diameter this wall of tissue keeps pace with it as does also the liver. The latter organ however in subsequent growth of its anterior or headward surface does not keep growing in continuity with the substance of the septum but becomes separated from it by a deep cleft, the region of continuity between liver and septum becoming thus restricted to a small area dorsal and close to the mesial plane. Similarly the region of continuity between the headward face of the septum and the wall of the sinus venosus which is at ﬁrst of relatively considerable dorsiventral extent becomes reduced to a narrow bridge of tissue.

In the Elasmobranchs the isolation of pericardiae cavity from the main splanchnocoele is only temporary. A median pocket-like extension of the pericardiae cavity spreads tailwards immediately dorsal to the sinus venosus in the substance of the mesodermal sheath covering the ventral surface of the oesophagus. This develops on each side a communication with the main cavity of the splanchnocoele which persists throughout life as a crescentie slit on the ventral surface of the oesophagus (Hochstetter, 1900). This secondary communication between pericardiac coelome and splanchnocoele is known as the pericardioperitoneal canal.

In Myxinoids, throughout life, and in Petromyzon, during the larval period, the rudiment of the wall separating pericardiae from splanehnocoelic cavity remains in the form of a simple bridge enclosing the duct of Cuvier so that the two cavities are in wide communication with one another.

In the Amphibia and Amniota the pericardiac cavity becomes telescoped back into the general peritoneal cavity, its hinder well becoming extended so as to form a thin membranous bag enclosing the heart and separating it from the other viscera.

Apart from the walling oil of the perieerdiac from the main peritoneel cavity there is found in the case of the Amniota a well-marked tendency for the latter cavity to undergo further subdivision, special portions becoming more or less completely wulled in by secondary fusions taking place between apposed portions of ‘the peritoneal lining.

FIG. ll0.——-Diﬁ'erentiation of the myotorne as seen in transverse sections of Lepidosiren larvae.

A, stage 30;B, stage 31 +: 0, stage 32; D, stage 35 —: E, dividing myoblasts of inner wall from stage 36. ml)’, myoblasts of inner wall; mt", myoblasts of outer wall; mf, contractile ﬂbrils; ma, vacuole. The contractile fibrils cut aemz-is

are shown as distinct black dots.

For example in Birds ‘ the mesodcrmal coating of the lung upon its ventral side becomes continuous (1) with that lining the body-wall laterally so as to enclose the portion of splanchnocoele dorsal to the lung as a pleural cavity, and (2) with that covering the surface of the liver, forming a ventral pulmonary ligament which serves to wall in a pulmo-hepatic recess lying between it and the mesentery. A third connexion, the origin of which is associated with the development of’ the abdominal air-sacs, forms the thin post-hepatic septum which stretches from the ventral surface of the lungs obliquely downwards and backwards to the ventral body-wall.

Amongst Reptiles somewhat similar arrangements exist, diFf'ering in detail in the diﬁferent groups.

The Myotomes

The developmental changes by which, in a gnathostomatous Vertebrate, the myotomes become converted into masses of muscle-fibres are excellently shown by Lzepidosiren in which the cellular elements are particularly large and distinct. In this animal the myotome is at first solid, but later on develops a small cavity or myocoele by the breaking down of its central cells. This niyoeoele soon becomes obliterated by its inner and outer walls coming together. The cells of the inner wall assume a more regular shape, taking the form of large parallelepipcdal cells (Fig. 110, A, mb’), ﬂattened dorsiventrally and stretching in an anteroposterior direction throughout the whole length of the myotome. The nuclei of these large cel1s——myoblasts or myoepithelial cclls—— divide, mitotically, so that they assume a syncytial character. Their protoplasm develops a longitudinally ﬁbrillated appearance and presently distinct cross-striped contractile ﬁbrils (mf) make their appearance in the protop1asm~—-each fibril running through the whole length of the myoblast or in other words from end to end of the myotome. The contractile ﬁbrils, which as seen in a transverse section are arranged in a :3 -shaped pattern (Fig. 110, A, onf), become more and more numerous and soon ﬁll up the inner twothirds of the myoblast almost entirely, there remaining only a relatively small amount of periﬁbrillar protoplasm between them (Figs. 110, B, and 11]).

The outer end of the myoblast does not for some time develop any contractile ﬁbrils but there appear in its protoplasm large vacuoles (vac) which form a broad clear band in horizontal sections—of much use as a landmark to indicate the outer limit of the inner wall of the myotome. The cells of the outer wall of the myotome take the form of elongated cylinders stretching throughout the length of the myotome and in their protoplasm longitudinal ﬁbrils make their appearance as in the case of the inner Wall myoblasts (Figs. 110, C ; 111, B). The longitudinal ﬁbrils become fused at their ends with connective-tissue septa formed by mesenchyme cells which wander in between consecutive myotomes.

1 For a well-illustrated account of the complicated arrangements in detail see Poole (1909).

Some such mcsenchyme cells also penetrate into the substance of the inyotome and settle down there to form. connective tissue. The cylindrical myoblasts of the outer wall undergo active multiplication (Fig. 110, C) so that it comes to be greatly thickened, composed of many layers of muscle-cylinders-—those towards the outer surface going on dividing actively while those further in towards the mesial plane increase much in size as they develop more and more ﬁbrils in their interior.

FIG. 1'.l1.—Ditl‘erent-iation of the myotomc as seen in horizontal sections of llepidosireu larvae.

A, stage 3'! ; B, stage 31 -}-. -mb’, myoblast of inner wall ; mh", myoblasts of outer wall ; m f, contractile ﬁbrils; mo, vacuoles; -y, yolk.

As the outer wall of the myotome continues to increase in thickness the myoblasts of the inner wall become relatively more and more insigniﬁcant. Eventually they divide up into musclecylinders like those of the outer wall so that it is no longer possible to distinguish the inner wall portion of the Inyotome from the outer wall part. The muscle-cylinders become the muscle-ﬁbres of the adult, the undifferentiated protoplasm between‘-tthe ﬁbrils persisting as the sarcoplasm the superﬁcial layer of which may be somewhat condensed to form the sarcolemma.

A point to be noticed, of much morphological interest, is that the inner wall myoblasts of Lepidosvlren are for’ a time (Fig. 110, A) in the form of typical myoepithelial cells such as are familiar in some of the lowest invertebrates. They are, as indicated in Chap. II., in continuity with the central nervous system by a protoplasmic tail-like extension of the cell-body closely resembling that which occurs in Nematode worms (Fig. 112). The peripheral portion of this remains as a mass of granular protoplasm on the surface of the muscle-ﬁbre—-—-the motor end-plate. The latter is therefore to be regarded as a portion of the muscle-cell which retains its protoplasmic condition rather than as a portion of the nerve-ﬁbre.

The mode of conversion of the embryonic myotome into the muscle-segment has been described as it occurs in Jlepidosvlren because of the two special safeguards against error which exist in that a11in1a1, (1) the large size of the histological units and (2) the fact that the boundary between outer and inner walls of the myotomes is marked by a clear and unmistakable landmark in the form of the vacuolar zone constituted by the outer portions of the inner wall myoblasts. lt now remains to indicate shortly the more important differFm. 112.-—Diagram of a motor ganglion- ‘Incas -111 detaill .Whmh are to be

cell in the spinal cord com,-mans found 111 descriptions of the process

through the substance of a nerve-ﬁbre as observed in others of the lower

8 muscle-cell

c.f, eoI|t_r3.c.tli.l'¢311:11.)l1l‘i'ls in 1pyoe11')ithelitl1ll‘cell; The chief of these concerns

';,t'(',‘1,’::';‘r’\‘,e_‘fu:;‘e_°” ’ ’”"’ "“"‘“°"'“’ ' ""‘ the fate of the outer Wall of the embryonic myotome. In Lep7Idosiren as has been stated the outer wall gives rise to muscle. In the case of Elasmobranchs and Granoids, Balfour stated explicitly that the outer wall of the 1]1yot0I11e similarly takes part in the development of muscle. Many authorities (Hertwig, Rabl, Maurer), however, deny that this is the case: according to them the outer wall plays no part in muscle-formation: it simply breaks up into amoeboid cells which contribute to the dermal mesenchyme. Hence these investigators term the outer wall of the myotome the “ Cutis-layer.” In the case of the Sturgeon, Maurer corroborates Balfour's statement that the myotome is composed of two layers of muscle-elements but according to him the outer layer is simply budded off from the inner and does not represent the original outer wall of the myotome as Balfour supposed. In the Amniota the myotome in early stages is almost square-as seen in a transverse section practically the whole of the wall IV MYOTOMES 205

next the endoderm representing the sclerotome. Cells proliferating from this invade the myocoele and completely ﬁll it up. It is only in later stages that the myotome becomes extended into the normal plate-like form by active growth at its inner (dorsal) and outer edges. Of the two walls of this stage the inner admittedly becomes converted into muscle-cylinders. The outer becomes loosened out into a mass of irregularly shaped cells and these are commonly believed to give rise to dermis. In view of what happens in Lepz'dos7I'rm, where accuracy of observation is so much more easily attained to, it seems advisable not to accept this as absolutely certain.

At the same time it may be allowed that there is no a. ymlori diﬂiculty in the way of admitting that portions of myotome which in one type of Vertebrate give rise to muscle, may in another have ceased to do so, for, as already indicated, a quite similar process of concentration of muscle-development in a localized portion of somatic mesoderm is a fundamental characteristic of the whole Vertebrate phylum.

The series of paired myotomes, each composed of a mass of longitudinal muscle-ﬁbres traversing it from end to end, forms the material out of which is formed the, often extremely complicated, system of voluntary muscles of the adult Vertebrate. The various myotomes as they increase in size become divided up into it may be numerous pieces and these are pushed hither and thither by processes of differential growth until the arrangement of the numerous adult muscles contrasts greatly with the simple longitudinal arrangement of the original myotomes. During the various displacements which it undergoes the individual muscle or fragment of myotome remains in organic connexion with its nerve-centre by means of its motor nerve and the course of these nerves in the adult frequently gives an important clue to the developmental migrations of the particular muscles. _

No attempt will be made here to follow out the evolution of the complicated muscular arrangements of the adult beyond a short sketch of the method in which the muscles of the ﬁns or limbs originate.

The median ﬁn is simply the extension of the body in the median plane and we should therefore naturally expect it to be muscularized by prolongations of the myotomes growing into it. The actual process is clearly illustrated in Fig. 113. In A a muscle-bud is seen to be projecting from the end of each myotome where a median ﬁn is developing-the upper group of buds belonging to the dorsal ﬁn, the lower to the anal. The buds diminish in size towards each end of the series and in the case of the dorsal ﬁn, towards its anterior end, there are a considerable number of abortive buds which never come to anything. The muscle-buds grow into the ﬁn fold and then become out off from the main part of the myotome to form the muscles of the ﬁn as is shown in B.

The paired ﬁns or limbs become muscularized by very similar, segmentally arranged, buds and it is necessary from the outset to bear in mind that this similarity need have no deeper signiﬁcance than that the paired ﬁns also necessarily obtain their muscularization from the segmentally arranged myotomes. The process as it occurs in the pelvic ﬁn of the shark Spinaw is illustrated by Fig. 114. In the 20 mm. embryo (A) the ﬁn rudiment is seen as a longitudinal ridge and a series of myotomes in the neighbourhood of this ridge are seen. each to be forming at its lower edge two projecting muscle - buds. These sprout out into the limb rudiment, assume an clon gated form and then become separated off from the myotome (0). Each bud now splits into two layers a dorsal and a ventral and each of these undergoes histological differentiation and becomes a bundle of muscle:ﬁbres—one of the radial muscles of the ﬁn: so %\ K that four radial muscles are derived from each

- _ , , in otome a dorsal and a Fm. 113.—-Muscular-ization of median mi 1!! Lepm'.I.).s'tcM.S‘. y _ .’ ( __ . _ . (After Schmalhausen, 191.2.) Ventral fro!" each of the

_ _ t _ two original buds. Such A, 1.3 mm. ; I3, :21 mm. The nmselu-buds and, in the lower . I 1 _ 1 _ . _. ‘ . figure, the llP.‘l'\'t'H (-.mu1e(_-ted with thmn are shown in black. 18 t’ l(-’ P1 Occbs In its “hum

outlines.

The existence of a disturbing complication of i this simple scheme is indicated by the adult arrangements,in as much as it can be shown that a single motor spinal nerve ('i.e. the nerve belonging to a single myotome) is related to more than the four radial muscles to which alone we should expect it to be related were the account which has just been given complete. This discrepancy is brought out particularly clearly by physiological experiments. Careful stimulation of a single spinal nerve very commonly causes three consecutive (dorsal or ventral) radial muscles to contract instead of only two, and in some cases apparently at still greater number. This seems clearly to indicate that the endorgans, in other words the muscle-ﬁbres, belonging to a particular motor nerve or myotome are in the adult not strictly confined within the limits of the two pairs of radial muscles corresponding to that motor nerve or myotome.

To those who believe in the organic continuity of muscle-cell and nerve-ﬁbre from an extremely early stage of development the idea obviously suggests itself that a shifting of some of the constituents from one muscle-bud into its neighbours takes place 1v MUSCLE-BUDS 207

during development. According to Mo1lier(1893) and Brena (1899) such a, process actually occurs. Broad enaetoxnoses or bridges make them‘ appearance connecting the varmue redial muscle rudiments

Fm. 114.——-Illustrating the omen-11132-izatior: of the pelvic Jin in .%;pirm.a-. (After Brmm, 1899.)

A, 20 mm. (70 llim-eOI1t'1‘I!1 SPgl:.8.}‘, B, .25 mm. (74 m..~:.)'. (.1. re mm. ; 1), :52 mm. The myotoxnes enindicated by Arahiu-n11nu:rals. The muscle-bode are shown in black, those within the tin rudnueut being nulnberotl with Romzm nuxnerole. The no: VE?'t]‘!lllk:-l are shown with double contour.

with their neighbours near their proximal ends. These connecting bridges persist for :2. short time and then disappear. According to the authors mentioned they are the expression of a. cellular interchange taking place between neighbouring muscle rudiments.

E. Miiller (1911) believes the connecting bridges in the case of Acant‘/mlas to be special developments of a syncytial network which lies between the buds from the commencement: he fails to ﬁnd in this animal any evidence of shiftin g of muscle-cells along the bridges. The matter appears to stand in need of further investigation. Already within the group of ]_+]l-asinobranchs we ﬁnd modification of the typical mode of Inuscularization of the ﬁns outlined above. In the case 01' the most anteriorly placed muscle-bud of the pectoral ﬁn of’ Spvinaa: the bud resolves itself into its constituent cells which separate before giving rise to muscle-cells. Again at tl1e anterior and posterior limits of the pectoral limb musculature in .Pm'stz'w/‘us and Torpedo the compact, stage of the '1inuscle-bud is eliminated entirely and the cells which muscularizc the ﬁn are budded off‘

Flu. 115.—-Diagram to illustrate the arrangement of mesoderm segments in the headregion of a. young Elaslnobranch embryo. (From a drawing by Agar.)

ut, otocyst; xplc, splanchnoeoele; 1., u, -v, w, occipital myotomes; 1, ‘.3, 3, anterior myotomes; I, II, ctc., visceral clefts; *, “ Fourth” myotome of van Wijhe.

separately from the myotome, wandering from their place of origin into the limb rudiment and there settling down (Braus).

Amongst Vertebrates outside the group ‘of Elasmobranchs such modiﬁcation appears to be the rule. Thus in Acipenser and apparently in Lacerta, typical muscle-buds arise singly from the myotomes concerned; In Teleosts Harrison ﬁnds muscle-buds in the pelvic ﬁn but a diffused origin in the pectoral. In Lung-ﬁshes and Amphibians the origin seems to be again diffuse and the same appears to be the case in Birds.

Mesoderm of the Head-Region

There are two important characteristics of the head-region of the Vertebrate ultimately connected with the muscular system: (1) loss of ﬂexibility, associated with the evolution of brain and skull and (2) special muscularity of the wall of the alimentary canal, associated with the presence of important movable skeletal structures enclosed in the substance of the visceral arches. These peculiarities ﬁnd their expression (1) in the tendency to suppression of the myotomes of the headIv MESODERM or HEAD ‘ 209

region and (2) in the retention, to a greater extent than in the trunk, of the muscle-forming capacity of that part of the mesoderm which lies ventral to the myotomes.

The mesoderm of the head-region shows the least amount of modification posteriorly where its relation to the mesoderm of the trunk is still clear. In the occipital region—the region between the otocyst and the occipital arch, which may be taken as the binder limit of the skull———-We ﬁnd a series of typical (“ occipital ”-——~ Fiirbringer) myotomes, the mesoderm ventral to which takes part in the lining of the splanchnocoele just as in the trunk-region. This series of occipital myotomes seems clearly to be undergoing a process of reduction. It is largest in such. comparatively primitive forms as Elasmobranchs. Again during ontogenetic development the series commonly shows progressive reduction. In Spinao; for example seven occipital myotomes make their appearance, but as development goes on the anterior three (t, u, 'v)1 break up and disappear, the fourth (Iw) does so incompletely, while the last three (as, 3/, 2) develop into deﬁnite muscle-segments though of small size. As each anterior myotome disappears those behind it become shifted forwards so that its place becomes occupied by the myotome originally behind it in the series. It will be realized that there is thus introduced a serious source of possible error which has to be carefully borne in mind in observations on the development of the oecipital region where the identification and correct reference of individual myotomes to their place in the series is of importance.

Anteriorly the series of occipital myotomes is prolonged forwards past the otocyst by a mass of mesoderm (" in Fig. 115) which was regarded by van W'ijhe (l883)—-who may be said to have laid the foundations of modern work upon the segmentation of the mesoderm of the head-—as the equivalent of a single (“fourth ”) myotome. It has already been indicated that the series of occipital myotomes is undergoing reduction from its front end backwards and it seems on the whole more probable that van Wijhe’s “ fourth ” myotome in the Gnathostomata is to be interpreted not as a single myotome but rather as the degenerate remnant of a series of myotomes. The number of myotomes originally present in this series does not appear to be capable of decision with any degree of certainty. Possibly it was very considerable and Froriep ﬁnds even in ontogeny (Torpedo) that during early stages (Stage “ D” of Balfour) as many as six distinct segments are recognizable in the region in question~—in other words that the series of myotomes commences not with t but with n, the anterior members of the series disappearing in turn as development proceeds. A point of

1 The hind end of the series-~-the occipital arcli——-being taken as a ﬁxed point while the front end varies, Fiirbringer has introduced the convenient method of

designating the individual occipital myotomes (or their nerves) by the terminal letters of the al ihabet-——the last being 2, the one next in front 3/, and so on. The myotomes behin the occipital arch are counted as belonging to the trunk and are

designated by numerals 1, 2 and so on (cf. Fig. 220).

interest is that the anterior limit of this series of recognizable segments agrees approximately with the anterior end of the definitive notoehord.

In front of the “fourth” myotome of van Wijhe we ﬁnd What appear to be fairly typical third and second myotomes, each continuous ventrally with the wall of the pericardiac portion of the splauchnocoele. Of these myotome III gives rise to the External Reetus muscle and II to the Superior Oblique. At the front end of the series we have the first orpremandibular or oculomotor myotome, peculiar in that it is fused with its fellow across the mesial plane and that it no longer shows any connexion with the splanchnocoelic mesoderm. It gives rise to the four eye-muscles supplied by the Third cranial nerve-«the Superior, Internal, and Interior Rectus, and the Inferior Oblique.

We have so far dealt only with the myotomes but the lateral or splanchnocoelic mesoderm is also continued well forwards into the head—region. Its more ventral portion forms the lining of the pericardi-ac cavity, while its more dorsal portion becomes traversed by the visceral pouches or clefts. The splanchnocoelic mesoderm ventral to myotomes II and III comes to form a stalk-like connexion between the myotome and the pericardiac wall (Fig. 115). This .sta1k is hollow in the case of myotome II- and lies in the mandibular arch: in the case of myotome III it is solid and lies in the hyoid arch. In both cases the wall of the stalk gives rise to the muscular apparatus of the particular arch——-—in the one case the masticatory muscles and in the other the hyoidean musculature which is destined to attain to such a development in the mammals as the musculature of the face.

The splanchnocoelic mesoderm corresponding to the myotomic mass behind myotome III (* in Fig. 115) is said to give rise to the musculature of the branchial arches. As the myotomic mass in question shrivels up during development, and the occipital myotomes move forwards to take its place, these myotomes come to overlie the splanchnocoelic mesoderm which gives rise to the branchial muscles. Consequently as will be realized the position of myotomes t, u, and v in relation to clefts III, IV, and V as shown in Fig. 115 is secondary, the myotomes having moved forwards before the formation of these clefts.

The above sketch has dealt with the cephalic mesoderm of Elasmobranchs but a similar scheme of development with minor variations in detail holds for other Vertebrates. Upon the whole it may be said that with upward progress in the evolution of the Vertebrata the segmentation of the mesoderm in the binder part of the head becomes more and more obscured. Right up to the highest forms however traces of it persist. In Fowl embryos of about the third day of incubation the series of obvious myotomes may often be seen to be prolonged forward (see Fig._ 236) by faintly visible blocks agreeing in size and exactly in series with the myotomes.

These blocks may be indistinguishable in ordinary thin sections but quite distinct in stained preparations of the whole embryo. It will require strong evidence to justify the refusal to give them the interpretation that at once suggests itself——that these slight condensations of the rnesenchyme are as it were the ghostly remnants of once existing myotomes which in Birds have ceased to become functional.

An important side issue of their presence to be borne in mind is that the slightly greater resistance of the more condensed portions of mesenchyme must necessarily exercise pressure upon the soft surface of the rapidly growing brain and produce a modelling of its surface which may be adequate to explain at least some of these appearances of segmentation of the brain-region which are included under the term neuromery. The blocks in question extend well forwards——-in the specimen

ﬁgured (Fig. 236) there are four distinguishable anterior to the middle of the otoeyst and they may be taken as additional evidence in favour of there being not one but a.number of myotomes represented in the region of van Wijhe’s “ fourth ” myotome.

It is of interest to note that in the Lampreys the blurring of the segments immediately posterior to the third of van Wijhe’s series seems not yet to have come about and there is an undoubted simple “ fourth ” myotome (Koltzoff, 1901). We may justiﬁably associate this with the low degree of cephalization in these creatures which has involved a persistence of, or more probably a reversion to, an apparently archaic condition of this myotome and its immediate successors in the series.

The relations of segments 1, II and III to the eye-muscles have been worked out in a number of Elasmobranchs and similar conditions have been described in Reptiles and Birds. Our knowledge of the holoblastic Vertebrates in this respect is still fragmentary. In the case of Lepidosiren and Protopterus the eye-muscles develop out of compact masses of mesenchyme in which it is impossible to recognize deﬁnite segments (Agar, 1907) while on the other hand in Uemtodus (Gregory, 1905) these segments make their appearance much as in Elasmobranchs.

Before leaving this part of the subject it should be pointed out that not all morphologists are convinced that segments I, II and III are actually serially homologous with the undoubted mesoderm' segments or myotomes of the trunk-region: the blurring of the mesoderm arrangements between them and the admitted myotomes, and more especially their late appearance in ontogeny, at a time when the anterior members of the occipital series have already degenerated, are brought as evidence against the more generally accepted view. The present writer does not feel inclined to attach great weight to these objections. (1) The break or blurring of the series immediately behind III seems adequately explained by the disappearance of functional muscles in this region and (2) the relatively late appearance of myotomes I to [II is explicable by the fact that the functional muscles derived from them are connected with the eyeball an organ which becomes complete and functional only at a 1'el:i.t.iw1y late pm'iod of development. p llY].‘0l3l{ANClIIAL on .lTvPoeLossA1. MUsoUJ.A'rUnE.——-In addition to the musculature already indicated the Vi‘-.rtel)raLe head possesses on its ventral side a system of hypobranchial muscles which have the appearance of a prolongation forwards of the longitudinal. muscles of the ventral body—wa1l. This hypohranchial musculature as a matter of fact does arise in ontogeny as a prolongation forwards of the anterior trunk and occipital myotomcs, as is well shown by Lepiclos-irren or .P’I"0t()7It6'}’?I.-.8’ Agar, 1907). About stage 29 the ventral ends of myotomcs 3/, z and 1 are seen to be growing out at their Ventral ’ ends into a long slender prolongation (Fig. 116). These -processes grow outwards in front of the pronephros and undergo complete fusion at their tips. The fused apical portion .c.h soon separates from the parent myotomes and grows forwards, on each side of the pericardiac cavity, until it reaches the hyoid arch. It now spreads ventrally until it meets its fellow below the pericardiac cavity. The common mass so formed becomes converted into a sheet of longitudinal muscle-ﬁbres, attached posteriorly to the shoulder girdle and anteriorly for the most part to Fm. 116. - |)ors_al view of anterior myo- the hyoid arch (coracohyoid muscle, tonnes of a Protopterus of stage 29. Fig 117’ cm._ hy) the branchial (Alter Agar, 1904.) _ . _ ’ . . «uh, ('()I‘a('.ul:_\uitl muscle‘ N notochord' ;n.f arches bfalng reduced In the ﬁshes muscle-bud Lo pectoral liinh,; )1-TL, 1)r«-iii:-plir-(is. In (l.ueSt10n' As Inuscle goes on with its development the anterior boundary of the portion belonging to myotome 1 becomes marked by a connective-tissue intersection, while in some specimens a similar intersection appears to demarcate 3/ from 2.

In other Vertebrates the hypobranchial or hypoglossal musculature appears to originate in the same way~——diﬂ'erence occurring only in the number of myotomes which take part. Five appears to be the most usual number (Scyllvlmn, Corning : Teleosts, Harrison).

Electrical Organs

The conspicuous sign of a muscle becoming active is that it changes its shape: an inconspicuous accompaniment of this change of shape is the production of a slight electrical disturbance. In the case of most electrical organs we have to do with portions of the muscular system in .which the function of contraction has been reduced to a subsidiary role or abolished entirely, while the production of electrical disturbance has become predominant.- We have here an excellent example of the principle of “substitution of functions,” which is constantly at work during evolution, the previously predominant functions of organs becoming subsidiary or falling into abeyance and being replaced by functions which were previously subsidiary.

The development of the ‘electrical organ can be conveniently studied in the case of the Skate (Ram) of which the most complete description has been given by Ewart (1888,1889, 1892). In this animal the electric organ forms an irregularly spindle-shaped body which lies embedded in the lateral muscles on each side of the tail region. It varies in size in different species and is distinguishable to the naked eye from the muscle by its more gelatinous appearance.

FIG. 117.-—-Side view of skull and myotomes of ];ep27dos-r'7'en., stage 38. (From Agar, 1907.)

Cartilage dotted, myotomes indicated by outlines, nerves black. u.n.p, antorbital process; uud, cups, auditory capsule; b'r.n, brachial nerve ; ¢..-. ,»h.n., nerve to dorsal portion of constrictor of pharynx ; cm‘. hy, coracohyoid muscles; by, hyoid arch ; I: _l/)l0{].'1t, hypoglossal nerve; M.y, M.z, M.1, M.2, ete., myotomes; -m.u.m'I, Mecke1's cartilage; -m1..~'. cups, nasal capsule; um-.. rm-ch, occipital arch; occ. rib, occipital I'll); ml, pectoral girdle; gum/, quadrute-; g/, 2, lIi!['\'¢-H2 l. 2, 1:, on-.. spinul llo'l'\'I'.\'; 2’. br, branch from 3 to hrachial nerve.

On examining transverse sections through ‘the tail it is seen that the electric organ occupies the place of the middle one of ﬁve superimposed portions into which the muscle is divided. And this clearly suggests, as Babuchin ﬁrst pointed out, that the electric organ is morphologically part of the muscular system. That this is actually so is placed beyond dispute by the facts of development. In an embryo of R. batvls about 7 cm. in length the position of the future electric organ is indicated by a slight modification of the muscle ﬁbres, inasmuch as some of these (Fig. 118, B) show a tendency to assume the shape of a club, the anterior end of the ﬁbre being slightly thickened. In contact with this thickened end is a mass of protoplasm crowded with nuclei. This represents the motor endplate which has assumed a terminal position.

In a slightly older embryo (Fig. 118, C) the club-shaped ﬁbre of the preceding stage has become further modiﬁed, the anterior end being now still thicker and the whole ﬁbre. having assumed the shape of a mac(-.. In the expanded head a portion of the mace the cross striation is becoming closer, while in the slender handle the striation is becoming blurred and in the portion next the head is almost disappearing. The end-plate forms a very deﬁnite layer of uniform thickness covering the truncated anterior end of the mace. It is crowded with large nuclei and to it pass nerve-ﬁbres which show a regular dichotomous branching. In the ﬁbre shown in Fig. 118, D, taken from the same 10 cm. embryo, the head of the mace is still more expanded as compared with the stem. The main portion of the head, in which the muscle striation had become closer, now forms a thick plate bent into a

FIG. 118.—-—Development of the electric organ of Ruin, Innis. (After Ewart, 1888.)

A Ml‘-1 '3 WU"! 41" "ml-ryes1i;:l1t-lyrwnr T um. in lvn<,:t.h: (I and I") r__rom an emlnryo about 10 cm. in le“‘n'“|§ 19. l'I'mn a .\'[u'*(_'.lIll(!ll :nlmul- H7 I'm. in lI"Il_‘.."t-ll. ml, :ul\-¢-.ol:u' laym; ::.I, o-lea-1|-it.-. layer; cl, e1€ClIl‘0l<'9'lllll«'ii I"!--./", nerve-lib:-es; .-.l, stI‘iat.ed 1.-uyvr: t. \'«-s!igi:nl 1'eIn:u'n.x' of n111.s'(:lu9.-libre.

saucer shape with its concavity posterior and composed of numerous closely packed lamellae. It forms what is termed in the fully developed organ the striated layer. On its anterior face the striated layer is covered by the end-plate, now known as the electric layer, while its posterior face is covered by a thick layer of richly nucleated protoplasm which, from the deeply pitted character of its posterior surface, is known as the alveolar layer. From this passes backwards the main part of the muscle-ﬁbre which shows symptoms of degeneration especially in the portion next the alveolar layer where it becomes vacuolated. Whether the alveolar layer represents, as seems probable, a localized thickening of the sarcolemma is not clear from the descriptions.

ln the fully developed condition (Fig. 118, E) the muscle-ﬁbre has become converted into the functional electroplax (Dahlgren, 1908) or electrical unit. What was the head of the mace in earlier stages is now expanded to form a broad thin circular disc, lying perpendicular to the long axis of the ,body—-the stem of the mace having degenerated into an apparently insigniﬁcant and functionlcss vestige (Fig. 118, E) or having disappeared entirely. The electroplax is formed of the striated layer which is almost ﬂat except round its edge where it is bent in a tailward direction. It is completely ensheathed in syncytial protoplasm, that on its posterior face forming the alveolar layer, probably nutritive in function, that on its anterior face forming the electric layer. Into the latter there pass the numerous end-twigs of the nerve-ﬁbres, the superﬁcial (Le. headward) layer showing a characteristic ﬁbrillation of the protoplasm in a direction perpendicular to the surface (nervous layer--Ewart) in contrast to the deeper portion in which the protoplasm is granular and nucleated (nuclear layer—Ewart). The taillike vestige of the posterior portion of the muscle-ﬁbre is directly continuous with the striated layer. With the latter it represents the contractile portion of the original muscle-ﬁbre, while the ensheathing protoplasm whether electric layer, or alveolar layer, or sheath of the tail-like vestige, is probably to be regarded as representing the superficial portion of the sarcoplasm.

As the muscle-ﬁbres pass through the above-described modiﬁcations, the connective tissue between them increases in quantity and becomes condensed between the electroplaxes in such a way that each electroplax becomes enclosed in a disc-shaped compartment. The walls of this ﬁt close to the electroplax round its edge while, on the other hand, the anterior and posterior walls are separated, especially the latter, by a wide space from the face of the electroplax. This space is occupied by connective tissue with sparsely scattered cells and a jelly-like appearance. That on the anterior side is traversed by the very numerous nerve-ﬁbres which branching dichotomously pass towards the electric layer, while that on the posterior side is traversed by blood-vessels.

During the earlier stages of development the electric organ increases i.n size, partly by the addiug- on to it of new electroplaxes formed at its periphery, but the marked growth which takes place in the Ol.‘;___f;LIl later on is due to actual growth of the individual units which form it. Thus comparing a skate of 180 cm. length with one of 45 cm. the indi\-'idual ele(3t1'Oplaxcs are found to have increased in size practically in the same proportion as has the body as a whole.

The above description deals with the development of the electric organ as it takes place in Rwia liatis. In other species of skate the process appears to be similar as regards its main features, but it is interesting to notice that the relative expansion of the front end of the muscle-fibre to form the electroplax is much less pronounced in certain species than is the case in R. bcmis.

Of the species so far investigated R. radiata shows the least advanced stage of evolution. In this species (Fig. 119, A) the electroplax is, as in various other species (e._q. R. celrcularis and R.

fullomca, Fig. 119), in the form of a cup rather than a disc. In 18. radiate the wall of the cup is very thick and retains throughout

life only slightly modi Flu. ll9.—~--Illustrating the adult condition 01' the electro- ﬂed muscle stnwture plax in ltaia. vrmliata, (B) R. Cirrzl/II-'/‘is, (C) Ii’. 1 it . 1 ' . ' fI.tNm4..LI_!(!,, and (D) R. b(I.ti.s'. (After Ewart, 1892.) 16 _ e ec no -ayer 13 relatively feebly devel oped, the thick alveolar layer is represented by hardly modified sarcolemma and the tail is only comparatively slightly degenerate.

The skate has been taken as the basis for the description of the development of the electric organ since the phenomena concerned have been particularly clearly worked out in this ﬁsh. In the Torpedoes the electric organ develops from muscles in the region of the visceral arches by very similar stages. As regards the electric organs of Teleosts om; knowledge is still very insiiﬂicient. _. In Mormyrids and in Gymnoms they are clearly modiﬁed portions of the lateral muscles as in the skate; in Ast')'0sc0]m.s (Dahlgren) they are believed to be derived from eye-muscles; while in ]l[a.lople'rm~us though generally believed to be modiﬁed skin glands they are believed by Dahlgren and Kepner (1908) to___ be more probably of muscular origin. Iv LATERAL MESODERM 217

LATERAL MESODERM.————’l‘he lateral mesoderm forms the lining of the splanchnocoele. Its superﬁcial layer persists throughout life as the coelomic or peritoneal epithelium, while its deep surface produces by proliferation abundant mesenchyme cells which forma connective-tissue backing to the epithelium. The development of muscleﬁbres, which is so characteristic a feature of the coelomic lining in the dorsal or myotomic region, is here to a great extent suppressed, this portion of the mesoderm no longer playing any part in the muscularization of the body-wall. It still however takes place in restricted areas, smooth or striped muscle-ﬁbres being developed in those portions of the mesoderm which invest particular organs such as heart and blood-vessels, alimentary canal with its appendages, oviduct. ‘The development of the musculature of the heart will more suitably be treated in the chapter dealing with the vascular system. As regards the muscles of the gut-wall we have little detailed knowledge, what there is being related mainly to the musculature of the skeletal elements contained in the visceral arches.

In the larva of Lepidosiren the important point has been established by Agar (1907) that the sheath of muscle which forms the constrictor of the pharynx is of double origin, its ventral and larger portion being a development of the splanchnic mesoderm covering the pharynx, while its dorsal portion arises as an outgrowth from one (3/) or more of the occipital myotomes. The fact that muscular tissue derived from myotomes may join the splanchnic muscle to form part of the muscular sheath of the alimentary canal is of importance (1) by impressing upon us that an apparently homogeneous muscular apparatus may really be heterogeneous —muscularized from two quite distinct sources, and (2) by indicating the possibility of splanchnic musculature being replaced by myotomic or conversely. Obviously the muscular sheath just mentioned might, by reduction of one or other of its component parts, become purely myotomic or purely splanchnic. As will be gathered later (Chap. VII.) the point is an important one from its bearing upon the discussion of certain problems of morphology.

RENAL 0RGANS.—-In triploblastic Metazoa the function of excreting nitrogenous waste products is commonly carried out by tubular organs to which Lankester (1877) gave the name nephridia. Under this term were included the excretory tubes of Chaetopods, Molluscs, Rotifers, Trematodes, Turbellarians and Vertebrates. Subsequent research soon brought to light an important structural diﬁ"erence as regards. the inner ends of these nephridial tubes in different groups of animals. In certain groups the tube possesses at its inner end an open funnel or nephrostome 1 which leads from the coelome into the cavity of the tubule, while in other groups the inner end of the tubule is without any coelomic funnel but is on the other hand provided with an arrangement of ﬂame 1 Goodrich terms such funnels “ coe1omostomes” and uses the word nephrostome in a special restricted sense. 218 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

cells. 'l‘he appreciation of this difference gave rise, not unnaturally, to a suspicion —-which would now appear to be unfounded——-that under Lankester’s name nephridium were included excretory tubes of two morphologically distinct types and the use of the word nephridium was often restricted to the one of these types in which the coelomic funnel was present.

Later researches brought out the fact that in some cases certain Polychaete worms——-the excretory tube may possess both flame-cells and coelomic funnel. And ﬁnally the hypothesis was developed———by Meyer and especially Goodrich (l895)—~—that the nephridial tube and the coelomic funnel were originally quite distinct organs with separate openings to the exterior. On this view the primitive excretory tube or protonephridium (Goodrich) was provided with ﬂame-cells at its inner end, while apart altogether from it and opening independently to the exterior was the coelomic funnel which formed the primitive exit for the reproductive cells. In the course of evolution there came about a fusion of the two structures, the

._ coelomic funnel becoming as it were grafted on to the nephridium

and in many cases shifted up the wall of the tubule right to its inner end. Such‘ a compound organ (N ephromixium— Goodrich) might retain for a time both ﬂame-cells and coelomic funnel-—-as in the Polychaetcs alluded to above—or the ﬂame -cells might, as is more usual, disappear leaving an excretory tube possessing at its inner end a coelomic funnel which shows no trace of its morphologically independent origin. To support the hypothesis which has just been outlined there is; brought in the evidence of embryology which testiﬁes (see Vol. I. p. 158) that the main part of the excretory tube is developed as an ingrowth of the ectoderm, while the coelomic funnel arises as an outgrowth of the mesoderm. «

This hypothesis has met with very general acceptance not merely with regard to the excretory organs of Annelids alone but also as a theory of the morphology of excretory tubes in general. As, however, the writer of this volume takes up a somewhat different standpoint it will now be necessary to' state shortly what that standpoint is.

The Word nephridium will be used in the original sense as meaning an excretory tube whether possessing ﬂame -cells or a coelomic funnel at its inner end.

Physiologically the open funnel and the ﬂame-cell appear to be associated primarily with two different sets of spaces. The funnel is associated with coelomic spaces and it serves to transmit to the exterior the products of the lining of such spaces—-ﬂuid, excretory, or reproductive. The ﬂame-cell is associated rather with the meshes of the (mesenchyrnatous spongework :1. it serves to ﬁlter off from these spaces watery ﬂuid containing excretory salts in solution. The activity of the “ﬂame” is in direct relation to the pressure of ﬂuid Within these spaces: if the pressure is.lowered by making a minute puncture in the body-wall the movement at once ceases——to IV RENAL ORGAN S 219

commence again when pressure is restored. This association of flame-cells with the spaces of the mesenchyme is seen in the more lowly forms in which they occur and it is therefore justiﬁahle to regard it as primitive in spite of the exceptional cases in which ﬂame-cells occur in coelomic cavities.

The question of the relative antiquity in evolution of coelomic funnel and ﬂame-cell is one which cannot be decided with certainty, depending as it does in tur11 on the unsolved question as to whether coelome or mesenchyme was evolved ﬁrst. Looking to the occurrence of mesenchyme in Coelenterates (ag. the Alcyonarians), in animals in which there is not as yet any closed-off coelome, the balance of probability seems to be on the whole in favour of the ﬂame-cell having originated first, in other words in favour of the Original nephridium being of the type called by Goodrich protonephridium. The fact that existing excretory tubes of this type arise from the ectoderm is also an argument for its antiquity, as it seems natural to suppose that priniitively excretory products were got rid of at the outer surface of the body.

In the primitive ancestral form the genital cells, formed by the lining of the enterocoelic pouches, would reach the exterior through the protostoma or primitive mouth but as evolution proceeded and the coelenteric pouches became separated from the enteron to form a closed coelomel another mode of exit would have to be evolved. The natural mode of such exit would be by rupture of the coelomic wall at its weakest spot. Such Weak spots would be provided at points where the cavity of the nephridial tube came into proximity with that of the coelome. At such points rupture would take place and the tendency would be for such a temporary rupture, at the time of maturity of the genital cells, to be replaced by a permanent ‘-’ opening from coelome into nephridium. This permanent opening would be the coelomic funnel (coelomostome, nephrostome in the original sense).

The coelomic funnel, though originally developed‘ to transmit the genital cells, would necessarily also serve as an exit for superﬂuous coelomic ﬂuid, and the ﬂuid so transmitted would necessarily serve incidentally to ﬂush out excretory matters passed into the lumen of the tube by the activity of its walls and would thus fulﬁl the function originally fulﬁlled by the fluid drawn in by the ﬂamecell. The function of the ﬂame-cells being in this way otherwise provided for they would tend to disappear.

The nephridial tube thus came to transmit (1) the reproductive cells and (2) the highly poisonous excretory products. There is, as it appears to the writer, ample evidence that under such circum 1 The argument involves as will be seen the assumption that the coelome was in its evolutionary origin enterocoelic. This assumption appears to be justified by the numerous cases in which the coelome so arises in ontogeny.

'3 An actual case where such a temporary rupture, brought about at parturition, has come to be a ﬁxed character of the species and develops independently of mechanical rupture, is seen in the “ median vagina” of certain Marsupials. 220 EMBRYOLOGY OF THE LOWER VERTEBRATES CI-I.

stances the tendency of subsequent evolution would be to separate from one another the paths to the exterior of the genital cells and of the poisonous excretory products respectively. It might fairly be anticipated for physiological reasons that there would be such a tendency but that it actually exists is demonstrated by the facts of comparative anatomy and embryology. Over and over again we ﬁnd cases where such separation has undoubtedly come about. For example in the evolution of the (lasteropoda the right nephridium has lost its excretory function and come to be merely a genital duct. In Vertebrates there are several familiar examples of parts of the renal system which have to do with transmitting genital cells becoming separated of?’ from those which retain a renal function.‘

The present Writer then’ believes the balance of probability to be in favour of the evolutionary origin of the type of nephridial tube commonly met with in coelomate animals, possessing a coelomic funnel or nephrostome at its inner end, having come about in the manner outlined above. The essential difference between the view here outlined and that developed by Goodrich is that it rejects the idea that evolution has brought about a more and more intimate connexion between originally independent genital funnel and nephridial tube as opposed to physiological probability. On the contrary it regards the funnel as having opened into the tube at the time of its ﬁrst appearance, the progress of subsequent evolution having been in the direction of separating genital funnel and nephridial tube and not of uniting them. Even in the ease of Polychaete worms the arguments against interpreting the anatomical arrangements in diﬁ"erent genera as illustrating evolutionary sequence in the reverse order to that believed in by Goodrich seem unconvincing and insuﬂicient to counterbalance the weight of physiological probability.

In the case of a tube leading from the coelome to the exterior the two ends are almost of necessity mesodermal and eetodermal in their nature respectively. Consequently the fact that the “nephromixium” has such a twofold origin in ontogeny does not appear to the present writer to constitute evidence of any particular weight that it actually arose in phylogeny by the fusion of two preexisting independent organs. As regards the proportion derived from the two layers the probability would be that the specially excretory portion was originally ectodermal——excretory products being naturally got rid of by the outer surface—and that the portion specially concerned with the getting rid of coelomic products would be mesodermal—-arising as a bulging of the coelomic lining.

Accepting as a working hypothesis that the nephridial system of tubes with their nephrostomes arose in the manner outlined above, it is important to bear in mind how greatly the system would be inﬂuenced in its subsequent evolution by the establishment of

”.'E.g.'«' the separation of the Miillerian duct from the kidney system or the separation of the renal collecting tubes from the Wolfﬁan duct. Iv , N EI’HRIDIAL' ORGAN S 221

circulating mesenchyme or blood. This would render possible the shortening up of the nephridial tubes and the more deﬁnite localization of the excretory tissue. Whereas the original ﬂame—cell type of excretory apparatus was d.iﬂ'use——the ﬂame-cells being scattered throughout the mesenchyme sponge-work———as it is still to be seen in the more lowly organized forms -it would now become compact, the waste-products being brought to it by the movements of the circulating blood.

N EPHRIDIAL ORGANS or V Ea'rEBnA'rEs.——Before passing on to the details of development of the renal organs in Vertebrates it is necessary to notice one or two points of general importance regarding the morphology of these organs within this particular phylum, and also to deﬁne precisely the sense in which certain technical terms will be used.

In the ﬁrst place the kidneys or renal organs of Vertebrates are built of tubules each of which is a nephridium according to the original deﬁnition of the term.

The conclusion, already arrived at, tllat the ancestral Vertebrate ~

possessed a completely segmented coelome, carries with it the further conclusion that in all probability a pair of nephridial tubes originally opened to the exterior from each segment. A characteristic feature however of the Vertebrates (with the exception of A7nph'£0:1c'us)‘is. that the nephridia open not directly to the exterior on the surface of each segment as in a typical Annelid but into a longitudinal duet which passes back along each side of the body and communicates at its hind end with the cloaca. The whole series of nephridial tubes on each side of the body is known as the archinephrosl and the duct as the archinephric duct.

In the embryos of Vertebrates development takes place from the head end backwards. We should therefore expect the nephridial tubules to appear in regular sequence from before backwards. It is however highly characteristic of the Vertebrate that the tubules, instead of developing in this regular sequence, develop in three batches one behind the other———an anterior, a middle, and a posterior. These constitute respectively the pronephros, mesonephros, and metanephros (Lankester, 1877). In many of the lower Vertebrates there is no separation between mesonephros and metanephros, the two forming a continuous structure which acts as the‘ functional kidney. Such a type of renal organ consisting of the series of tubules corresponding to mesonephros together with metanephros may conveniently be termed the opisthonephrosﬁ

Of the four types of kidney just mentioned the first-——the pronephros-—forms the functional kidney in larval Vertebrates. It is well seen in the larvae of Crossopterygians, Actinopterygians, Lungfishes, and Amphibians, while, as Sedgwick ﬁrst pointed out, it is

1 Archinephron, Lankester (1877). l’rice’s term Holonephros is also frequently

used in the same sense. _ _ ’ In analogy with the use of the word opisthosoma In the group Arachnida. 222 EMBRYOL()GrY OF THE LOWER VERTEBRATES CH.

reduced in forms with richly yolkcd eggs where the development is not larval. Its reduction or disappearance in the last-mentioned forms may probably be due to the facilities afforded for getting rid of excretory products by simple dill'usion from the blood circulating on the surface of the yolk-sac into the surrounding medium.

The opisthonepliros f0_rms the functional kidney in the adults of most if- not all anairlnilatic Vertebrates.

Distinct mesonephros and metanephros are found in the Amniota --the mesonephros being functional during the later embryonic period and in the Reptiles during the ﬁrst few months after hatching,

FIG. 120.———Rena1 organs of the Frog (Rana temporaria) as seen from the ventral side after the ventral wall of the splanchnocoele and the portion of the alimentary canal contained within it have been removed. (After Marshall, 1893.)

A, 12 mm. tadpole. ; 13, -10 mm. tadpole; C, frog at time of metamorphosis. A, dorsal aorta; Am, nor! in root; Iv‘, fatty body; gl, glomerulus; up, ()piStlHnlupl|l'()s; -)1», pI'ull(*.p]n'ns_

while the metanephros forms the deﬁnitive kidney of the adult, its excretory activities being reinforced during the ﬁrst few months in the case of Reptiles by the still functional mesonephros. _ THE P.RONEPHROs.—-—A typical functional pronephros is well seen in a frog tadpole of about half an inch in length (Fig. 120, A, gm). It consists of a massive organ lying dorsal to the anterior portion of the splanchnocoele on each side. It consists mainly of a much convoluted tube the anterior portion of the archinephric duct, and into this there open three segmentally arranged pronephric tubules also, except the anterior one, much coiled and twisted. While the organ as a whole is retroperitoneal, vie. outside the coelomic lining, there exists an opening leading from the splanchnocoele into each tubule ———the nephrostome (Fig. 121, ns). Duct and i3'l1l'_)l'lleS are lined with cubical, or almost columnar, epithelium and in the neighbourhood IV PRON EPHROS 223

ot' the nephrostome the cells become pigmented and carry powerful flagella. At the lip of the nephrostome the lining epithelium of the tubule is continued into the ﬂattened epithelium lining the sp1anchnocoele which is richly ciliated in its immediate neighbourhood.

The archinephric duct is continued back from the pronephros along the coelomic roof to open at its hinder end into the cloaca.

The pronephros has very characteristic relations to the bloodvascular system. The tubules serve to transmit to the exterior the ﬂuid secreted by the coelomic epithelium and a patch of this epithelium, lying on the roof of the splanchnocoele at its mesial side and facing the nephrostomes, has its secretory activity much exaggerated. This specially secretory epithelium has its area increased by bulging into the splanchnocoele, the bulging portion enclosing a vascular skein connected - with the aortic root. This bulg-‘X I/, ing structure is known‘ as the glomerulus (Figs. 120 and 121, gl). The anterior convoluted part of the archinephric Z. . duct and the tubules " opening into it have other relations to the vascularsystem,fortheir surface is bathed by the blood of the posterior cardinal sinus which

FIG. 121.—~Transverse section through a 12 mm. Tadpole at the level of the pronephros. (After Marshall, 1893.)

forms a system of ir , gl, glommulm; mf, mlv‘sl)ln0; I, lung; li, liver; M, medulla regulal Spac.eS between oblongata; .\, uotm-luml. us, nephrostome; oos, oesophagus; them. T1118 d011b1e up, operculum; gm. })I‘0lu-pllros.

relation to the blood system is doubtless correlated with the double function of the organ. It serves in the first place to get rid of watery ﬂuid secreted by the coelomic epithelium, and with this function the glomerulus with its aortic blood supply is concerned; secondly it has to extract poisonous waste products from the circulating blood, and this is done by the wall of the tubule acting on the venous blood which bathes its surface.

DEVELOPMENT or THE PRONEPHROS IN HYPoGEor111s.—}[ypogeophis, a member of the Gymnophiona, will be taken as an example of the mode of development of the pronephros for the following reasons:

(1) In this as in other Amphibians the pronephros still becomes an actively functional organ. Consequently the probabilities are in favour of its developmental processes having departed less from the 224 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

ancestral methods than in the case of those Vertebrates (e._q. E1asmobranchii, Ainniota) in which the organ is modiﬁed to the extent of being reduced to a functionless rudiment.

(2) Its histological texture is comparatively coarse and the general structural arrangements in the embryo are so distinct as to eliminate to a great extent risk of observational errors.

(3) Its development has formed the subject of a particularly careful and complete investigation (Brauer, 1902). '

D —--a.n.a7. FIG. 122.-——Early stages in the development of the pronephros of II?/pogeop/ufs. Each figure represents a longitudinal section, so arranged as to pass outwards through the

uephrotoines, cutting them across, and viewed from the dorsal side. (After llraucr, 1902, slightly siniplilii-cl.)

A, from an cinln-yo with 15 ll\I_:.~4u(lm'lIl seginents; B, 12 .~".«'-g.:IlIvIIl..\' ; (‘, 16 segments; D, 27 scginmiits. u..n.vl. ill'('lllIlH1Dll!‘l('. duct: ‘nu, ]n‘tIlll-}.’.l‘l(' Lulmla-. Tlw Honnui ll_'.El1l‘I'.\‘ are placed in the lll‘.]blll‘P(‘(’)8lt’.S.

The ﬁrst signs of the pronephros make their appearance-—in embryos with about 9 or 10 mesoderm segments——in the form of bulgings outwards of the outer or somatic Wall of the nephrotome of segments IV and V. These outward bulgings are the rudiments of the pronephrie tubules. A third soon appears in segment VI (Fig. 122, A, of. also Fig. 123, A, pn). The three rudiments grow actively i.n length pushing their way tailwards along the body just external to the nephrotomes. They come to be in close contact and presently fuse to form a rod-like structure (Fig. 122, B) which continues to extend backwards towards the tail and becomes tubular through developing a cavity secondarily, in its interior. This, at first solid, rod-like structure (Fig. 122, C, a.n.d) is the rudiment of IV - - PRONEPHROS 225

the _a.rchinephrie duet, which thus owes its origin to the fusion

FIG. 123.——Deve1opment of pronephros of Hg/pogeopluls as seen in transverse sections. (After Brauer, 1902.)

A, embryo with 22 segments; B, with 29 segments; C, with 44 segments. A, dorsal aorta; end, endoderm; gl, glomerulus; mm, lateral mesoderm; mc, myocoele; my, myotome; N, notochord; no, nephrocoele; ns, nephrostome; p.c, peritoneal canal; pn, pronephric tubule; sac, spinal cord; sol, sclerotome; splc, splanchnocoelo.

together of the outer ends of the tubule rudiments belonging to segments IV, V, and VI.

'VﬂT. TT 0 226 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

Additional tubule rudiments to the number of about 8 arise in order behind those ﬁrst laid down. They arise in exactly the same way "as the first (Fig. 122, B, Vll) but as the archinephric duct rudiment has already grown past their point of origin they become joined on to the duct by their outer ends undergoing sec(mda'ry fusion with it. Each tubule rudiment grows actively in length so that it eventually forms a much-coiled tube connecting the nephroeoele or coelomic cavity of the nephrotome with that of the duct.

In the meantime the nephrotome is undergoing important changes apart from the tubule rudiment. Its cavity, the nephroeoele, from being a mere slit with its floor and roof in contact, becomes widely dilated and it becomes cut off from the dorsal part of the segment which forms the myotome and sclerotome (Fig. 123, O). The nephrotome also becomes gradually constricted off from the lateral mcsoderm but in this case the separation either never becomes completed (Fig. 123, 0, 11.0) or if it does so, is merely tempt»rary————communication being soon re-established at the point where the constriction took place. The nephroeoele is thus, even in the fully developed pronephros, in open communication with the splanchnoooele by a more or less narrow channel the peritoneal canal (Fig. 123, C, 12.0), the splanehnocoelic end of which forms the peritoneal funnel.

As the nephrotome and tubule go on with their development there arise characteristic relations with the blood-vascular system. An intersegmental branch from the dorsal aorta passes to each nephrotome, causing its ﬂoor to bulge into the nephroeoele (Fig. 123, C, gl) and form a conspicuous projection——-the glomerulus——— which later on ﬁlls up most of the nephrocoelic space. From the glomerulus the vessel passes (as the was ejferens) into a network of blood-spaces lying between the coils of the tubule and belonging to the posterior cardinal vein.

The fully formed pronephros of Ilypogeopltis is composed of about a dozen segmentally arranged units each developed in the way described. It is to be noted however, that the last three of these units never become fully developed and further that behind the last as well as. in front of the ﬁrst unit of the functional pronephros each segment has its typical nephrotome though this never proceeds with its development. In other words the pronephros of Ifypogeopluis possesses at its anterior and posterior ends a number of units more or less reduced or vestigial.

It is also of interest to notice certain variations which occur in connexion with the relations of the tubule to the nephrotome. In What may be termed the typical arrangement the nephrostome opens from the dilated part of the nephroeoele (Fig. 124, A). Frequently however it has become shifted on to the constricted peritoneal canal (Fig. 124, B). When it does this there are apt to arise very misleading appearances as illustrated by the accompanying ﬁgure, whereby on the one hand the tubule appears to lead directly from iv PRONEPHROS 227

s the splanchnoeoele, the chamber containing the glomerulus appearing

to be a sid_e branch (Fig. 124, C‘), or on the other hand the pronephric chamber appears to form the dilated end of the tuhulewhile the peritoneal canal appears to form a side branch (Fig. 124, D).

In connexion with what has been said it is important that the student should get clear in his mind from the beginning (1) that the cavity into which the gloinerulus projects (known as the cavity of the Malpighian body in the more highly evolved types of kidney) is simply a more or less completely separated off portion of the

J

FIG. .l.24.—-Illustrating variations in the relations of neplirocoele, tubule and peritoneal canal in the pronephros of 1/3/pogcoplufs.

a..n..d, archinepln-ic duet; 'n.«_', l](‘.])hI‘OCO(‘l(‘.; ns, nephrostome ; p.f, peritoneal funnel; 1., t.uhu1e.

coelome (nephrocoele) and that neither it nor the peritoneal canal is to be regarded as a portion of the tubule, and (2) that the actual tubule commences at the nephrostoine or opening leading into it from the nephrocoele. The word neplirostoine throughout morphology nieans an opening leading from coelome into n.ephridiuin. It is necessary to accentuate this because in many embryological writings the term nephrostoine, or nephrostomal canal, is applied to the peritoneal canal which is not an opening leading from coelome into nephridiuiii but simply a communication between the splanchnocoelic and the nephrocoelic portions of-_ the ‘coelome. A clear appreciation of these points is of help in facilitating the comprehension of a difficult chapter in Vertebrate morphology.

.q.n.d. 228 EMBRYOLOGY OF THE_LOWER VERTEBRATES CH.

clan other Vertebrates possessing a functional pronephros the appearances seen in early stages are readily reconcilable with those described above for IIyp0_qe0 plmls, and we ma.y

a take it that, apart

from variations in detail, this represents the normal mode of development of the organ. Cnossorrnmrerr. —- In Crossopterygians, so far at least as Polyptew-us —— the only member of the group investigatedis concerned, the ﬁrst rudiments of the pronephric tubules are in the form of projections which pass outwards and backwards from the external side of each of the anterior nephrotomes (Fig. 125, A). The number of these tubule rudiments presumably varies, seven being seen in one specimen and nine in another. Apparently the tubule rudiments become fused at their outer ends to form a solid mass—-the rudiment of the archi Fm. 125.-—'l‘ransverse sections of Pol}/1;te1'us-—stages 20, 23, nephric duct. In the and 28—passing through the rudiment of nephrostome Stages Shown in Fjg-_ B which is seen projecting outwards from the wall of -. tlie nephrocoele. ‘ 126’ A and B’ ﬁve

tubule rudiments are

A, dor.»-:11 not-ta: Purl, coelomo-: rm. --nt-e-rit°‘ca\'it.v:_'m.u, !'ll:}'O(_'.-l'H‘l(‘2 Seen Passing at their N, n()t',()(_-lmrtl ; M12. In-1')l|I'()t:m-.l¢'-. ; ,-_v'. 2', Iﬂnil 4-rum l!.{n'dIlI:ll \'I*Ill ; R)./(‘-, . split re1')1'osmnti1i;: sphmchnocoele. Outer end-S In to the IV ’ PRONEPHROS 229

duct rudiment. As development goes on however the tubules belonging to nephrotomes I, III,- and IV, those labelled A, C and D in the ﬁgure, become reduced in size and ﬁnally

A B C D - E FIG. 126.——Dorsa.l view of p|'mIr:pln‘0s of l’nlyp?eru.s- at stages 20, 23, 24+ , 25 untl ttl)I_n,I.t ‘Z8.

(1..'n..d, amlniiu-pln'ic. «luct. 'l‘lu- tubule rudiments are indicated by letters, the nephrotomes by l{0m:u1 numerals. '

disappear, while B and E on the other hand increase in length and become the functional tubules. The anterior end of the archinephric duct becomes gradually modelled out of the solid rudiment ‘already referred to in the way indicated in the ﬁgure.

FIG. 127.—Pa.rt of a longitudinal v('-.1‘tic:tl section through the series of nephrocoeles in the pronephros of I '0/ypI¢.'I‘H.5‘——8tIlge 24 + .

a,.'n..d, arcliiuephric duct; eat, ectoderm; end, undoderm; no.3, nephrocoele “ B"; M. F, nephrocoele “ F."

After it has assumed its deﬁnitive tubular form this front part of the archi.nephric duct commences to grow actively in length, it becomes thrown into complicated coils and forms a large fraction of'the entire bulk of the pronephros in its early functional stages. 230 EMBRYOLOGY OF THE LOWER VERTEBRATES

FIG. 128. ——Renal organs of the right side of a Protopterus larva of stage 34. (From a reconstruction by M. Robertson.)

a.n.d, archinephric duet; op, opisthonephric tubules; pn, pronephros. The capital letters indicate nephrostomes and the figures metotie * mesoderzn segments.

“ Metotie ”=poste-rior to the otocyst. CH.

In its later functional stages the pronephros reaches relatively enormous bulk, occupying the whole thickness of the body-wall, but in these later stages the two tubules become much elongated and coiled as well as the duct itself.

The nephrocoeles belonging to the various nephrotomes which develop tubules form a series of closed cavities lying in a row one behind the other (Fig. 127, mzb’, 97.0.11’). They are for a long time. in Polg/p/erus, the only coelomic spaces which are Widely open (Fig. 125, B). As development goes on the nephrocoeles connected with the functional tubules (B and E) become more and more dilated, their wall becoming thinner as they do so, and the lloor bulging into the cavity to form the glomerulus. Eventually the cavity of the nephrocoele becomes continued ventrally, a split spreading downwards to form the splanchnocoele, into which’ the nephrocoele opens freely. The portions of splanchnic Inesoderm to which the glomeruli are attached, 'i.e. the ﬂoors of the original nephrocoeles, become folded in towards one another, as the splanchnocoelic cavity dilates, to form the dorsal mesentcry so that the glomeruli are eventually borne by the mesentery one on each side.

Meanwhile the nephrocoeles belonging to the tubules which atrophy gradually shrink up and disappear, and as they do so the two large functional nephrocoeles increasing still more in size meet and their cavities as well as their gloineruli become continuous. N o deﬁnite constrictions (peritoneal canals) are formed between nephrocoeles and splanchnocoele, unless possibly during late stages, but the dorsal portion of the more posterior nephrocoele becomes cut off from the splanchnocoele by another method-—the free edge of the glomerulus coming to fuse with the somatopleure so as to form a ﬂoor to the nephrocoele. Iv PRONEPHROS 231

This posterior nephrocoele is still in wide communication with the splanchnocoele indirectly by way of the anterior nephrocoele.

DIPNOI.——In Lepidosiren and .P7'0t0pte7"u.s1 the fully functional pronephros of the larva possesses usually two tubules (Fig. 128, B and D). These are the surviving members of a series of tubule rudiments extending through at least the anterior 4-7 segments but probably extending much further back. The tubules which become fully developed are normally “ B” and “ D ” 'i.e. those corresponding to the second and fourth mesoderm segments. Thus the second tubule does not correspond with the second tubule in the fully developed pronephros of Pcilyptems. The tubules appear to originate (cf. Fig. 129, A, B) as in H3/pogeoplmls except that the outgrowths from the nephrotomes are solid as in I’0lypte'rus and such is the case also with the archinephric duct rudiment.

The nephrocoeles of the two main pronephric tubules undergo fusion as in Polypterus so as to form a large proncphric chamber on each side. This is continuous with the pericardiae portion of the splanchnocoele and the two glomeruli as usual become fused together to form a compound glomerulus? lu Lepiciosrrowt the fusion of the pronephric chambers takes place before the appearance of the glomerular rudiments. These appear first on the ﬂoor of the continuous cavity (Fig. 129, C, gl) and very soon undergo fusion themselves. By dif1'erential growth the root of attachment of the glomerulus becomes gradually shifted towards the mesial plane and dorsally (Fig. 129, D and E) so that it comes to hang down into the pronephric chamber or nephrocoele from a point in close proximity to the dorsal aorta.

The pronephric chambers are at ﬁrst perfectly continuous with the splanchnocoele which spreads outwards from them. Later on the. pronephros becomes greatly enlarged and bulges across the sp1anchnocoele until it comes in contact with the mesodermal sheath of the oesophagus. Fusion then takes place (at the point marked with * * in Fig. 129, E) between the surfaces in contact so that the glomerulus comes to be enclosed in a secondary pronephric chamber, which however remains freely open to the splanchnocoele at its hinder end. The glomerulus becomes ﬁrmly slung diagonally across this chamber by its tip undergoing fusion on the ventrolateral side of the chamber with the mesoderm investing the pronephros.

In Uemtodus (Semen, 1901) the pronephros probably develops in a manner similar to that described in the case of the other two Lungﬁshes. The organ in its first stage is a solid projection of the mesoderm the appearance in section being similar to that ﬁgured for Lepidosiren. The portions of the rudiment corresponding to the individual tubules are in such close apposition as to be at ﬁrst indistinguishable (as is often the case in the other two Lung-ﬁshes):

1 A large part of the investigations upon which this account is based were carried out by Miss Muriel Robertson in the University of Glasgow during 1904. 2 The word glomus is often used for such a compound glomerulus. 232 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

it is only when nephrocoeles begin to appear (in the regions of the

ﬁrm. as shown in trzu verse sections.

Fm. 129.-—-Development of the proneplwos in Lepz'u’z;'

A, stage 21; B, stage 21 ; (3, same 24+. u.'n.¢1, zu-chincphric duct; and, uudoderm; ant, enterlu cavity; gl, g1ouwrulu.~:; I.m,1ate.ml n1usmlm'In; -my, myutmne; N, notochord; ow, nephrocoule; pn, pmnephric tllblllv; s.«.:, spinal cord ; .~.-a-I, -*.-c.lex-oton1e.

ﬁfth and sixth segments) that the segmented nature of the rudiment IV PRONEPHROS 233

becomes apparent. The fully functional pronephros has two tubules on each side, corresponding to the segments above mentioned: it may be presumed that these are the survivors of a once greater number, though there is no record of other rudiments having been actually observed.

FIG. 129A.——Development of the pronephros in Lepidus'£-re-n as shown in transverse sections.

D, tags 30: E, st:u,:e 31+. A, dorsal aorta; a.n.d, archinephrie duct; end, emloderm; gl, glamorulus; li, liver; -my/, myotome; N, notochord; nc, nephrocoele; oars, or-sopliagus; pn, pronephric tubule; p.'v.c, posterior vena cava; splc, splanehnocoele.

ACTINOP'l‘ERYGII.———Tl1e acquirement of a thorough knowledge of the development in the more primitive members of the group—the ganoids———is an essential preliminary to the proper comprehension of the development of the more highly evolved Teleosts but-unfortunately our knowledge of renal development in the qanoids is still far from complete. 234 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The tubule rudiments appear to arise in normal fashion, as outgrowths of the lateral Wall of the nephrotome. These outgrowths show the familiar variation of being sometimes hollow sometimes solid. Thus in Amia according to Felix (1904) the anterior three rudiments are hollow pockets while those farther back are at ﬁrst solid.

Tubule rudiments make their appearance from segment III to segment Xlll but here as elsewhere only relatively few of these complete their development and are to be found in the pronephros at the height of its functional activity. Thus in a six-day Acipenser larva Jungersen found six functional tubules vvhile in Amid Felix ﬁnds only a single tubule functional. In the latter case the tubule

opens from a large pronephric chamber apparently formed by the fusion of at least three nephrocoelcs. The tubule belongs originally

to the most anterior of these and corresponding to it there is present a single open peritoneal canal. Later on this becomes replaced functionally by another peritoneal canal situated farther back. In Lepidostcus the functional pronephros has at least three tubules each with its nephrocoele (Felix, 1904).

As in the case of the Lung-ﬁshes the dorsal part of the splanchnoeoele in the pronephric region becomes floored in by the approximation of the mesial surface of the pronephros to the lateral surface of the oesophagus (cf. Fig. l29A, E) so as to form a secondary pronephric chamber. In Lepwldosteus this forms a widely patent cavity with which the ﬁrst nephrocoele becomes completely merged and which remains ventrally in continuity with the main sp1anchnocoele by a narrow richly ciliated tubular channel. In Acipenser the ﬁrst nephrocoele undergoes a similar modiﬁcation while the remainingl ﬁve are fused with one another but isolated from the splanchnocoe e.

TELEOSTEI.-—The development of the renal organs has been worked out in detail in the case of the genus Salmo by Felix (1897). In this case the myotomes are already separate from the more ventrally situated portions of the mesoderm at a very early stage. The first rudiments of the pronephros are in the form of a series of somewhat conical, segmentally arranged, solid projections from the median edge of the lateral mesoderm towards the mesial plane. These projeetions—ﬁve in number (segments 3-7) in a 26-day Trout—-are probably to be regarded as nephrotomes which have been precociously separated from the myotomes, if indeed they ever were continuous. These ﬁve nephrotomes soon come into intimate contact so as to be no longer distinguishable. They now together form a continuous mass of mesoderm the so-called pronephric fold. The dorsal and outer portion of this mass becomes nipped off to form the anterior portion of the archinephric duct except at one point where a connecting isthmus remains to form a tubule. The mesial portion of the mass becomes the wall of the single pronephric chamber. Iv PRONEPHROS 235

The whole mass is at first solid, the cavity of duct, tubule, and pronephric chamber, developing secondarily.

'l‘he cavity of the pronephric chamber is for a time continuous with the split-like splanchnocoele, but it soon becomes constricted off from it and forms a completely closed cavity. Bearing in mind the segmented condition of the pronephric rudiment in its ﬁrst stage of‘ development and the process of fusion of successive nephrocoeles which takes place in Ganoids, we may conclude that the pronephric chamber of the Teleost probably represents a number of nephrocoeles fused together. The single pronephric tubule is very possibly the same member of the series as that which occurs in Amie although this has not yet been actually determined.

A remarkable peculiarity found within the group Tcleostei is that in a few genera (e.g. Fierczsfer, Zoarces, Lepadogaster) the pronephros retains its renal function throughout life (of. Guitel, 1901, 1902).

AM1>H1n1A.——In Amphibians other than Gynmophiona the pronephric rudiment first becomes visible-as a solid swelling of the somatic mesoderm at the level of the anterior mesoderm segments (Rana segments 2-9, ’l’m'ton alpestrm'.s~ 1-6, Mollier). Though at first no segmentation is _to be detected by the ordinary methods of observation in this swelling it is reasonable to interpret it as representing morphologically a series of closely apposed or fused nephrotomal projections like those of 1-"I3/pogcophis. This pronephric rudiment gradually becomes demarcated off from the rest of the mesoderm by a split which becomes apparent on its ventral side at first laterally and then spreads inwards.

The rudiment new forms a thick flap (of. Lung-ﬁshes, Fig. 129, A and B) hanging down on the outer side of the mesoderm, and continuous with the somatic mesoderm along its dorsal and median edge. Segmentally arranged coelomic splits make their appearance along the line of attachment of the pronephric flap and these we may interpret as incipient nephrocoeles. The split already mentioned as demarcating the pronephric rudiment ventrally spreads round its median edge, so as to detach it completely from the (nephrotomic) mesoderm except at Segmentally arranged points where a connecting isthmus remains as the nephrostomal end of a tubule. The pronephric rudiment now undergoes a kind of modelling process similar to that occurring in Crossopterygians and Lung-ﬁshes, its outer portion being gradually cut off from behind forwards so as to form the archinephric duct, while the part nearer the mesial plane forms the recurrent portion of the duct with the tubules connected with it.

The whole rudiment is at first solid. The earliest coelomic spaces to appear are the nephrocoeles and from these split -like extensions spread outwards in each tubule rudiment, while towards the outer margin of the rudiment the continuous longitudinal cavity of the archinephric duct develops.

Of the tubule rudiments, as usual, only a few become functional-— 236 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

in Anura commonly 3, in Urodeles commonly 2 (in Amphqluma 3 according to Field). Probably here as elsewhere the number is really a variable one. As the tubules develop they show active increase in length so that they become much coiled and the same applies to the part of the archinephrie duct lying in the pronephric region.

It is only when they ﬁrst appear that the nephrocoeles show a segmental arrangement: later on they become merged in the general splanchnocoele. Along the inner wall of the dorsal portion of this cavity, 13.3. the portion which represents the fused nephrocoeles, the glomerulus develops as a continuous laterally projecting fold of splanehnie mesoderm. Usually the portion of the body cavity containing the glomerulus becomes for a time incompletely shut off from the rest to form a secondary pronephrie chamber as in Lungﬁshes, the mesoderm covering the lungs undergoing fusion with that covering the bulging surface of the pronephros. The secondary

pronephric chamber may in turn be subdivided by the edge of the”

glomerulus fusing with the mesoderm covering the pronephros.

MEROBLASTIC Vslrrsnnxrns.-As a r11le, in the Meroblastic Vertebrates the pronephros never becomes a functional organ, and correlated with this it shows a reduction in its structure. Possibly, as already indicated, this may be due to the presence of the large yolk-sac with highly vascular surface in contact with the external medium, which will facilitate the getting rid of excretory material by diifusion outwards.

ELASMOBRANCHII.-——1n Elasmobranchs the ventral ends of certain of the anterior mesoderm segments, usually commencing with segment VII, become dilated to form vesicular cavities (van Wijhe, 1889) which are probably to be interpreted as nephrocoeles. The tubule rudiments appear as thickenings of the somatic wall of these nephrocoeles which grow outwards and being in close apposition form at their outer ends, apparently by fusion, a solid continuous pronephric swelling. The tubule rudiments make their appearance in sequence from before backwards.

Different workers vary in their statements as to the number of rudiments in different forms [Sop/llrium, 5———Riickert, 3~—van Wijhe; Pristiurus, 5--Riickert, 4—Rabl, 3—van Wijhe; ]t’a/ta clwvata, 5——van Wijhe; R. alba, 8——Rabl; Torpedo, 7——~Riickert (Fig. 130)] from which we may conclude safely that the number of tubule rudiments is very liable to variation both as between different species and different individuals. This variability may be taken in correlation with the fact, observed by van Wijhe, that in P1"'ist'i'u/rus dilated nephrocoeles made their appearance from segment I to segment XIV, gradually diminishing in size towards the end of the series, although tubule rudiments appeared only in 3 segments. Both phenomena indicate that the pronephros in Elasmobranchs as in other groups has undergone reduction from a once much greater anteroposterior extension.

In a comparatively late stage the tubule rudiments develop their 1v PRONEPHROS 237

lumen. The pronephric swelling extends backwards into the archinephric duct. N 0 glomerulus develops but segmentally arranged branches of the dorsal aorta appear on the right side corresponding in number and degree of development with the pronephric tubules. These give rise either one of them (Riickert, van Wijhe) or by fusion together (Rabl) to the root of the vitelline artery but are termed by Rabl pronephric arteries.

The pronephros undergoes rapid degeneration and eventually

pothlipg is; left of it but the coelomicfunnel of the Miillerian duct see ‘e ow .

SAUROPSIDA.—-In the fowl pronephric tubule rudiments develop to the number of about 12, in the form of solid outgrowths of the somatic mesoderm at the level of the nephrotomes although, except in the case of the most anterior, the mesoderm is not yet segmented at this level at the time when the rudiments appear. In at least some cases segmental dilatations of the otherwise split-like coelome occur opposite the tubule rudiments and are no doubt to be interpreted as the nephrocoeles of the corresponding tubules. The ﬁrst tubule rudiment makes its appearance in embryos with 8 or 9 segments, its position showing considerable variation (usually segment 4, 5 or 6).

The successive tubules appear in , _ _ rapid succession——almost synchronously. 1‘ “,§§~",‘?i“g’1’.’t’ At about the 19-segment stage the Torpedo. (After Riickert,1888.) myotomes become separated from the a,vn,d,archinep]1x-ic duct; z,,.,, mom. nephrotomes, the latter remaining in "0-'-<01: rm-1. etc-. pronephric tubules. continuity with the lateral mesoderm ‘""’ "“"‘b‘”"“‘ ‘““‘ and their cavities (nephrocoeles) with the s lanchnocoele. About the same starre the baekwardl ro'ectin tip cl‘) each tubule rudiment undergoes tiusion with its dligcegsor llgl the series and thus gives rise to a continuous longitudinal rod—like structure——the rudiment of the archinephric duct (Felix, 1904). The archinephric duct in its anterior portion thus would appear to develop i.n a manner essentially the same as that found in the Grymnophiona and so many other of the lower vertebrates.

The hinder end of each tubule rudiment, as well as the archinephric duct itself, is at ﬁrst solid. The deﬁnitive lumen makes its appearance (about 20-segment stage) secondarily in the form of discontinuous chinks which gradually become continuous and spread backwards.

Pronephric glomeruli develop in the Bird though at a late stage 238 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

when the pronephros is already degenerating. They were discovered ﬁrst by Balfour and Scdgwiek (1878) in the Fowl where they vary in number from about 3 to about 7. They may, a.s so commonly occurs in pronephrie glomeruli, undergo a less or greater amount of fusion with one another and also with the anterior glomeruli of the opisthonephros. The whole pronephros in the Bird undergoes rapid atrophy and by the sixth day of incubation has usually in the Fowl completely disappeared except the glomeruli which may still be detected for a day or two longer.

in the Reptiles also a rudimentary pronephros makes its appearance but degeneratcs without becoming functional. The nephrotomes or protovertcbral stalks, at ﬁrst solid, develop a patent cavity or nephrocoele. ]n a varying number of segrncn ts (in Lizards 6-8, commencing with segment V) proncphric tubule rudiments develop as outgrowths of the somatic wall of the nephrotome after the ordinary fashion and fuse together at their outer ends to form the archinephric duet.

THE ARCHINEPHRIC DUcr.——-As has already been indicated, it is characteristic of the Vertebrate that its ncphridial tubes no longer open directly to the exterior, but that, on the contrary, they open into«a longitudinal duct on each side -—the archinephric duct———which in turn opens into the alimentary canal towards its hinder end. The first steps in the evolution of the archinephric duct have passed beyond our ken and to decide as to how they came about we have to balance probabilities on a basis of somewhat scanty embryological and anatomical data. Two obvious possibilities present themselves ——(1) that the row of segmentally arranged nephridial openings came to be sunk beneath the general surface in a longitudinal groove and that this groove became covered in to form a longitudinal duct, and (2) that the external opening of each tubule became shifted backwards so as to open into its successor in the series and so give rise ﬁrst to a common opening with it and later to a common longitudinal duct (Fig. 131) in the way exempliﬁed by the posterior kidney collecting tubes of male Elasmobranchs. On which side the balance of probability lies will be apparent on considering the developmental facts so far as they are known to us at present.

It will be recalled that in Hypogeophas-, according to Brauer, the anterior portion of the archinephric duct arises by a number of pronephric tubule rudiments bending tailwards at their outer ends and undergoing fusion together. The fused portion forms the duct rudiment and it proceeds to extend backwards by independent growth until eventually it reaches and fuses with the wall of the cloaca. It is only a small portion of the duct close to its anterior end which is formed by the direct fusion of tubule rudiments-——the tubules farther back growing out and fusing secondarily with the already formed duct.

If we turn to other Vertebrates we ~ﬁnd considerable evidence for believing that Ifypogeophis presents to us a mode of development IV ARCHINEPHRIC DUCT 239

of the archinephric duct which. is relatively primitive. In a number of Vertebrates there appear to be distinct traces of the formation of the front end of the archinephric duct by fusion of the outer ends of tubule rudiments in a manner essentially the same as that which holds for fly/pogeoplznls. As will have been gathered from the preceding pages this is the case with such different groups of Vertebrates as Elaslnobranchs, Crossopterygians, Lung-ﬁshes, Reptiles and Birds.

Fm. 13l.—--Diagram illustrating a possible mode of evolution of the archinephric d11ct.

A, the coelomiu eompm-tment-s are bulging t-I)\\'{l.l‘(l!~‘. the neplwidial tubes; 13, the colnpartinentis hax-‘e come to open into the nephridial tubes and the ﬂame-cells have disappeared ; C, l), the externa‘. openings of the nu-'-phridia are beconiing shifted backwards so as to give rise to the archinephric duet E, the archinephric duct is completely formed and commtmieates with the enteron through one 01 the segments retaining, or revert-ing to, its primitive enter-o(2oelic. connexion. a.n.d, 8.!‘('.llinHpln'i(' duct; c.f, eoelomic funnel; (uwl, coeloniie cavity; cl.u, cloncal opening of arc.hinephI'ic duct: ect ectoderm; end, endorlenn; _/'.r_-., flame-cell ; n, nephridial tube.

If we are justified in looking upon this mode of formation of the duct in ontogeny as relatively primitive, i.t obviously affords strong support to the second of the two above-nlentioned hypotheses as tc the evolutionary origin of the archinephric duct: the bending back

of the tubule rudiments would then be interpretable as a developmental reminiscence of the backward shifting of their external open ings which took place during phylogeny. 240 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

.The independent backgrowth of the remainder of the duct in Hypogeophis is probably to be regarded as a case of accelerated or precocious development to allow the anterior tubules to become functional at an early stage of development before those farther back have developed.

As regards the ontogenetic development of the main part of the duct in other Vertebrates we ﬁnd the most divergent statements and it seems clear that this divergence can only be explained by the actual facts not being always the same.

In the Sauropsida it is admitted that the main part of the duct is formed as in Hg/pogeophvls by independent backgrowth. Amongst the Anamnia the same is said to be the casein Elasmobranchs by Balfour and by J-labl, and in Al;/tes according to Gasser, but other authors describe two other methods of formation as occurring.

The ﬁrst of these is found in Elasmobranchs according to van Wijbe, Beard, Riickert and others. According to these investigators, the archinephric duct makes its ﬁrst appearance as a longitudinal ridge - like thickening projecting inwards from the ectoderm. This becomes split off as a solid ectodermal rod which develops a cavity secondarily and forms the archinephric duct. Such a

mode of development would be of great morphological interest [as it would lend decided support to the View that the archinephric duct originated in evolution as an ectodermal groove———it being a

common ontogenetic modiﬁcation that what is morphologically a groove develops ontogenetically in the form of a solid ridge-like ingrowth. It has however to be borne in mind that there exists a serious source of possible error in making observations upon the archinephric duct in early stages. The duct lies between ectoderm and somatic mesoderm——the two cell-layers mentioned ﬁtting close round it. During the various processes to which the embryo is subjected preparatory to being cut into sections the ectoderm usually separates slightly from the mesoderm, and the archinephric duct tends to adhere ﬁrmly to one or other of these layers. This is the case more particularly at its tip, where it is pushing the ectoderm and mesoderm apart as it grows back and is therefore in particularly intimate contact with them. It is exceedingly difficult in studying sections to distinguish with certainty between such intimate contact and actual organic continuity. In cases where the hinder part of the duct is adherent to the ectoderm an appearance is produced which simulates closely a development by splitting off from the ectoderm.

As a matter of fact C. Rabl’s very careful investigations (1896) fail to conﬁrm the ectodermal origin of the duct in Elasmobranchs and upon the whole in the writer’s opinion there does not appear to be any longer justiﬁcation for accepting it as actually occurring.

The other mode, by which the extension of the archinephric duct backwards has been described as taking place in the Anamnia, is that the duct becomes split off from the.underlying somatic mesoderm. It is necessary again to bear in mind the caution expressed Iv ARCHINEPHRIC DUCT 241

above but making full allowance for this it seems impossible to escape the admission that in many forms (Petromg/zon, Lung-ﬁshes, most Amphibians, Teleosts and probably actinopterygian Ganoids) the duct is prolonged backwards by a process of this kind.

It being accepted that in a ‘number of Anamnia a large part of _the archinephric duct arises in development by being split off from the mesoderm, we are faced by the problem how this mode of development is to be correlated with the mode of development by fusion of the outer ends of tubule rudiments. It may be suggested that what has happened is that the development has been accelcrated—--as often happens ——by skipping over the early stages._ The mode of development in question may have been derived from the more primitive mode by the omission of the separate tubule stage and the passage at once to the stage in which the tubule ends are fused into a continuous structure.

In some cases however the primitive mode of development has undergone a further modi- . ﬁcation. This is exempliﬁed a by P0l.7/Ptwus (Graham Ker1'a Fm. 132.——'1‘ransverse section through Polyptems 1907) where the hinder portion of stage 23 at level of cloacal opening.

Of the duct appears l}O be a.n..d, opening of archinephric duct into cloaca; cl, for]-“ed conversion ope-.i|i1Ig of clo:u-u lo exte.ri01:; end, nlinn.-utlzu-_\-' I(‘.'ll1{ll of the series of nephrotorneso :_;l(:;.l‘l]l‘lll-‘Ill-, my/, m__\o1ome, .\,1|utoc-lund. .~.«-, spllllll These are not segmented but

form a continuous structure which becomes converted directly into the archinephric duct. .

In whichever way the archinephric duct completes its extension backwards, it eventually comes to open into the cloaca. This is, in the great majority of Vertebrates, described as coming about by fusion of the previously freely-ending tip of the duct with the cloacal wall. It is obvious that such a process cannot correspond with what happened during evolution as the duct must have had its posterior aperture throughout in order _to perform its function.

It is possible that a clue to the evolutionary origin of the communication between archinephric duct and alimentary canal is given by Polypterus. It has already been mentioned that in this animal the hinder part of the archinephric duct arises by bodily conversion of the row of fused nephrotomes. Fig. 132 shows that the opening of archinephric duct into the alimentary canal presents a striking

VOL. II R 242 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

resemblance to the primitive communication of mesoderm segment with enteron, and it is suggested that it actually is this primitive communication which has remained patent while in all the other segments it has disappeared.

The two archinephrie ducts open at ﬁrst separately into the cloaea, one on each side. In some groups of Vertebrates however their terminal portions become gradually approximated and eventually fused together into an unpaired dorsal vesicle which may undergo various modiﬁcations. In Elasmobranchs it forms the urinogenital sinus which bulges forwards and on each side becomes prolonged into the sperm-sac. In Lung-ﬁshes it forms the cloacal eueeum: in Teleostei the urinary bladder.

It is noteworthy that in the adult Lung-fish the communication of the kidney duets with the caecum is close to the posterior opening of the latter, so that a small amount of shifting would cause these ducts to open into the cleaca independently of the caecum.

This suggests a possible evolutionary origin of the allantois. It is conceivable that a caecnm similar to that of Lung—ﬁshes arose by a fusion of the terminal portions of the kidney d11cts"mmtml, instead of dorsal, to the alimentary canal and that the ducts then came to be emancipated from the caecum which remained as a ventral diverticulum of the cloaca to form the allantois. We have no definite evidence as to the evolutionary origin of the allantois and it is well to bear in mind the possibility here indicated in addition to the simpler and perhaps more probable hypothesis that the allantois was from the beginning simply a bulging outwards of the ventral cloacal wall as it is actually in ontogenetic development.

DEGENERATION or TIIE PRONEPHROS.—-—The role of the pronephros

as the functional renal organ is usually conﬁned to comparatively

early stages in development and at the end of this period, when its function is being taken over by the opisthonephros, the pronephros commences to undergo characteristic degenerative processes which normally culminate in its almost complete disappearance.

In the frog (Marshall and Bles, 1890) these processes become apparent in the tadpole of about 20 mm. in length. The archinephric duct becomes more or less obstructed behind the pronephros and as ﬂuid continues for a time to pass into the tubules the latter become greatly distended in places, their lining cells assuming a cloudy appearance, the cell boundaries becoming indistinct and their inner surfaces losing their smooth outline and becoming ragged. The whole organ shrinks in size, becomes invaded by leucocytes, the nephrostomes close, one after the other, and by the end of the ﬁrst year the whole organ with the adjacent portion of the archinephric duct has practically disappeared. 3

MULLERIAN DUc'r.—-—Throughout the series of gnathostomatous Vertebrates, with the exception of the teleostomatous ﬁshes, the oviducts are admittedly homologous. They-—the Miillerian ductsare above all characterized by the fact that they open freely into the IV MtiLLER1AN DUCT 243

splanchnocoele at their anterior end by an open funnel (ostixmn iubwe). There exists in some of the more archaic ﬁshes what appears to be distinct evidence that the Miillerian duct has been evolved out of the tubules and duct of the pronephros and it will therefore be convenient to consider this evidence now.

The Elasmobranchs are the ﬁshes in question. I11 Torpedo (Riickert, 1888) as the pronephros degenerates its tubules become reduced to the three hindermost. Of these three the two posterior degenerate while the other—tubule E-;pcrsists and its enlarged nephrostome becomes the coelomic funnel of the Miillcrian duct. Other workers (e.g. van Wijhe and Rabi), working on other J£lasmobranchs (P'r'isti'u/rus), trace back the coelolnic funnel of the Mullerian duet also to an opening derived from the pronephros and nephrostomal in its nature, but they believe the opening to he formed not by the persistence of a single enlarged nephrostomc but rather by the fusion of three or four nephrostoines together. That it is morphologically a single nephrostome is however rendered more probable by what we now know regarding the development of the pronephros in those of the more archaic ﬁshes in which it develops as a functional organ. It will be recalled, for example, how in 1’0l-3/pterrus tubule E (like B) becomes enlarged as compared with A, U, and D. A prenephric tubule enlarged in this manner in correlation with purely excretory needs would provide an obviously adequate beginning for the evolution of a funnel for the transmission of the eggs like that at the front end of the Miillerian duct.

While the funnel of the Miillerian duct is nephrostomal in origin the main part of the duct is developed in the Elasinobranchs (Semper, 1875; Balfour, 1878) from the arehinephric duct. The latter undergoes a process of splitting from before backwards into a dorsal and a ventral tube, the latter being at first a solid thickening of the ventral wall of the arehinephric duct. Of the two tubes so formed the ventral is continuous with the pronephric funnel, while the dorsal carries the openings of the kidney tubules farther back in the series: the former becomes the Miillerian duct, the latter persists as the functional duct of the opisthonephros (Fig. 133, C, W.d).

This mode of development is satisfactorily explained by the assumption that the relatively archaic ﬁshes in which it occurs are repeating the process by which the Mullerian duct arose in evolution. Such a splitting of an originally common duct into two, so as to separate the routes by which two different products reach the exterior, is probably of frequent occurrence in evolution. Good examples are seen in the splitting of the common genital duct of hermaphrodite gasteropods (e.g. the ordinary snails) to form a separate oviduct and vas deferens. It appears then justiﬁable to accept as a working hypothesis that the Miillerian duct arose in evolution by being split off from the arehinephric duct and that its coelomic funnel is a persistent pronephric funnel.

Turning to Vertebrates other than Elasmobranchs, well-marked 244 EMBRYOLOGY OF THE LOWER VERTEBRATES

CII.

differences are found to exist between the phenomena as described for different groups and even for members of the same group by

P“ --as. D 0 S 2 1' ~ : 1 Md. ‘ I

.1 I U.l‘l.d. W.al. .-J‘ ~ -. A B C - C/.

Fm. l33.—-Arrangement of archinephric duct, etc., in embryos of l’7'istizmts. (Based on Rab1’s ﬁgures.)

A, male 17 mm. ; B, female 19 mm. ; 0, female 27 mm. a.n.d, archinephrie duet; cl, cloaca; M.d, Muller-ian duet; as, coelomic opening of Mullerian duct; gm, pronephrie nephzostome; W.d, duet of opisthonephros.

different observers. While some of these may be due to observations being pushed to within the limits of probable error it is impossible to avoid the conclusion that great differences do actually exist in the details of development of the Miillerian duct.

It is possible on general embryologieal principles to arrive at an idea of the kind of variations which might be expected to show themselves from the supposedly primitive mode of development.

1. The Miillerian duct might continue to arise in an unmodiﬁed manner by splitting from the archinephrie duet, its funnel being a persisting nephrostome.

1].. In correlation with the fact that the one derivative of the archinephric duct (duct 01' the opistho— nephros) is I‘equi1‘e(l to be functional at a very early period, while the other (Miillerian duct) does not function until adult life, there would be a tendency for the two duets no longer to keep exactly abreast in their development but to become separated, the Woliﬁan duct developing relatively earlier, the Miillerian relatively later. T 0 enable this to take place, the primitive stage in which the two ducts were still one would tend to be more and more curtailed until it was eventually eliminated and the two duets were independent from the beginning.

III. The independently arising Miillerian duct might retain the mode of extension backwards by intrinsic growth, eventually reaching and fusing with the wall of the cloaca.

IV. Its separation from the somatic mesoderm might take place relatively

later than its extension backwards so that it would arise in development eomparatively late, as a longitudinal ridge or fold gradually separating off from the mesoderm from before backwards. Iv MITLLERIAN DUCT 245

A survey of the phenomena as described for the various subdivisions of the Vertebrata shows that as a rule they may be ﬁtted without diﬁiculty into one or other of these types of modification.

Thus in the Amphibia some, especially of the older observers, described the extension of the Miillerian duct as taking place by splitting off from the archinephric duct as in Elasmobranchs, others as being due to independent intrinsic growth, still others as a process of folding or splitting off from the splanchnococlic epithelium.

()ne of the most careful modern accounts (H. Rabl, 1904) based upon?

the phenomena observed in the relatively primitive Urodela (Salmmmdm) states that the funnel. is the persisting “second” nephrostome of the pronephros and that the portion of duct behind this arises as a thickening of the coelomic epithelium—-the cells first assuming a columnar shape, then becoming arranged in several layers to form a ridge projecting into the subjacent connective tissue, and ﬁnally becoming split off as a solid rod. Only the anterior portion of the Mnllerian duct is formed .in this way, the rod-like rudiment so formed proceeding to grow back independently to form the hinder part of the duct.

In Reptiles and Birds the ostium is described as originating as a pit in the eoelomio epithelium, which we may look on as a delayed and modified nephrostome, and the extension backwards as taking place by independent growth. The cvagination of the epithelium to form the pit is, as is usual in such cases, preceded by the epithelium becoming somewhat thickened.

The mode of origin of the Miillerian duct has not yet been worked out in detail in the Ganoids and Lung-ﬁshes. In ordinary ﬁshes (Teleostei) the conditions are peculiar and will be dealt with along with the development of the ovary.

As regards the further development of the Miillerian duct, it should be noted that its completion and opening into the cloaca is commonly delayed till a comparatively late stage-——-often till a period but shortly before sexual maturity.

Though primarily rctroperitoneal the Miillcrian duct comes, with increasing growth, to bulge into the splanchnocoele, pushing inwards the peritoneal lining which comes to surround it as a sheath containing muscles, blood-vessels, etc. Its lining epithelium becomes glandular and specialized to minister to the nutritive and protective needs of the egg in ways which differ in the different groups.

Various modiﬁcations make their appearance in later stages. Very frequently the coelomic opening becomes shifted by the addition to the tube of a secondary extension formed from the peritoneal lining. In Elasmobranchs this shifting is towards the mesial plane and except in a few species leads to complete fusion so as to form a single median opening for the two oviducts. Again the hinder ends of the Miillerian ducts are in many cases approximated and they too may fuse to form a terminal unpaired portion.

In the case of the Birds the right oviduct lags behind in 246 EMBRYOLOGY ()F THE LOWER VERTEBRATES CH.

development from about the eighth day of incubation; it never opens into the cloaca, and it persists in the adult as a functionless vestige.

There is considerable probability that the genital p0reS——paired openings leading from the hinder end of the splanchnocoele directly into the urinogenital sinus (Cyclostomata) and through which the gametes pass out——are to be looked on as Miillerian ducts in the last stage of reduction, the whole duct having disappeared except its hinder opening. Whether there is any evidence bearing on this in their ontogeny is not yet known.

The Mullerian duet goes through the early stages of development in the male as well in the female. It usually however never opens into the cloaca and it soon becomes reduced to a vestige. This may persist to a greater or less extent as an individual variation or as a normal characteristic, 12.1]. in the male El-asmobranch or Lung-ﬁsh well-marked vestiges remain in the adult, and so, still more markedly, in some of the Aniphibia such as the Bufonidae and some of the Gymnophiona.

OI’ISTIIONE1’liROS.——-Here again l3rauer’s excellent account of the development in II”:/pogeoplmls (1902) may be taken as a basis of our description. The opisthonephros in this amphibian is composed of segmentally arranged units extending from segment 24 to segment 100. Each unit is identical in composition with those of the pronephros, consisting of a tubule and a chamber (Malpighian body) containing a glomerulus and communicating with the splanchnocoele by a peritoneal canal. As in the case of the pronephros, each unit arises in development from the nephrotome or protovertebral stalk, the tubule rudiment being in the form of a diverticulum of the lateral or somatic wall of the nephrotome, the blind end of which comes in contact and fuses with the wall of the duct secondarily. Again as in the case of the pronephros, the nephrotome becomes completely separated from the myotome. It also becomes constricted off from the splanclmocoelic mesoderm, incompletely in some cases, a narrow communication——the peritoneal canal ~-- remaining open between the nephrocoele and the splanchnocoele, but more usually completely. In this latter event a new peritoneal canal is developed secondarily in place of that which has been obliterated, a diverticulum growing out from the wall of the nephrotome which meets and fuses with the splanchnoeoelic epithelium.

.There are differences in detail between the development of pronephros and opisthonephros, e.g. the tubule rudiment makes its appearance relatively later in the case of the latter—at a period after the nephrotome has become constricted off from the splanchnocoelic mesoderm. A further difference lies in the fact that there takes place in the opisthonephros a great increase in the number of its tubules——secondary, tertiary, etc. tubules being added to those of the original series. These arise in characteristic‘fashion. An outgrowth arises from the posteromedian portion of the nephrotome and Iv OPISTHONEPHROS 247

becomes constricted off as a small round vesicle with thick wall, composed of tall epithelial cells, and a small lumen. This is a secondary nephrotome. It remains for a time without change but eventually behaves very much as the original (primary) nephrotome, one wall becoming invaginated to form a glomerulus, and pocket-like outgrowths giving rise, one to a tubule rudiment, the other to a peritoneal canal. An important difference in detail is seen in the behaviour of the duct, which sends out a tubular projection of considerable length to meet the secondary tubule. This outgrowth arises from the duct some distance behind the point where the primary tubule opens into it.

The secondary nephrotome in turn buds off a tertiary nephrotome which again behaves as before and its tubule is met by a projection from near the tip of the outgrowth of the duct which has already developed in relation to the secondary nephrotome. Consequently secondary and tertiary tubules open into the arehinephric duct by a common collecting-tube formed of this outgrowth.

Apparently new generations. of subsequent nephrotomes may go on being formed in a similar fashion each from the preceding one until there may be as many as eight in a single segment, all of them, except the primary, opening into a common collecting-tube.

The degree of development reached by the opisthonephric units is different in different parts of its length. They attain full development in the manner above described in the region of segments 50100. In the region in front of this (segts. 30-50) the secondary nephrotomes and their derivatives never become functional and their rudiments degenerate. Still further forward (segts. 24-29) even the primary units as a rule degenerate without completing their development.

Apart from differences in detail it is clear that the primary units of the opisthonephros present the most striking resemblance to those of the pronephros and the evidence that they are serially homologous seems convincing.

Normally there is a gap of a few segments between the hind end of the pronephros (segt. 15) and the front end of the opisthonephros (segt. 24) but Brauer found that even in these segments there makes its appearance a distinct nephrotome, with the vestige of a glomerulus, although it does not proceed with its development. Consequently the units of pronephros and opisthonephros (primary) are to be regarded as members of a once continuous series. That this series once extended back beyond the present limits of the opisthonephros is indicated by the fact that distinct nephrotomes are present in segments 101-104, but as was the case in the intermediate zone between pronephros and opisthonephros these do not proceed to develop tubules.

ELAsMoBRANCHII.—-—It will be convenient now to consider shortly the development of the opisthonephros in the Elasmobranch ﬁshes as they have provided the material for a large proportion of the most 248 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

important work dealing with the morphology of the Vertebrate kidney. It was in the opisthonepbros of Elasmobranchs that Sedgwick (1880) made his classical discovery—- which forms the foundation on which our presentéday knowledge rests-—that the nephridial tube of the Vertebrate is a development of the coelomic Wall, of that part of it which we now call nephrotome or protovertebral stalk. Since the date of Sedgwick’s work the op1sthonephros of Elasmobranchs has formed the subject of detailed studies by Riickert, Babl, van Wijhe, and other well-known investigators: Owing to the lower end of the myotome in these ﬁshes becoming displaced in a lateral direction, through the accumulation of mesenchyme between it and the mesial plane, the protovertebral stalk becomes rotated outwards so as to assume a nearly horizontal position (Fig. 13-5-I, A, rat), the originally dorsal end of the stalk becoming now

0. Rab], 1896) ; B, variation observed in ’I'o'rpedo (after Hiiekert, 1888).

A, dorsal no1't;n.; a.n.d, :u-iuhineplnic duet: act, ect.od¢'.-r-m; my, n'l)'Ol30lllI'!: wt, nephrotmm-; splc, splunclnlocoele; t, tubule rudiment.

external, and the originally external side coming to be ventral. The duct (a.n.d) thus comes to lie ventral to the nephrotomes instead of being on their outer side as was the case originally. The nephrotomes become isolated from the myotomes by their ends next the myotomes breaking up into mesenchyme. The result is that the nephrotomes now form a series of blindly ending pocket-like projections of the coelomic epithelium which curve outwards dorsal to the duct.

Each pocket has an epithelial wall and it is noticeable that the somatic portion of the Wall is markedly thicker than the splanchnic, the cells of the former being taller and more columnar in shape. As development goes on it is found that the thicker more columnar celled portion of the wall of the pocket extends for some distance on to its dorsal wall, and this is interpreted by Riickert and Rab] as meaning that the somatic epithelium is spreading inwards towards the mesial plane, replacing splanchnic epithelium as it does so. In View of what we know regarding the development of other groups it seems more reasonable to explain the appearance as being expressive Iv OPISTHONEPHROS 249

of outward growth on the part of the somatic wall of the nephrotome——-the terminal portion becoming the tubule rudiment. Riickert (1888) ﬁgures a remarkably interesting variation which he came across in Torpedo in which the separation of nephrotome from myotome had been delayed. In this (Fig. 134, B) the nephrotome still forms a distinct stalk continuous with the myoteme and the tubule rudiment is visible as a pocket-like projection of its somatic wall, agreeing exactly with the assumcdly primitive type of tubule rudiment as it occurs in the pronephros of one of the lower holoblastic Vertebrates. To correlate this specimen with the normal condition all that is necessary is to imagine the portion of the stalk next the myotome to have disappeared by becoming resolved into mesenchyme. The rest of the stalk together with the tubule rudiment would then remain as a curved blindly ending pocket‘ the tip of which would represent the tip of the tubule rudiment. This curved pocket-like structure increases in length, its tip comes into contact with, and later fuses with, the dorsal wall of the- duct and it is in this way converted into a short tube opening at its inner end into the splanchnocoele and at its outer into the duct. The tubular structure so arising does not retain its simple tubular shape but undergoes the series of changes shown in Fig. 135. Its cavity dilates in the middle to form the deﬁnitive nephrocoele, the cavity of the Malpighian body (mb); its splanchnocoelic end becomes relatively narrow to form the peritoneal canal (p.c)9: its outer end becomes also relatively narrow and it is this outer portion which undergoes an immense increase in length and becomes the functional tubule.

Opisthonephric rudiments appear in the fashion above indicated throughout the greater part of the length of the body where the splanchnocoele is present. They commence behind the pronephros (about the 8th or 9th segment) and extend back to the cloaca or a few segments posterior to it. In the latter case the postcloacal rudiments do not come to anything. Their occurrence is to be looked on as a reminiscence of a period when the alimentary canal and splanchuocoele extended farther tailwards. It is to be noted also that the group of tubules at the front end which subserve a genital function in ‘the male similarly appear only as transient rudiments in the female.

The portions of the opisthonephros which perform an active renal function increase much in bulk and this, as elsewhere, is brought about not merely by the great increase in length of the individual tubules, but also by the addition of numerous new tubules, each with its Malpighian body etc., of the second, third and so on, order. Probably (Balfour, 1878) these arise by a process of budding of the nephrotomes of a similar type to that which occurs in Hypo 1 Care should be taken to avoid the not uncommon error of referring to the whole of this structure as the “ tubule-rudiment.”

’ Attention has already been drawn (p. 227) to the undesirability of applying the misleading adjective “ ncphrostomal ” to this canal. 250 EMBRYOLOGY OF THE LOWER VERTEBRATES CH‘. geoph/is though in the case of the Elasmobranchs the details seem to be more obscure and the descriptions are conflicting.

The male Elasmobranch is an excellent example of a Vertebrate in which the nephridial system is responsible for the function of conveying to the exterior both the renal excretory materials and the reproductive cells and we find a well-marked tendency to separate

A f l _. \ H B /, Pf -J Inc. M ‘pr C ’ m 5

Pf-J

Flo. 135. -——Illustrating the later development of a segmental unit of the opisthonephros in male Pristmrus. 'l‘he ligure is in each case a view from the mesial side.

(After C. Rab], 1896.)

A, 15th unit of 17 mm. embryo; 1!, 15th unit of 2225 mm.; C, 15th unit of 25-3 nun. ; 1), 25th unit from same embryo as C. m.b, Malpigluau body; p.c, peritoneal canal; p.f, peritoneal funnel; t, tubule.

each opisthonephric tubule leads has already

the routes of those two products towards the exterior. This separation is brought about by the shifting backwards of the openings of the collectingtubes of the posterior, purely renal, part of the opisthonephros, so that instead of being spaced out along the course -of the duct they come to be coincident with its opening into the urinogenital sinus. This backward shifting is most pronounced, and it also makes its appearance earliest in ontogeny, in the most anterior of the tubules in question. It is accompanied by a fusion together of the terminal parts of the collecting-tubes into a continuous ridge-like projection of the dorsal wall of the duct, in which the individual lumina are for a time greatly reduced or even completely obliterated. Eventually, as a rule, the ridge splits up and the terminal parts of the collecting-tubes regain their individuality forming a group of distinct tubes, varying in number in different forms from about 4 (Spmax) to about 15 (Acant/w'a,s), and converging so as to open close together into the urinegenital sinus. In some cases the splitting apart is not ‘complete and more or fewer of the tubes may be united together to form a longitudinal “ureter.”

The mode of origin of the Malpighian body—— the deﬁnitive condition of the nephrotome ——- from which been indicated. It

is for a time rounded in form (Fig. 135) but eventually one portion of its wal1—varying greatly in position———comes to bulge inwards to form the glomerulus containing a loop of blood-vessel.

The peritoneal canal during development lengthens out consider ably (Fig. 135, D) and becomes narrower.

This narrowing is most

marked in the posterior third of the opisthonephros and in this we Iv OPISTHONEPHROS 251

see what is probably the expression of a general tendency for the portion of coelome containing the glomerulus to become more and more completely isolated from the main splanchnocoele as the renal unit becomes more and more highly evolved. Eventually, in the adult of the majority of Elasmobranchs, the peritoneal canal becomes completely obliterated, but in a considerable number of others 1 this happens, if at all, only towards the anterior and posterior ends of the opisthonephros so that the greater part of the organ retains open peritoneal funnels throughout life. Bles (1897) has made the interesting suggestion that there is a physiological correlation between the persistence of open peritoneal funnels and the absence of abdominal pores —— secondary perforations of the wall of the splanchnocoelc in the neighbourhood of the anus which make their appearance, at a late period of development, in various Elasmobranchs and other Vertebrates.

URODELA.——The third type of development of the opisthonephros amongst the more primitive Vertebrates is found in the Amphibians, especially in the Urodeles. The excellent account given by Furbringer (1877) still forms a thoroughly adequate basis for the description.

The Amphibians possess, as has already been shown, a large and highly developed pronephros amply suﬂicient for their excretory needs during early periods of development. In correlation with this there is marked delay in the development of the opisthonephros, the myotomes having already become separated and their stalks or nephrotomes breaking up into mesenchyme before the opisthonephric units make their appearance. The rudiments of these units-—-the ne];hretomes——become reconstituted in the midst of the mesenchyme as solid cellular strands which may retain their metameric arrangement (Amplmluma —— Field, 1891; anterior segments in Trrvlton, Amblg/stoma, etc.) but usually have completely lost it. Each of these nephrotome rudiments is a solid strand of cells which curves outwards dorsal to the duct. In the anterior region where, as is specially clear in ’l’mIt_on, the inner end of the strand is for a time continuous with the lining of the splanclmocoele, the general arrangement is clearly the same as that of the Elasmobranch (cf. Fig. 136, A, with Fig. 134). The splanchnocoelic end of the nephrotome disappears for a time while the main portion develops a cavity in its interior and becomes converted into a vesicle with epithelial wall lying immediately dorsal to the duct (Fig. 136, B). This vesicle becomes elongated in a mediolateral direction (? by active growth of its outer wall) and then assumes a characteristic curvature first f\- and then in-like in shape (Fig. 136, C). The mesial end of the un gives rise to the Malpighian body, the remainder to the actual tubule, its outer end undergoing fusion with the wall of the duct (Fig. 136, D). The tubule grows rapidly in length and is forced into complicated

1 E.g. Cestracion philippi, Rlwina squatina, Scyllvium. camZcula,S. stellarr, {’r1Istiu'r'us mclcmostomus,‘ Spinax miger, Acanthiws vulgaris, Scymmos lwlclmla.-—(Bles, 1891). 252 EMBRYOLOGY OF THE LOVVER VERTEBRATES

CH.

coils and windings as it does so (Fig. 136, E) while the Malpighian body dilates and its dorsal wall becomes invaginatcd to form the glomerulus. _

As a rule the primitive continuity of nephrotoine with the splanchnocoelic lining disappears in the Amphibian as already i11dicated, but it becomes re-established by a peritoneal canal developing secondarily (Fig. 136, D) as an outgrowth, arising in Urodcles usually from the neck of the Mialpigliiaii body and in .Anura from a point farther down the apparent tubule, which grows towards and fuses with a thickening of the coelomic epithelium. Such displacements of the communication between Malpighian coelome and splanchnocoele are probably of a similar nature to those mentioned in the case of the pronephros of H3/pogeoplz/is (see p. 226 . - o

)In those parts of the opisthonephros which are actively renal in function, z'.e. the hinder portion in Urodeles and the greater part of the whole length in _Anura, there takes place great increase in bulk, associated with the development of generations of subsequent tubules. Such secondary, tertiary, etc. tubules make their appearance amongst the mesenchyme in the form. of cellular strands which resemble closely— both in their original appearance and in the series of

Flu. 136.——'.l‘ransv¢-rse st-ut.ioi1s showing changes pass th_r0ugh various" Stilgcs in the «levelopment of ———those from which the primary

the opistlionepbros. 1877)

A, Triton alpestris: B, 14 mm. ; C, D, Salamandra maculata, 17 mm. ; E, Salamandra mxwulata, 21 mm. ; F, Sa.la.ma/ndm mac-ulata, 25 mm. A, dorsal aorta; a..n.d, atchirlﬁpllric duct: 9. gonad: gl, glomerulus: n, nephrotome; nc, nephrocoele; p.c, rudiment of peritoneal canal; splc, splanchnocoele; t. tubule; tl $2, I3, primary, secondary, and tertiary tubule rudiments.

(After Fiirbringer,

Salmmndm maculata,

elements arise (Fig. 136, F, £3). Eventually the secondary tubule comes to open into the primary tubule, the terminal section of which thus forms a collecting-tube common to both, while the tertiary tubule similarly comes to open into Iv OPISTHONEPHROS 253

the secondary. As the various generations of tubules go on with their development, undergoing the same histological differentiation and increasing enormously in length, they become inextricably mixed up together to form the compact fully developed opisthonephros of the adult.

Eventua.lly, in the Urodele, the duct is slightly displaced outwards so as to leave a distinct gap between it and the opisthonephros across which pass the terminal parts of the collecting-tubes. In the male Urodele the openings of these become, as a rule, shifted backwards to the hind end of the duct as in Elasmobranchs.

The Amphibia alone among tetrapod Vertebrates retain the relatively primitive feature of possessing open peritoneal funnels in the adult, and they can be excellently demonstrated with their actively moving flagella by examining the slender anterior portion of the excised and still living kidney of a female Urodele in normal salt solution under the microscope. In the anterior genital portion of the opisthonephros of the male they as a general rule (not in Spelerpcs, Spengel) remain however obliterated. _

In the Anura (N ussbaum, 1886) the peritoneal canals at an early stage of larval life lose their connexion with the Malpighian body or tubule and establish a secondary connexion with the blood spaces between the tubules, thus affording a route by which the ﬂuid in the splanchnococle is returned to the blood, analogous to that provided by the lymphatic system in higher Vertebrates.

AMN1OTA.———In the Amniota the opisthonephros of the Fishes and Amphibians is represented by the mesoncphros and metanephros-— and it will be convenient to consider the mesonephros ﬁrst.

MESONEPHROS OF BIRDS:--AS has already been pointed out one of the marked differences between Amplmloams and the Craniata is that in the latter segmentation is no longer apparent at any stage of development in the ventral or splanehnoeoelic region of the mesodcrm. The Amniota show a further accentuation of this difference inasmuch as the loss of mesodermal segmentation has extended so far towards the dorsal side as to involve the region of the nephrotomes. In the early embryo of the bird the nephrotomie part of the mesoderm has the form of an unsegmentcd mass—the intermediate cell-ma.ss——showing more or less distinct traces of being composed of a somatic and a splanchnic layer continuous with the corresponding layers of the splanehnoeoelic mesoderm and of the myotome. Although the intermediate cell-mass no longer consists of discrete nephrotomes, traces of its primitive segmental nature persist in its connexions with the segmentally arranged myotomes and in the fact that its connexion with the lateral mesoderm is not continuous in a longitudinal direction.

As regards the mode of origin of the actual mesonephric units differences exist, as was shown long ago by Sedgwick (1880), which are of much interest owing to the fact that the less modiﬁed mode of development found at the front end of the series is readily correlated 254 EMBRYOLOGY OF THE LOWER VER'l‘EBRATES OH.

with that which is found in the Anamnia, while the more highly modiﬁed mode of development occurring posteriorly is equally readily correlated with what happens in the Inetanephros of the Amniota. In the a.nterior region (approximately segments 12-15) the intermediate cell-mass is compact, rccognizably two layered, and the split which separates the two layers may he obviously continuous with the splanchnoeoele (Fig. 1317, A). It separates at an early stage from the myotome, but it remains continuous at intervals with the lateral

Flu. 137.——Sections illustrating the development of the xnesonephros in Birds. . (A and B, after Sedgwick, 1881 ; C, D, E, after Schreiner, 1902).

A, 22-segment chick at level of the 15th segment; B, 34-segment r_-hick at level of 13th or 14th segment (combined from two sections); 0, 45-segment duck at level or :2‘.Hh so-;;u1~-rut; I), 45-sogrm-.nt duck at level of 25th segment; E, 45-segment duck at level of 24th :-u-_-,-nu-nt. .—l, dorsal aorta; u..'u-.d, archinephrieduct; glam, glomerulus; no, nephrocoele; nr, nu-pin-otmm-.; pm, pm-itonuul (::u'1ul; p.c.-v, posterior cardinal vein; splc, splanchnoeoele; t, tubule rudiment.

mesoderm. The intermediate cell-mass becomes closely apposed to and very soon directly continuous with the duct by a narrow isthmus in each segment——the tubule rudiment (Fig. 137, A). Ventrally, 7J.e. near its junction with the splanchnoeoele, the split between the two layers of the nephrotome dilates and forms a deﬁnite nephrocoele which opens into the splanchnoeoele by a wide peritoneal canal (Fig. 137, B, 19.0). The tubule rudiment develops a lumen leading from nephrocoele into duct} and the dorsal wall of the nephrocoele becomes

1 The opisthonephric duct in the Anmiota is known as the mesonephric or Wolﬁan duct as its function 13 restricted to draining the mesonephros or “ Wolfﬁan body.” IV _ MESONEPHROS 255

invaginated into the cavity to form a glomerulus (glam) which may become much enlarged so as to extend right out into the splanchnocoele.

As the process of development is traced back into the second region of the mesonephros (stretching approximately from segment 16 to 19 or 20) a distinct modiﬁcation becomes apparent. The intermediate cell-mass in this region becomes loosened out into mesenchyme, and amongst this loose tissue what may be termed the definitive nephrotomes make their appearance secondarily in the form of roundish condensations of cell elements. Each of these becomes more and more sharply marked oil from the surrounding mesenchyme, its cells assume a radial arrangement, and presently a small rounded cavity appears in the centre. This cavity dilates and the result is a hollow vesicle with a wall composed of a single layer of cells————the deﬁnitive nephrotome.

In the third, hinder, region of the mesonephros, extending from about segment 20 or 21 backwards, the -process in the Fowl, though not in the Duck, has undergone the further modiﬁcation that the intermediate cell-mass is from an early period completely isolated from the peritoneal epithelium. The peritoneal canals have here completely disappeared, except for faint vestiges, the cells of the peritoneal epithelium still showing here and there traces of the same arrangement as they have further forwards where they are passing into a peritoneal canal. Apart from this separation from the peritoneal lining, the process is similar to that already described. Here also the intermediate cell-mass becomes separated out into loose mesenchyme in which the deﬁnitive nephrotomes make their appearance secondarily.

An important feature in the above described processes of development is the obliteration of the primitive segmentation of the nephrotome region. When the definitive nephrotomes become visible, and so bring into view the metameric segmentation of the mesonephros, a further modiﬁcation becomes apparent in that the mesonephric segments, except towards the front end of the series, are more crowded together than are the_ primitive mesoderm segments as represented by the myotomes (Sedgwick, 1880). Thus in the Duck Schreiner (1902) found in the region of myotome XX, 4 or 5 mesonephric rudiments, in that of myotome XXV-7, in that of XXVI -—-9, in that of XXVII as many as 13.

As development proceeds, the mesonephric elements become still more crowded together inasmuch as from segment 21 or 22 backwards “ subsequent” nephrotomes make their appearance in the mesenchyme. These closely resemble in appearance the primary nephrotomes, with which they are at ﬁrst in close proximity if not in actual continuity, and they develop in succession one over the other, each series forming a vertical row over its primary nephrotome. The number of subsequent tubules is greatest posteriorly where there are commonly four to a segment. 256 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

The later development. of the individual nephrotome of the mesonephros takes place in the Birds along lines exactly similar to what takes place in lower forms such as the Amphibia. The tubule rudiment originates as an, at ﬁrst solid but later pocket-like, outgrowth of the lateral wall of the nephrotome (Fig. 137, C, t). The tip of this presses against the mesial wall of the duct and, as the tubule grows in length, fusion takes place and the lumina of duct, tubule rudiment and nephrotome—which together form a characteristic m-shaped structure as seen in a transverse secti0n——bec0me continuous (Fig. 137, D and E). The portion of the m nearest the mesial plane represents the nephrotome in the strict sense, tie. the forerunner of the Malpighian body, and has assumed a watch-glass shape, its dorsal wall being involuted into the cavity as the rudiment of the glornerulus (glam).

The further development of the mesonephric unit, Which need not be followed out in detail in this book, consists in (1) the immense growth in length of the tubule, which leads to its becoming inextricably intertwined with its neighbours, (2) the histological dill'erentiation of its wall, and (:5) the differentiation of the Malpighian bod .

lt should be mentioned that where the tubules are much crowded together they do not all establish a communication with the duct in the typical manner above described. Some, even of the primary tubules, come to open into neighbouring tubules. In the ease of the subsequent tubules, some open into the duct in the typical fashion, others open into neighbouring tubules, while the majority become connected with pocket-like outgrowths from the duct. These outgrowths are greatly developed in some birds (of. Duck, Fig. 138, B), becoming much elongated and taking the form of branched collecting-tubes into each of which open a series of subsequent tubules (cf. H3/pogeoplmls), the whole condition distinctly foreshadowing arrangements presently to he mentioned in the metanephros.

The mesonephros acts as the renal organ only for a short period during the early stages of development. In the Fowl it begins to develop about the end of the second day of incubation, it reaches its maximum about the 7th or 8th day, and almost immediately thereafter begins to show signs of degeneration as the renal function becomes concentrated in the metanephros. The mesonephros never completely disappears though it ceases to be of any importance as a renal organ: its persistence is correlated with the fact that this portion of the opisthonephros has already in the forerunners of the Amniota important functions connected with reproduction. Its modiﬁcation in relation to these functions will be gone into later.

METANEPHROS OF BIRDS. —- The continuous mass of mesenchymatous tissue representing the nephrotomes or protovertebral stalks does not cease at the binder limit of the mesonephros at segment XXX: it is continued on through segments XXXI, XXXII, Iv METANEPHROS 257

XXXIII. and XXXIV to the level of the opening of the duct into the cloaca. The nephrotomal tissue in the segments mentioned remai'ns for a time passive but eventually it gives rise to the deﬁnitive nephrotomes of the metanephros. The metanephros is therefore ontogenetically as was indicated long ago by Sedgwick (1880) in its origin simply a tailward continuation of, the niesonephros. In the terminology used in this hook it consists of the greatly enlarged posterior segment or segments of the opisthonepliros. The development of the metanephros is inaugurated by the appearance of the rudiment of the

ureter or meta— \ai\82l8-9l.u/ nephric duct. This

arises as an out growth (Fig. 138, H,

M’) from the dorsal

wall of the nieso nephric duct near , its posterior end.

The outgrowth ex tends in a dorsal

direction and then

spreads out at its _. tip, pi‘ojecting very mn

slightly .ta1lwards 95 ' M I 2, | M l 9, , ,0 , ,, 1 ,, . ,3 i but growing much more actively in a headward direction along the outer side of the hinder or n,leba'n0Phrlc POI.‘ Fm. 138.-—ReL-onstructed outlines of hind end of mesoneplitic 171011 Of the n91)l1r0* duct and ureter in Bird embryos as seen from the left side. tomal mesenchyme. (After S<="I'ei"*=r» 1902-)

latter becolnes A, duck embryo with 48 segiiieiits; B, duck eHll)I‘)() with 50 st-5.-,iin-nts; secondarily (about 0, duck embryo, lO'75 nini.; I), fowl viubi-_\o, l3'.’» mm. )Il.’It.,l1N‘SO-. _ _ in-phros; W7‘, ureter. 'llie Arabic iiiinierals iiidicate the position ot the end 0t the the nlPSO(l6I‘m segnii-nts.

day) marked off’ by

a distinct break from the mesonephric portion. About the same time the dorsal wall of the actively growing ureter begins to develop pocket-like outgrowths (Fig. 138, D). These increase in length, branch repeatedly, especially the hinder ones, and become collecting-tubes. As this takes place the nephrotomal mesenchyme becomes condensed into small portions, one of which ensheaths the growing tip of each branch of the collecting-tubes. In these terminal caps of mesenchyme deﬁnitive nephrotonies gradually come into view, similar to those of the mesonepliros. In other words the deﬁnitive nephrotome is at ﬁrst a mere rounded cellular mass. This develops a lumen and, as the latter dilates, assumes a vesicular form, and ﬁnally the actual tubule makes its

VOL. II S 258 EMBRYOLOGY OF THE LOWER VERTEBRATES GIL.

appearance as an outgrowth which fuses secondarily with the tip of the collecting tube. The Malpighian bodies begin about the ninth day to develop their special characteristics in a manner similar to those of the mesonephros. An important point to notice is that the metanephros differentiates from behind forwards instead of in the opposite direction as does the mesonephros.

About 24 hours (the exact time varies greatly) after the ﬁrst appearance of the ureter the part of the mesonephric duct between it and the cloaea becomes incorporated in the cloaca so that inescnepbric duct and ureter come to have independent openings into the oloacal cavity.

As the metanephros goes on with its development it comes to be situated in great part dorsal to the Inesonephros but it will be understood that this topographical relationship is secondary. At first it is completely posterior to the mesonephros. Even the ureter is in its lirst stage localized about segment X XXIV (Fig. 138, 'B) and its extension forward as far as segment XXV or even farther is purely secondary. It will be noticed in Fig. 138 how exactly the ureter in its first beginnings resembles one of the pocket-like outgrowths of the duct which in the mesonephric region develop into collecting-tubes, and it seems scarcely possible to avoid the conclusion that the metanephros of the Fowl is simply the enormously hypertrophied nephridial apparatus of a single segment, the ureter being a greatly elongated collecting-tube with an immense number of subsequent tubules opening into it.

OPISTIIONEPIIIZOS IN OTHER GROUPS OF VER'l‘EBRA'l‘ES

CROSSOPTERYGII.—-—0ur knowledge of the early stages of development is still fragmentary being based upon three specimens of I-’ol3/pterus (stages 32, 33 and 36) obtained by Budgett (Graham Kerr, 1907).

In the youngest of these stages a number of the opisthonephric units have made their appearance in the form of rounded cell masses arranged segmentally in the mesenchyme ventral to the myetomes, those which are best developed possessing a distinct lumen.

In the specimen of stage 33 these rudiments have become elongated forming thick curved masses, one end of which is closely applied to, or even fused with, the dorsal wall of the duct. The lumen is restricted to the end farthest from the duct, which represents the deﬁnitive nephrotome, while the part which extends towards the duct-—the tubule rudiment——is solid.

In the 30-mm. larva described by Budgett (1902) the opisthonephros commences about 4 segments behind the pronephros and stretches through about 39 segments with from two to ﬁve Malpighian bodies and tubules in each segment. On the right side of the body 18 of the Malpighian bodies——rou_ghly one to each segment -—communicated with the splanchnocoele by a nearly straight peri1V OPISTHONEPHRQS 259

toneal canal. The earlier developmental material does not suffice to show deﬁnitely whether or not, as is probable, this peritoneal canal is a secondary connexion with the peritoneal epithelium. The peritoneal funnels exist only for a time during larval life : in specimens 90 mm. in length they had disappeared. In 0a.lr(.7mIchth3/s (Lebedinsky, l895) the peritoneal canals have been found still persisting in a larva of 15 cm.

AUTINOPTERYGIAN GANon>s.-—-Here again the deﬁnitive nephrotomes appear a.s solid masses of cells arranged segmentally. The gap separating them from the pronephros is in the more primitive Sturgeons about 3 or 4 segments, in the more highly evolved Amia 16 or 17 (J ungersen, 18923-4). 'l‘he rudiment grows in length, develops a lumen secondarily, joins on to the duct by its lateral end while its mesial end dilates to form the Mzilpigliiaii body——all in the usual fashion. At a late period—after the Malpighian bodies have already assumed their characteristic features-——they develop peritoneal canals as outgrowths from their walls which meet and fuse with the peritoneal epithelium secondarily. Later on the peritoneal canals become again obliterated and appear to be absent in the adult except in the case of Amia.

TEI.E()S'[‘EI.——I11 the Teleostean ﬁshes, as is indeed the case to a certain extent in all the members of the Teleostomi, the 0pisthonephros is delayed in development in correlation with the prolonged functioning of the pronephros. According to Felix (1897) in the Trout the ﬁrst opisthonephric units or deﬁnitive nephrotoines begin to make their appearance about '70 days after fertilization as rounded clumps of cells, in the centre of which a small lumen appears. These lie immediately dorsal to the duct, in the connective tissue trabeculae which at this stage of development traverse the cavity of the interrenal vein. These rudiments appear lirst about the middle third of the duct and gradually spread backwards, those in front being segmental in position while those farther back are no longer segmental and fuse together into irregular masses. Each rudiment grows actively in length and goes through the usual series of changes before joining up to the duct.

To the primary units just described are added secondary and tertiary units. These develop exactly as do the primary except that in the case of the tertiary set the tubule may fuse either with the duct directly or with an already developed tubule.

As the tubules increase enormously in length they become inextricably entangled together extending even across the median plane so that the substance of the two kidneys becomes continuous through the interrenal trabeculae. It is further characteristic of the teleostean kidney that there takes place in it a great development of round-celled pseudolymphoid (Felix) tissue. This forms a packing tissue between the tubules and appears to be formed by proliferation from the walls of the interrenal venous spaces.

The opisthonephros extends back for a short distance behind the 260 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

cloaca. This postcloacal portion drains into a pocket-like prolongation which grows back from the duct, usually on the right side only.

A remarkable peculiarity has been observed in certain Teleosts (I3e11a.(log(1ste7-, (fruitcl, 1901) in which, correlated with the persistence of the enlarged glemcrulus of the pronephrcs, the Malpighian bodies of the opisthcnephros have, at least in the adult, completely disappeared.

l)IPNo1.—ln the Lung-fishes the development of the opisthonephros closely resembles that in Ampliihia. ln Leynldosi/ren and 1"r0to;ntm'u.s' the units appear as rounded, at ﬁrst solid, masses independent alike of myotome and of peritoneal epithelium. In Promprrrus they commence about segment 14-18 but in some specimens they ll.]_|])('.:l.l‘ to be represented by slight eondensations of Incsenchyme right forwards as far as the hind limit of the pronephros. The rudiments extend hack to about segment 36 73.6. to about one segment in front of the cloaca. They are roughly segmental in position and remain so during the greater part of larval life. Each rudiment gives rise to a typical Malpighian body and a tubule which joins on to the duct secondarily. There does not appear to be any trace of peritoneal canals developed although they are for a time present in Ccmtorlus.

The development of the primary units is followed by the development of subsequent ones but the origin of these has not so far been worked out.

In Protoptm-w.s, though not in I167)/I:I]0r5‘?lT67l/, the hinder ends of the kidneys become continuous across the mesial plane and this fused portion becomes gradually marked off conspicuously by its pale colour the cortical region of the paired kidneys becoming crowded with amocbocytes laden with melanin which settle down there and give it a coal-black appearance.

REP'I'ILIA.--In the Reptiles we ﬁnd, as we should expect, that the process of development follows upon the whole the same lines as in Birds but at the same time shows various features in which the condition remains more primitive. Thus in Lacnrta Schreiner (1902) ﬁnds that, except in the hinder portion of the opisthonephros, the units arise directly from typical nephrotomes or protovertebral stalks. These become isolated from the peritoneal mesoderm and then from the myotoine. Each develops a lumen and becomes vesicular and its‘ lateral wall gives rise to an outgrowth which becomes the tubule rudiment and fuses with the duct. N o peritoneal canals are developed, though vestiges of these may appear——a vestigial peritoneal funnel appearing as a slight projection from the splanchnocoele into the ventral end of the nephrotome (Lacerta), or the latter remaining for a time connected with the peritoneal lining by a solid stalk representing the peritoneal canal (Angwis).

In the posterior segments the nephrotomes are no longer distinct: they form a continuous mass of mesenchyme stretching uninterruptedly from segment to segment. In this, cellular condensations occur which give rise to definitive nephrotomes and these also are no longer strictly segmental, there being about 2 to each segment from segment 25 to 3 .

The deﬁnitive nephrotomes pursue the normal course of development. The first to appear are towards the ventral edge of the nephrotomal tissue but later other subsequent units appear in succession more dorsally. .

General Embryology of the Renal Organs of Vertebrates

The main problem connected with the morphology of the renal organs is that which deals with the serial homology of its constituent elements. Lankester (1877) clearly implied this homology when defining his terms archinephros etc. while, looking at the natter from a more strictly embryologieal standpoint, Sedgwiek

l88l) formulated the View that pronephros, mesonephros and metaiephros are simply successive portions of a single elongated ancestral excretory organ possessing a d11ct and segipentally arranged, serially iomelogous, tubules.

In discussing this archinephros theory it is necessary to bear in nind the following points :-—

(1) The names pronephros, mesonephros and metanephros accordng to their original definitions signify three sets of renal structures 'orming a succession along the length of the body in a tailward lirection :-—(a) an anterior or headward Set, (b) a middle set and (c) l. posterior or tailward set respectively. It is inadmissible by the

erms of the original deﬁnition to use them in any other sense and .0 do so is bound to lead to confusion.

(2) In addition to the anteroposterior series of renal units there nay develop a sequence of elements within the same body segment-5.6. the development of the primary unit may be followed by the groduction of a series of subsequent un-its, secondary, tertiary, luaternary and so on, probably derived originally from the primary lephrotome by (5. process of budding. The extent to which such subsequent units may develop differs greatly in different animals and 11 different segments. In the pronephric region there are commonly 10118, in the opisthonephros of Hy/pogeoplmls there may he as many as awenty in a segment, while it is possible that the metanephros of the Bird is to be looked on as a gigantic mass of subsequent tubules aelonging to a single segment.

It is obvious that in comparing renal elements of different parts )f the series care must be taken that the comparisons are made Jetween elements of the same order, and it is further obvious that a langer to be guarded against is involved in the theoretical possibility )f the suppression of the tubules of one order—say the primary ,ubules—in some particular region.

The comparison of mesonephros with pronephros involves then these two fundamental questions

(1) Does the mesonephros contain a set of units of the same order is those of the pronephros ?—i.e. in this case p7'7Jmcw~y elements and (2) Are these elements serially homologous throughout the length of pronephros and mesonephros?

Froni the facts of development as stated earlier in this chapter it is clear what the answer to these two questions must be. lt has been shown that in 11;:/pogenplm and other forms the ﬁrst tubule to appear in each segment of the opisthonephros arises as a direct outgrowth from the nephrotome exactly in the same way as the pronephrie tubule: it is clearly then a primary tubule, and its Malpighiau body, arising directly from the main part of the stalk, is also primary. The evidence then seems conclusive that in II;/po/;z’n}2lz;-z3.~= the pronephric and opisthonephric tubules form a homologous series, and naturally if this is true of I[‘(/})U.(/e0I)]’I1.s it is, in all probability, true of other Vertebrates.

Yet the View has been strongly advocated and is still held by many morphologists that there is no precise homology between the units which build up pronephros and opisthonephros. Riickert, van Wijhe, Field, Semen, Boveri, Felix, have been among the more important protagonists of this view. They have brought forward such arguments as the following :

(1) While the pronephric tubule arises as an outgrowth of somatic mesoderm, the mesonephric is derived partly from somatic and partly from splanchnic.

(2) The pronephric tubules arise relatively early and in continuity with the archincphric duct, the mesoncphric tubules arise much later and in discontinuity with the duct.

(3) The glomerulus of the pronephros is unsegmcnted and lies in the general splanehnocoele: that of the mesonephros is segmental and lies in a special chamber the cavity of the Malpighian body.

These arguments however do not appear any longer to have the weight which formerly attached to them.

(1) The evidence of Hypogeop/nfs that opisthonephric tubules arise as outgrowths of the somatic wall of the nephrotome just as do the pronephric tubules seems quite convincing.

(2) In II3/poge0p7m.'s all the pronephric tubules except the first three join up to the duct secondarily precisely as do the opisthonephric tubules. Further the precocious completion of the archinephric duct is a physiological necessity, in view of the early functioning of the pronephric tubules, and this in turn involves as a necessary consequence that the tubules behind those which ﬁrst function become joined to it secondarily.

(3) The glomerulus of the pronephros is segmental and the pronephric chambers are also segmental at ﬁrst in some of the more archaic forms and the unsegmented condition is purely secondary.

Another line of argument is directed not against the view that pronephros and mescnephros are built up of serially homologous units but rather against the strict homology of the functional parts of these units. Thus it is stated that in the’ region of the pronephros in addition to the main tubules there occur rudiments of other 1v RENAL ORGAN S 263

tubules which resemble more closely those of the mesonephros and similarly that in the region of the mesonephros, in addition to the ordinary tubules, there occur vestiges of another set of tubules resembling more closely those of the pronephros. Consequently, of the set of potential tubules (primary, secondary etc.) which is repeated in each segment, it is not the corresponding member which becomes the functional or main tubule in the pronephric and opisthonephric regions respectively. 'l‘o the present writer the various observations which have been brought to support this argument do not appear to be anything like so convincing as the very clear evidence afforded by Jly/pogeoplm: and he consequently holds that in the present state of our knowledge there is no adequate reason to refuse to accept the precise homology of the first-appearing (“primary”) tubules of the opisthonephros with those of the pronephros.

The idea of the primitive continuity between mesonephros and metanephros is less open to attack than that between the pronephros and the anterior (mesonephric) portion of ‘the opisthonephros. Apart from the evidence of embryology we ﬁnd in various of the lower vertebrates (Elasmobranchs, Urodeles) an elongated opisthonephros in the adult which shows in the clearest possible manner an incipient stage in the differentiation of the organ, into an anterior genital region and a posterior renal region, of precisely the same kind as we believe to have taken place in the Anmiota.

Further we have seen that in actual ontogeny the tubules of mesonephros and metanephros arise from an at first perfectly continuous mass of nephrotomal mesenchyme. As regards the minor problem whether one or more primary tubules still persist in the metanephros among its immense mass of subsequent tubules there is, as yet, no adequate evidence.

Accepting then the idea of the arehinephros as a sound theory of the primitive condition of the renal system of Vertebrates we may sketch out the probable course of the modiﬁcations which have come about in its development somewhat as follows.

Primitively its tubules developed——in accordance with the development of the body-segments generally—in regular sequence from before backwards.

The disappearance of segmentation in the ventral portion of the coelome enabled the early-formed tubules-——those towards the head end—to drain the whole length of the splanchnocoele. Correlated with this these tubules became greatly enlarged and their eﬂicieney greatly increased.

This high development of the anterior tubules to drain the whole splanchnocoele enabled them to cope with the entire excretory needs of the developing animal for a prolonged period and the tubules behind them in the series being unnecessary were either delayed in their appearance or ceased entirely to develop.

Thus a gap arose separating oil‘ the precociously developed tubules as the pronephros. Within the pronephros itself there was a tendency for functional activity to become specially marked in certain tubules these becoming enlarged in comparison with the others. The increase in size of pronephrie tubules was accompanied by increase in the size of their glomeruli, which consequently came into contact and fused together.

As the pronephric tubules drained the whole splanchnocoele the peritoneal canal leading to their nephrocoeles became wider and wider until at last they ceased to be marked off from the rest of the splanclmocoelc.

The opisthoncphric tubules the renal functions being still fora time undertaken by the proncphros -developed in regular sequence from before backwards. With the acquisition of new outlets for lluid in the splalichnocoele, such as abdominal pores, or connexions with lymphatic or blood vessels, the peritoneal canals leading from it into the nephrococlcs (Malpighian bodies), in which the secretion of coelomic fluid was specially concentrated, became gradually reduced and finally disappeared, there being no longer any physiological need for them.

Witliiii the series of opisthonephric tubules, the excretory function became more and more concentrated in the segments nearest the cloacal opening. In these segments the opisthonephros increased in bulk owing to the specially active budding processes which gave rise to successive generations of subsequent (secondary, tertiary, quaternary and so on) tubules. ,

'l‘he ﬁnal stage in this process was reached in the Birds, where renal activity ‘became concentrated in a single segment close to the cloaeal opening. In this segment an immense hypertrophy of the opisthonephric elements took place, successive generations of tubules being added on in front. Thus the opisthonephrie mass belonging to this segment came to extend hcadwards dorsal to the anterior portion of the opisthonephros (mesonephros) and became the definitive kidney or metanephros.

Olucm or THE N EPHRIDIAL DUcTs.—As already pointed out the nephridial tubes in eraniate Vertebrates open primitively into a longitudinal archinephric duet-the presence of this duct being the most conspicuous feature which differeritiates the renal system in Vertebrates from the presumably ancestral condition as exempliﬁed by Annelids, where the tubules open separately upon the external surface.

Two possible ways in which this duct may have originated in evolution have already been indicated and it has also been indicated that on the whole the balance of probability seems to be in favour oi the view that it came into being through the backward shifting of the external opening of each tubule till it beca.me coincident with the next behind it.

Those who take this view usually assume that the archinephric duct originally opened posteriorly upon the outer surface of the body and that its opening became secondarily shifted into the cloaca. lint as already pointed out there is no embryological support for t.his view. Everywhere the archinephric opening is at first within the endodermal part of the alimentary canal and this suggests that the communication of duct with cloaca has come about in some other way. The evidence of 1’oly/ptems suggests as already indicated that the opening into the cloaca represents the persistent primitive communication of a mesoderm segment with the enteron. It is quite conceivable that a secondary communication between archinophric duct and gut may have come about in this Way, in correlation with the pronephric -part of the archinephros reaching the actively functional condition at a period when the mesoderm segment at the level of the anus had not yet been completely separated from the endoderni. Once this secondary opening was established it would be a natural consequence for the post-anal portion of the nepliridial system to atrophy and disappear.

The hypothesis indicated in this description derives the nephridial apparatus of the Vertehrata from an ancestral condition resembling that characteristic of Annelids —-—the main difference being that in the Vertebrates the nephridial tubes open into a longitudinal duct which at its hinder end communicates with the alimentary canal. It is of great interest then to ﬁnd even within the rou of the Annelida clear expressions 0% the? tendency for the nephri- ‘Fm’ .13-9'-Diagmm of am

_ _ posterior end of the body of dlal l}l1b0S l';O open 1l'll)O such 3. duct. Allolobephom (I-n£i1;(tv.'. as seen best marked case of this known up to the “Om tl1°}‘°1‘-W -7‘i‘l° to SNOW present appears to be thatof the Earthworm the re1“h°m °i thc Mal

, _ organs (shown in black) All0l0b0])}L0?'a cmtzpae (leS(3I‘lb6(l R088. according to Rosa (1906).

(1906). Here (Fig. 139) in the posterior portion of the body the nephridial tubes lead into a longitudinal duct which fusing posteriorly with its fellow opens into the alimentary canal on its dorsal side and near its posterior end. In other words in this particular case an arrangement precisely like that of the vertebrate has been evolved out of an ancestral condition in which segmentally placed nephridial tubes npmuml independently upon the outer surface.

In regard to theorigin of the typical metanephric duct or ureter as seen for example in a Bird there are two obvious possibilities. If the mctancphros represents a number of nephridial segments its special duct may have originated by such steps as are represented by the adult conditi_on in male Urodeles and male Elasmobranehs tie. by the openings of the collecting-tubesinto the original duct becoming displaced backwards. Or on the other hand if the metanephros 266 EMBRYOLOGY OF THE LOWER V ERTEBRATES CH.

represents the greatly enlarged tubule system of a single segment the ureter would probably have arisen simply by the enlargement of the collecting-tube of that segment. When one studies the facts of development as now known (see p. 258 and especially Fig. 138) the balance of probability appears to be decidedly in favour of the second of these hypotheses representing the method by which the ureter has actually arisen in phylogeny.

TIIE (a‘r0NAD.——Thc great mass of the cells which constitute the body of a Vertebrate or any other of the higher Metazoa are specialized for the performance of particular functions in the ordinary life of the individual, and, correlated with this specialization, such cells have lost the power of giving rise to reproductive cells or gametes. The main mass of the body constituted of such specialized cells is known technically as the soma. At one or more points in the body there remain however patches of cells which have not uncle.1'gone this specialization for ordinary vital functions an d which retain the power of giving rise under favourable circumstances to gametes. The sum total of such cells constitute the gonad. The word gonad is commonly used in a loose sense as an equivalent of ovary or testis but it should be borne in mind that each of these organs contains a large proportion of immigrant tissues ——connective tissue, blood, nerves and so on——-which are strictly speaking part of the soma.

The problem of greatest general importance attaching to the development of the gonad of Vertebrates is that which concerns the origin of the.cells (gonocytes) which constitute it.

And the interest of this question rests especially on the fact that in certain invertebrates the germ-cells have been traced back to blastomeres specially set apart at early stages of segmentation. All the probabilities seem to indicate that such a process if it occurs in the animal kingdom at all, is of a fundamental character and that indications of the same process may be conﬁdently looked for in other groups.

The most however that we seem to be justified in asserting to be deﬁnitely established for Vertebrates is that genital cells are derived from the mesoderm of the coelomic wall. Apart from the actual facts of observation such development of gonocytes from coelomic lining ﬁts in well with general morphological ideas. It is clear that we must believe that in the simplest diploblastic ancestor of the Vertebrates the gonocytes were derived from epithelial cells. It is also clear that, on the view that the Coelomata passed through an Actinozoan-like stage during their evolution, we must regard it as probable that during that stage the gonocytes were situated, as in existing Actinozoa, in the endodermal epithelium lining the pockets between the mesenteries~ ~an epithelium which, on that view, is represented by the endoderm of the enterocoelie pouch of an Amphioxus embryo and by its derivative the coelomic mesoderm of an adult Amphioxus or other Vertebrate.

In Amphioxus the gonad of the adult shows special peculiarities which mark it off from all other Vertebrates. Bearing in mind however that the general arrangement of the mcsoderm of the adult Amphioxus, which also shows striking peculiarities, is preceded by a condition in ontogeny which there is reason to regard as more nearly primitive than occurs in any other V ertebrate—~-the possibility at once suggests itself that this may also be the case with the gonad. Consequently it becomes important to enquire what are the early conditions of the gonad in Amp/w'owu3 and whether it is reasonable to interpret the conditions in the more typical vertebrates as being modiﬁcations of those illustrated byAm,pIwl09c'us.

The earliest so far recognized stage of the gonad (Boveri, 1892; Zarnik, 1904) consists of a thickened portion of coelomic epithelium at the ventral end of the mesoderm segment 03.6. in the region where at an earlier stage the segmented part of the mesoderm was continuous with the portion which loses its segmentation. The thickening lies close to the headward boundary of the segment and within its ventral angle. As the segment has already become nipped oil‘ from the lateral mesoderm it is not possible to say from actual observation that the thickening belongs to the splanchnic rather than the somatic wall though this is probable from the condition in the more typical vertebrates. The genital thickening is repeated over a number of segments (from about the 9th or 10th to about the 34th or 35th--Zarnik).

There are then three important points to be gathered from the study of the origin of the gonad in AI/:1,plmI0a:'as:-~ (1) It arises as a thickening of coelomic epithelium ’i.e. it shows the mode of origin characteristic of coelomate animals in general,

(2) It arises close to the boundary of segmented and unsegmented mesoderm, and

(3) _It arises on the dorsal side of that boundary.

ln the more typical Vertebrates the ovary or testis ﬁrst becomes clearly recognizable as a rule in the form of a longitudinal ridge—— the genital ridge-——-which runs along the dorsal wall of the splanchnocoele on each side, at a varying distance from the line of attachment of the dorsal mesentery, and projects into the splanchnocoelic cavity. The genital ridge commonly extends over a greater antero-posterior extent than does the functional gonad later on--6.9. in the Salmon of the 185th day it extends from about the level of the fourth trunk myotome back to behind the anus (Felix). '.l‘he restricted portion of the ridge which is destined to develop into functional ovary or testis is termed by Felix the gonal portion to distinguish it from the portions in front (progonal) and behind (epigonal) which remain sterile.

The relatively great anteroposterior extent of the gonad during early stages in its development is probably to be regarded, along with the greatly elongated condition in the adult of some of the more archaic Vertebrates, as evidence that at one period of evolution the gonad extended throughout the whole length of the splanchnocoele.

As development goes on the genital ridge‘ increases in depth and is new termed the genital fold. Tliis is composed of peritoneal epithelium covering a supporting and, later on, vascular core cl’ mesonchymatous connective tissue.

The rudiment of the actual gonad in the strict sense consists of a thickening of the peritoneal epithelium covering the genital foldthe germinal epithelium. This thickened germinal epithelium may extend over both mesial and lateral surfaces of the genital fold as in most Amphibians, Reptiles and Birds or it may be restricted to its lateral (I(:lz.t/2,y/up/uls S? , most 'l‘eleosts) or median (Elasmobranehs except in very early stages, Icltthy/opI:.'z's (5) surface.

(_)f the more primitive holoblastic Vertebrates the Amphibia are the only group on which detailed observations on the origin of the gonad have been recorded. We shall accordingly summarize the early stages in the development of ovary and testis in this group and where possible in its more primitive subdivision the Urodela.

Fig. 1-10 illustrates the earliest stages of the gonad so far identiﬁed in Urodeles, as described by Schapitz (1912) for the Axolotl. Fig. A is taken from an embryo in which the protovertebral stalk or nephrotome is not yet completely restricted oil’ from the inyotolne. On its outer side is seen the rudiment of the arehinephrie duct. The stalk is continuous ventrally with the. lateral or splanchnocoelic mesoderm. In its inner portion certain of its cells (tag. the two adjoining cells in the ﬁgure in which the nucleus is shown in a darker tone) are beginning to show recognizable indications of nuclear and cytoplasmic features which are characteristic of the gonad later on. It will be borne in mind that the wall of the protovertebral stalk is morphologically part of the coelomic wall to which therefore these gonad cells also belong. In the sections shown in B and C the mass of cells showing these peculiarities has become more and more distinctly marked 011' from the lateral niesoderm (mes) and may now be spoken of deﬁnitely as the gonad. In the stage illustrated by D the lateral mesoderm is seen to be spreading inwards towards the mesial plane ventral to the gonad and it is beginning to show here and there a distinct split separating its somatic and splanchnic layers. In the later stages (E, F, G) this split becomes a patent cavity-—the splanchnocoele (spIc)——and the gonad is seen to lie on the dorso—lateral side of this, separated from the actual cavity by the somatic layer of peritoneal epithelium. In the last stage ﬁgured (G) the gonad is causing a slight bulging of the peritoneal lining into the splanchnoeoele: this bulging is the incipient genital ridge (gm).

During the earlier of the stages illustrated the gonocytes gradually acquire the superﬁcial histological characters of germ-cells. The cell-body is larger than that of the other cells, it remains full of yolk particles, and in the spaces between the latter are to be seen ﬁne granules of dark pigment. The nucleus is elongated or lobed in shape, the chromatin distributed in ﬁne particles so that the nucleus as a whole stains less deeply than do the nuclei of other cells, and IV GONAD 269

large round nucleoli are present, frequently corresponding in number

With the lobes of the nucleus. , The embryonic gonad during the stages which have been described

Fin. 140.——Origin of the gonad in Am-b/_'I/.s'l(_mI(.l. (After Selmpitz, 1912.) A, 7-8 (lays after fertilization ; B, 1.0 days ; '3, 12 days ; D, 18 days ; E, 17-18 days ; F, 19 days ; G, just after hatching.

u.-ml, :ll‘('llill|‘])ll[‘i(' duct; 1], ;.';mI:u| ; g.r, 3.-“u-nitul 1-i:l;_;-n-; nu-s, Intvml nu.-soul:-rm: -my, m_vntr_mw: .s-pie, spinm-lnu)uoo-la‘. .-\1.- -“ in Fig. I) is st-o-n Inn‘ «of HM‘ hc:l\'i|,\' _\H|l<|‘<l vvlls \\'llil'll .-m- int.«-rim-It-ml by smut. ats a.eeessur_y gu|i0eyt;e.~'.

is not as a. rule continuous from end to end. On the contrary it consists of 'l.S(')l£Ll)(3('l pleces and these HI numux-' <'-.:1.ses Show distinct traces of metaineric arrmigenient, the pieces being directly opposite the mesoderm segments. The discontinuity becomes less marked in the later stages but even in an 18-day embryo Schapitz found the gonad still consisting on one side of the body of metamerically arranged blocks while on the other it had become a continuous strand, except for a single small isolated piece posteriorly.

From what has been said it seems clear that the gonad of the Urodela is a derivative of the coelomie wall lying close to the boundary between the segmented and the unsegmented (lateral) portions of the mesoderm. As in early stages it consists of blocks with a roughly segmental arrangement it would appear to lie on the dorsal or nephrotomal side of the boundary mentioned. There is no apparent reason for declining to interpret this early segmented stage of the gonad as a persistent trace of a primitive segmental arrangement like that of Awtplmloams.

'I‘he tendency for the segmented condition to disappear in the typical V ertel:»1'-ates is adequately explained by the gradual dorsalward encroachment of the unsegmented splancbnocoele. The boundary between segmented and unsegmented (lateral) mesoderm has altered much in position during the course of evolution, and there is no adequate rea.son to suppose that this boundary is not still a lluctuating one and if it is so we may expect varying traces of the original segmented condition to present themselves during development.

'l‘he gonad has been described as being paired throughout but it may be mentioned that various observers have noticed an unpaired condition at one or other period during the early stages of development. This appears to be adequately interpreted as a secondary fusion similar to that occurring between the right and left opisthonephros in a Teleost or in 1-’9'ot0_pte'ru.s rather than as a primary condition.

We have traced back the gonad to its first recognized beginnings in one of the relatively primitive holoblastic Vertebrates. Before passing on to its farther development it has to be noticed that there exists a considerable volume of evidence pointing to the existence of additional germ-cells which arise independently of the coelomie lining and some of which migrate into the germinal epithelium and may give rise eventually to functional gametes. It is not proposed to describe this evidence as it has not as yet, in the present writer’s opinion, reached the stage of being convincing. It does not appear to have been satisfactorily demonstrated that the supposed extra gonocytes are really gonocytes at all rather than somatic cells. What is needed to provide such a demonstration is a careful study by skilled cytologists of the nuclear features of these cells, to determine whether there are any deﬁnite nuclear characters (such as Boveri discovered to be present in the gonocytes of Ascaris me_qalocephala) showing them to be beyond doubt gonocytes and affording a means of tracking them down in their supposed migration. Mere shape and staining capacity of the nucleus as a whole, or presence of IV GON AD 271

nucleoli, do not seem sufliciently deﬁnite characters as these are probably directly related to volume and metabolic activity of the cell. Cytoplasmic features—<>f which much use is made in this connexion~—such as richness in yolk or roundness in shape are also unreliable. As regards the ﬁrst of these, the study of the development of embryos rich in yolk brings out clearly the fact that the cells in particular tissues do not, by any means, all keep pace with one another in their developmental processes. Individual cells lag behind, and one of the commonest characteristics. of such cells is that the yolk stored up in their cytoplasm remains unaffected for some time after that in the neighbouring cells has been completely used up. Obviously in such a case richness in yolk, even when occurring along with greater size due to less active division, does not constitute evidence of any weight as regards difference in morphological nature between the heavily yolked cell and those round about it. Again there is reason to believe that yolk may be stored up secondarily in particular cells or portions of tissue of a developing embryo as a preparation for future needs quite apart from the actual germ-cells.

As regards approximation to a spherical shape, it should be remembered that there is a usual tendency for irregularly shaped or branching cells, such as those of ordinary mesenchyme, to assume temporarily a rounded form at the period during and about mitosis. Such cells are apt to assume an appearance misleadingly like that of young germ-cells. .

The various features above indicated occurring together are sufficient to give a characteristic appearance to the cells in the main gonad but they form hardly deﬁnite enough criteria to prove that cells elsewhere are germ-cells in face of the strong probability that the Whole mass of germ-cells in the body are of a common origin.

GENITAL Rrncn AND GENITAL FoLn.——The genital ridge was left as a slight bulging inwards of the peritoneal epithelium covering in the gonocytes. As development goes on the ridge becomes converted into a prominent fold ~ -the genital fold. The peritoneal epithelium at ﬁrst passes continuously over the surface of the strand of gonoeytes but soon a change comes about in their relative positions the gonoeytes coming to be incorporated in the thickness of the epithelium which may now therefore be spoken of as germinal epithelium. The gonoeytes are to be seen ﬁrst along the free edge of the fold (Fig. 141, A) and this during subsequent development swells out greatly and forms the functional ovary or testis, while the proximal portion acts merely for suspensory purposes. The gonoeytes increase in number by mitotic division but are also reinforced from small apparently indifferent cells lying between them (Fig. 141, 0, gal). We may take it that these small cells are in all probability to be interpreted as cells of the original gonad which have lagged behind in development, though it is naturally difficult from mere observation to make certain that they are not ordinary peritoneal cells. At a particular stage in development (between 26 and 33 mm. in Rana teInporm"ia~~—Bouii1) the total number of gonocyteslin the gonad undergoes a remarkable reduction, e.g. from between 200 and

Flu. 141.-I)eve1opnient of the gonad in Amphibia as seen in transverse sections. (A, atter Schapitz, 1912 ; B—E, after Bouin, 1901.)

A, lar\-'n01'Axu10t| (.~'hnlII3/stowm m.ea:ica.na) ; B, Rama te-nzpm~m~z'.a, 20 mm. tadpole; C, 24 mm. tadpole; 1), 33 mm. tmlpolu; E, 35 mm. tadpole with hind legs C0n1plett*.ly developed. fol, nucleus of folliclecell; 0-}; Kﬁnmll "Old: {I-8. Wnititl Stlﬂlltli Kit‘, ££0110('}'tﬁ: gel, transitional stages showing conversion of apparently lmliﬂ‘ex-out cells into gonocytes; mes, inesenchyme; som, somatic mesoderm; spl, splanchnic meso<l<-i'ni; 3/, yolk.

250 in a frog tadpole of 26 mm. to between 37 and 46 in a tadpole of 33 mm. (Bouin). During this process individual gonocytes IV GEN ITAL ORGAN S 273

degenerate and in many cases they appear to be shed into the splanchnocoele leaving behind them the spaces or follicles in which they lay walled in by indifferent cells. The meaning of this phenomenon is obscure but a suggestion is made regarding it below.

A period of active mitotic division of the gonocytes new sets in which leads to the formation of solid masses of gonocytes projecting down into the interior of the young genita.l gland (Fig. 141, E). These gonocytes are the ancestral oogonia or spermatogonia as the case may be.

Up till now the genital fold has been spoken of as if it were merely a fold of the coelomic epithelium. As a matter of fact the fold very soon becomes invaded along its attached edge by immigrant mesenchyme cells forming a solid connective tissue core or framework which serves both a supporting and later, when it develops blood-vessels, a nutritive function to the developing germ—cells. The penetration of nests of gonocytes into this in the form of solid downgrowths of the germinal epithelium may be interpreted as representing an ancestral increase in area of the fertile portions of the germinal epithelium -—the increase being originally brought about by the formation of hollow downgrowths into the vascular stroma, and these downgrowths having secondarily lost their cavities. The otherwise mysterious degeneration and shedding of large numbers of gonocytes already referred to may probably be regarded as a means of providing room for the localized parts of germinal epithelium which undergo this active proliferation.

Ul{INOGENl'l‘AL NETWORK. A characteristic feature of the Vertebrate is the set of tubular channels—-—va.sa. eﬁ‘erentia—-which in most of its subdivisions connects the testis with the opisthonephros and is frequently to be recognized in more or less vestigial form in the female sex as well.

Apart from variations in detail, these channels may be said to pass from the cavity of the testis to the cavities of the Malpighian bodies of the opisthonephros. They are clearly recognizable during early stages of development as solid strands of cells lying within the genital fold (Fig. 141, 1), gs), the cavity in their interior developing secondarily. As regards their ﬁrst origin the majority of observers state that they are first recognizable at their renal ends and have therefore interpreted them as outgrowths from the coelomic epithelium of the Malpighian body, i.c. from the nephrotome. Other observers seeing them make their appearance gradually in the core of the genital fold and reaching the Malpighian body secondarily regard them as differentiating in situ from the mesenchyme, while still others have adduced evidence in favour of the cells which give rise to the strands being budded off from the peritoneal epithelium close to the attached base of the genital fold. The disparity between the statements of different observers is most reasonably to be attributed to actual variation in the mode of development. It may be assumed that originally the connexions of the genital portion of the peritoneal epithelium with the peritoneal funnels, or with the nephrostomes, were in the form of open eiliated grooves or gutters on the surface of the peritoneum, that later on these became closed in to form tubular channels, and that in actual ontogenetic development in the modern amphihia the development from the coelomic epithelium has become obscured except for traces now at one end now at the other.

At their distal ends the cell-strands in the male can be traced gradually farther and farther into the genital fold until they come into immediate relationship with the cell-nests of gonoeytes. Tn the female of Urodcla and Anura. the strands do not spread so far into the genital fold, nor are they, even in early stages, so well developed as in the male.

The fatty body is developmentally simply a portion of the genital l'old which becomes specialized as a store-house of fat. In Anura it is the progonal portion which undergoes this differentiation while in Urodeles and Gymnophiona the rudiment of the fatty body is continued backwards as a ridge along the mesial face of the genital fold throughout its extent.

The fat is stored in the connective tissue of the organ, the fat cells being usually interpreted as immigrant mesenchyme cells which have invaded the rudiment by its base of attachment. It has also been suggested that these fat cells are peritoneal in their origin (Abramowioz, l9l3)~——a suggestion of obvious interest in view of the general tendency in the animal kingdom for potential germ-cells to undergo degeneration in order to provide nourishment for the germcells which become functional.

Testis

The development of the functional testis out of the genital fold is seen in peculiarly simple and diagrammatic form in the Gymnophiona. Here the strands of the urinogenital network, as they sprout into the interior of the testis, anastomose together along its axis so as to form a central canal——-around which, embedded in the stroma of the organ, lie the rounded nests of gonocytes. Fusion takes place between each gonoeyte-nest and the wall of the central canal and then each nest develops a cavity in its own interior and becomes a hollow ampulla opening into the canal at its inner end.

Various modiﬁcations of this simple scheme are to be found. ln Grymnophiona themselves ampulla-formation becomes suppressed except in localized regions between successive vasa efferentia, so that intervening portions of the testis are sterile and form merely thin tubular connexions between the bead-like fertile portions. Again the ampullae vary in shape: they may be elongated and tubular (Discoglossas) or, as in the majority of cases, ﬂattened against one another by pressure. The “ axial” canal again may lie close to the surface: it may become greatly branched, as in most Urodeles, or may form a complicated network as in most Anura.

Ovary

In the differentiation of the ovary (Bouin, 190]) the most important points to be noted are the following. As regards the germinal epithelium the most conspicuous feature is of course the immense increase in size, accompanied by the storing up of yolk in the cytoplasm, exhibited by those gonocytes which are to become functional eggs. Synchronously the indifferent cells of the germinal epithelium in their neighbourhood become converted into follicular cells, having for their main function the ministering to the metabolic needs of the growing egg-cells. The intervening portions of germinal epithelium, which do not undergo this modiﬁcation, retain their germinal character and provide successive batches of eggs in successive breeding seasons.

The cellular strands of the urinogenital network assume, as in the male, a tubular form, their wall becoming a cubical epithelium. As in the male the ovarian ends of these channels come into close relation with one another and fuse to form a central canal. In the Anura. the fusion together to form an axial cavity appears to be less complete than in the male, a number of isolated central cavities being formed one behind the other. Fusions which take place later lead merely to reduction in the number of these central spaces (in Izfu//2.a from about 12-15 down to about 5-7-——Bouin). With further development, and the functional egg-cells increase in size, the epithelial walls of these spaces become thin and ﬂattened. Eventually they become pressed together and the cavity is reduced to a mere slit. The portions of the tubes lying nearer to the attachment of the ovary become vestigial.

The presence of these axial cavities in the ovary, homologous with the central canal into which the microgametes are shed iii the male, is of great morphological interest. It suggests the possibility that at one time the eggs were shed into this central space and therefore that the condition now holding in the Vertebrata, where the eggs are shed into the open splanchnocoele, is to be interpreted as a reversion to, rather than a persistence of, the primitive method of shedding the eggs.

Leaving on one side the elaboration of histological detail which is not dealt with in this volume, the development of ovary and testis shows in its main features great uniformity throughout the gnathostomatous Vertebrates, and may therefore be dismissed with a few general remarks. Everywhere we see the gonad consisting at an early stage of a localized patch of coelomic epithelium in contact with nutritive and supporting mesenchyme: everywhere We see this coming to project into the splanchnocoele as a more or less prominent ridge or fold.

A conspicuous feature is the widespread tendency towards increased localization of the actual functional areas of the germinal epithelium. This is seen on a small scale in the development of cellnests of gonocytes separated by indifferent or sterile portions: it is seen again in the restriction of fertility to a relatively small anteroposterior part of the genital fold, long progonal and epigonal portions becoming sterile: it is seen again even in the actual differentiated 276 EMBRYOLOGY OF THE LOWER VERTEBRATES CII.

testis where a considerable length towards the posterior end may lose its fertility and assume a merely conducting function,‘ or where such sterile portions may be repeated at regular intervals throughout the length of the testis (Gymnophiona).

This concentration of the activity of the gonad may affect its bilateral symmetry. In Elasmobranchs both ovaries may be present and functional (]3we'n1,a7'gus,Notidanusg7“?§s(ms), or one may be functionally inactive ((}entroph07'us, ’/’r_y_¢/on), or as happens in the majority, one, usually the left, fails to complete its development and is reduced to a more or less insigniﬁcant vestigc. A similar reduction of one ovary takes place in many Tcleosts. In the Birds the right ovary ceases its development at an early period and soon disappears entirely in the majority of individuals, although exceptions are of comparatively frcquent occurrence.

Ovnnr AND (_)vioUc'r or TELEOSTOMA'1‘(_)US Fisnus.-——1n the most archaic of existing teleostoniatous FiShes——tl1e Urossopterygian ganoids——the ovary is in the form of a typical genital fold which sheds its eggs into the splanehnocoele, from which in turn they pass out by a Miillerian duct. Consequently we may take it, in the absence of convincing evidence to the contrary, that the ancestral condition of the ovary and oviduct in the teleostomatous Fishes did not differ from that in other primitive gnathostomes such as Elas1no— branchs, Dipnoans, or Urodeles. A peculiarity however of the Crossopterygian oviduct as compared with that of the other groups mentioned is seen in the reduction of its glandular activity, and this reduction——which ﬁnds its physiological expression in the reduction of tertiary egg envelopes——-probably gives a clue to the subsequent evolutionary history of the oviduct within the group Teleostomi, in the majority of which the whole Mullerian duet has apparently been reduced to the verge of complete disappearance.

As regards the ovary itself there has come about secondarily-— perhaps in correlation with increase in number and diminution in size of the eggs——a condition in which the eggs are not set free in the general splanclmocoele but are shed into an ovarian cavity, the wall of which is in complete continuity with that of the oviduct. The ovarian cavity is formed by the walling in of the portion of splanehnocoele which lies along the fertile (usually lateral) face of the ovary. The precise method of enclosure differs in detail in different Teleosts. In some (e._q. I-’e'rca, Gaste7'osteu.s-, Acerdna, Zoaroes) the ovigerous surface of the genital fold becomes invaginated, or overgrown by ﬂaps which eventually meet and undergo fusion (Fig. 142, A): in others (ag. Uyprinoids) the free edge of the genital fold meets and undergoes fusion with the wall of the splanehnocoele (Fig. 142, B).

Which of these two types of development is the more nearly primitive cannot be stated with certainty but the balance of probability seems on the whole in favour of the ﬁrst mentioned, for the formation of folds or grooves of the fertile surface of the genital fold, so as to give increased area, is a very usual phenomenon, and the formation of a single longitudinal groove would readily lead to the ﬁrst—1nentioned condition. 011 the other hand the replacement of this condition by the second is also readily understandable.

1 Lepidasiren and Protopterus (Graham Kerr, 1901); 1’olyptorus and probably Teleosts (see below).

The ovary passes without a hreak into the oviduct which is simply the posterior sterile portion of the genital ridge in which a cavity develops .=econdarily——not always continuously-——~from hefore hackwards. The oviduct dilfers greatly in length in different Teleosts: in some (Zowrces, 03/cloptcw/.s) the ovary itself may stretch right back to the genital pore.

Although the above description fits in with the normal conditions, there are various Telcosts in which the processes of fusion connected with the ovary do not take place and in which the ovary remains as a genital fold hanging down. into the splanchnocoele,

I:‘u.'. 1l2.—l)iagranx illustrating the conversion of the genital fold into a closed ovary in the Teleostean lishes.

e.g. in the case of the Salmon fusion of the ovarian edge with the body wall takes place anteriorly for a short distance and again in the posterior sterile region, but the greater part of the fertile region of the ovary hangs free. In such cases the eggs are shed into the splanchnocoele and pass to the exterior by genital pores (compare Cyclostomata, p. 246).

Unfortunately we are still in almost complete ignorance regarding the development of ovary and oviducts in the Ganoids. From the little we do know it would appear that in Lepidosteus (Balfour & Parker, 1882) the ovary hecomes enclosed in the same manner as in Cyprinoids (Fig. 142, B). Posteriorly it is continuous with the oviduct as in 'l‘e1eosts generally. In the other Ganoids the ovary retains the form of a genital fold hanging down into the splanchnocoele while the oviduct is provided anteriorly with a coelomic funnel. The position of this funnel, far removed from the front end of the splanchnocoele, is sometimes used as an argument against the homology of this opening with the ostium of a true Miillerian duct, but such an argument carries little weight as we know from the higher vertebrates that the ostium of an undoubted Mullerian duct is liable to undergo secondary shifting into such a position. Again the fact that the opening lies on the mesial side of the ovary is adduced as an argument in the same sense but in this case We have delinitc embryologioal evidence from 1’0lypte'ru,.s' (Budgett, 1902) that this position is secondary, the early rudiment of the duct lying external to the ovary and immediately ventral to the Wolflian duet as is the case with a typical Miillcrian duct. Consequently there is no suilicient reason to doubt that these oviducts with open ostia in ganoids are really Mullerian ducts.

l’HYI.()GENY OF 'J‘i+JI.1«;0sTEAN 0VmUC1‘.—--Tlie facts of development show clearly that the main part of tho Teleostean oviduct is of the same morphological nature as the ovary with which it is continuous. It arises from the hinder part of the primitive ovary which has become sterile and assumed a merely conducting function.

The main diflieulty connected with the morphology of the organ is that of accounting for the joining up of the part of the oviduct of ovarian origin with the eloaca or exterior. Balfour suggested that this had come about by the oviduct becoming fused with the lips of the “abdominal pores.” As an objection to this was adduced the observation by llyrtl that in ﬁlo’/'/zzyrus abdominal pores exist along with oviducts. This objection disappears, however, if we remember that in Balfour's time there were confused together under the same name two different types of aperture-——true abdominal pores and genital pores. Substituting genital pores for abdominal Balfour's view seems still the most feasible. The probability seems to be that the main steps in the evolution of the Teleostean oviduct were as follows : (1) The primitive oviduct or Mullerian duct underwent gradual atrophy becoming gradually shorterl until eventually nothing was left but its external opening—~—-the genital pore. This process would doubtless be correlated with the loss of its glandular function and this in turn may have been connected either with the adoption of pelagic spawning, in which special tertiary investments for the eggs were no longer required, or with a special development of primary envelopes within the group. A stage was thus reached which is represented by the condition in Salmo. Of course We do not know whether Salmo has retained this condition or has reverted to it: the latter is more probable.

(2) The portion of splanchnocoele along the ovigerous surface became enclosed so as to form a cavity which served to conduct the shed ova back into the neighbourhood of the genital pore. Anteriorly the ovarian surface abutting on this cavity remained fertile, while posteriorly it became sterile, so that the posterior portion of the cavity performed merely a conducting function (oviduct).

(3) The lips bounding the posterior end of the oviduct from being merely in proximity to the genital pore came to be completely fused with the edges of the latter opening which consequently became the oviducal aperture. '

1 We may see early stages in this process illustrated by the Ganoids Amie and Acipenser.

Urinogenital Connexion

We have already summarized for the Amphibia the course of development of the urinogcnital network —the system of tubes or vasa, eﬁ”erent'ia which connect testis and kidney and which serve as the outlet for the sperm. It is now necessary to glance at some points in the general morphology of this system of tubes. It has already been indicated that at their genital end the tubes become merged together in the axial cavity of the testis. The latter we must regard as morphologically an isolated portion of splanchnococle into which the spermatozoa are shed although it is no longer traceable to splanchnocoele in actual ontogeny. It has also been suggested that the tubular channels were probably originally open grooves of the peritoneal lining which became converted into closed tubes as the gonad became isolated from the main splanchnocoele. .

’l‘he vasa efferentia frequently show a tendency, more or less pronounced, to anastomose together into a network. In the Amphibia it is a very general though not invariable rule that anastomosis takes place close to the edge of the kidney, forming the longitudinal “marginal canal” which is conspicuous in most Amphibians. A similar marginal canal is formed in Elasmobranchs.

In taking a general view of the system of vasa cfferentia we ﬁnd that one of its characteristics is, as in the case of the gonad itself, a tendency to increased localization of its functional portions. Thus during ontogeny in Amphibians the vasa eﬁerentia towards the hinder end of the series become blocked and non-functional, or disappear entirely, leaving only those at the anterior end functional. This process reaches its limit in such forms as Alytes or Discoglossus where only two or a single member of the series persist.

Similarly in Elasmobranchs the number of functional vasa efferentia becomes reduced to a few at the anterior end of the series (0cntr0ph07'us 9, Scyll'£'u/m 6, Acantlmlas 4-6, Pristiurus 3, Jllustelus 2-3, Rwia 1). The same happens in Amniota.

In the Dipnoi on the other hand the localization takes place at the hind end of the series, the functional vasa efferentia being reduced to about half-a-dozen (Lepidosiren) or to a single one (Protopterus). '

Another phenomenon which makes its appearance is the simpliﬁcation and shortening of the route by which the spermatozoa pass from the vasa efferentia towards the exterior. Primitively the vas efferens opens into an otherwise normal Malpighian body containing its glomerulus and continued into a functional renal tubule. This condition may persist (Rama esculenta, Buj'o), or the glomerulus may disappear (R. temporaria), or ﬁnally the whole Malpighian body and its tubule may be shortened and widened and converted into a simple tubular continuation of the vas efferens towards the opisthonephric

FIG. 143.—Diag1-am illustrating the ex olution of the genital duct of the male Teleost. A, condition in which the sperms were shed mtu the .».planchur)coe-19 and passed outwaids tlu ough t-he nephrostomvs and tubules of the Op1St:h0I1PphI‘0~ ' B, test].

sh'ut qﬁ‘ from s'planchnocoel_e and_c0rm_numcatmg with the (.'{l.Vlt1t‘S. of certain lfutalpighian bodies scattered throughout. t3he_1ength of kidney and te~ti~. (exempliﬁed by ficzpenser, ebc.): C, t.est1s_ divided into functional (T1) and sterile (1?) portions and vasa elferentia reduced to a few in hinder portion of T, (exempliﬁed by I-91”’1°3_”'¢’n)1 D. "833 efferentla reduced _to a_§mgle one ap hind end of '12 (exempliﬁed by I'ro!uptcrus): E, the communication; between the hind end of T2 and the °P1Sth0n€PhUC duct has become slmphhed (exempliﬁed by Polypterus or young Teleost). :1 «vona-1 (portion of lining of coelome); and, duct» of opist;ho—

nephros ; p.e, peritoneal lining with nephrostomes opening through it. ‘ ' O

duct (Dis00gl0ssw,.9, Alg/tes, anterior vasa eiferentia of B07/2b'i7uttor). In A13/tes the single vas efferens with its continuation becomes completely emancipated from the kidney tissue and lies in the adult some distance from the anterior end of the kidney.

The same phenomenon is seen in Elasmobranchs and Amniotes where the opisthonephric tubules connected with the vasa efferentia never reach the length and complicated convolution of the normal tubule and the Malpighian bodies either degenerate (S03/llium., Pristzlur/'u.s', Birds) or are eliminated entirely from ontogeny (Skates).

On the other hand the simpliﬁcation of the route from testis to Wolllian duct may come about in a diﬁ'erent fashion, as is seen in Amia, where the opening of the vas eﬁ"erens has become shifted from the Malpighian body down the course of the tubule, in some cases till it has come to open directly into the duct.

A careful study of the method by which these various modiﬁcations eome about during ontogeny is greatly needed.

Application of the general priI1(3ipleS- outlined above seems to afford a probable explanation of the remarkable arrangement in Teleostean ﬁshes, where the testis is continued back into a special sperm duct which opens to the exterior near the opening of the kidney duct (Fig. 143). '

The presence of a urinogenital network along the whole length of the testis in Ganoids (Acipenser, Lepvidostems) justifies the assumption that the ancestral Teleost possessed this primitive arrangement of the network. In the case of Poly/pterms the testis is continued back as a duct which opens into the urinogenital sinus formed by the hinder ends of the Wolfﬁan ducts. The duct, however, is not, except near its termination, a simple tube but contains a network of cavities continuous with those of the testis. It is in fact not a simple duct but a portion of the testis which has become sterile.

Similarly in the case of various typical Te_leosts it has been shown (J ungersen, 1889) that the duct is formed by the hinder part of the genital ridge, that it contains for a time a network of cavities continuous with those of the functional testis——-that it is in other words the modiﬁed and sterile hinder portion of the testis-—and, ﬁnally, that posteriorly it opens into the Wolfﬁan duct.

N ow the method by which the condition met with in Polypterus or in the young Teleost has arisen is probably indicated by what has happened in the two Lung-ﬁshes Lepirioszrevt and Protoptems. In the former the testicular network is reduced to the extent that only about half a dozen vasa efferentia persist at the hind end of the series. In P'rotopte'rus these are still further reduced to a single vas eﬁ°erens which passes from the hinder end of the sterile portion of the testis—“ sperm duct ” of the older descriptions——-into the hind end of the kidney and communicates with the Wolﬂian duct through the hindermost kidney tubules.

The only further step needed from the condition exempliﬁed by Protoptemos to that of Poly/pterus or the young Teleost is that the communication between the sterile or duct portion of the testis and the Wolflian duct should come to be by a direct tubular prolongation of the vas efferens instead of by tortuous kidney tubules. That such a shortening up and simpliﬁcation of the channel from testis to Wolﬂian duct does actually tend to come about in evolution is demonstrated by the precisely similar series of modiﬁcations which have occurred in the Anura at the front end of the urinogenital network. In those not only has the series of vasa efferentia become reduced to a single (anterior) member in such a form as Discoglossus but the kidney tubule interpolated between it and the Wolﬂian duct has become shortened and widened, so that there exists simply a single tube leading from the testis and continued at its other end into the Wolllian duct.

Giving consideration to these various points it appears to be justifiable to relate the probable evolutionary history of the sperm duct of the Teleost in the following terms :—

I. Primitively the elongated testis communicated with the Wolfﬁan duct by way of ((t) a series of vasa efierentia distributed along its length, and (b) the tubules of tlie opisthoncphros.

II. The posterior portion of the testis became sterile and functioned merely as a reservoir and duct.

III. The vasa. etferentia became reduced to those connected with this sterile region and ﬁnally to the hindermost one of these.

IV. The channel formed by this together with the kidney tubules into which the spermatozoa passed from it became shortened and widened until it reached the condition of a simple tube leading from the hind end of the testis into the hind end of the Wolfﬁan duct.

V. The ﬁnal stage was reached by the opening of this tube into the Wolﬁian duct becoming shifted back until its opening to the exterior came to be independent.

Suprarenal Organs

The organ familiar to students of the Amniota and especially of the Mammalia under the name Suprarenal or Adrenal is now generally recognized as being not a single organ but an organic complex formed by the union of two originally separate elements-—the medullary substance and the cortical substance. These two elements arise quite independently i11 ontogeny, the medullary substance being derived from the sympathetic ganglia, while the cortical substance arises in the form of a number of thickenings of coelomic epithelium on the roof of the splanchnocoele between the two kidneys. That this independence in early stages of ontogeny is a repetition of a condition which occurred during phylogeny is indicated by the fact that in Fishes the two elements are still independent. The names medullary and cortical substance, reierring as they do to a topographical relation which occurs only in mammals, are obviously unsuitable from the point of view of comparative morphology and it is becoming customary to apply other more characteristic names. The medullary substance in mammals and what corresponds with it in other Vertebrates has a very characteristic chemical or physical reaction, in that it takes on a deep yellow or brown colour when treated with salts of chromic acid. Hence it is convenient, and usual, to apply to it a name expressive of this reaction-— such as Ghromophile (Stilling), Chromaﬁine (Kohn) or Phaeochrome Poll).

( The cortical tissue has also characteristic features——-in particular the fact that its cytoplasm contains numerous granules of lipoid or fat-like substance, soluble in Ether, Xylol, etc., staining deeply with various Aniline stains, and giving the characteristic black with Osmic Acid. For masses of this tissue the name Interrenal organ may be used (Balfour) which although a topographical term like cortical substance has the advantage of being correct for vertebrates in general during at least the early stages of their development.

Of the more primitive groups of gnathostomatous Vertebrates only the Elasmobranchs and the Amphibians have been studied carefully in regard to the development of these organs and we shall consequently use them as illustrating the general mode of development which, with variations in detail, holds throughout the groups dealt with in this volume.

ELASMOBRANe111I

The Interrenal organs are here interrenal in position through life, forming either one (Sharks) or a pair (Skates and Rays) of elongated bodies lying in the region of the mesial plane and extending for some distance opposite the hinder part of the opisthonephros.

In S’c;2/llvlmn (Poll) the interrenal makes its first appearance (7 mm. embryo) in the form of a number of irregularly distributed thickenings of the splanchnic mesoderm in the region of the root of the mesentery, just ventral to the dorsal aorta. The possibility of metameric arrangement in the very earliest stages does not seem to be absolutely excluded but there is no evidence of this so far. The rudiments are most numerous in the genital region but they occur as far forwards as the hind end of the pronephros and back as far as the cloaca. The rudiments of the two sides, projecting towards the median plane, meet and become continuous, and as anteroposterior fusion also comes about, the rudiment takes the form (10 mm. embryo) of a cellular rod lying beneath the dorsal aorta and above the mesenterie root, and for a time still continuous with the splanchnic mesoderm which gave it origin. For a time there is close apposition, amounting to apparent continuity of tissue, between this red and the opisthonephric nephrotomes lying on either side of it, but it is doubtful whether any special morphological signiﬁcance is to be attached to this. In embryos of 16-28 mm. in length the interrenal organ gradually becomes separated in a tailward direction both from the coelomic epithelium and the ncphrotomes, and assumes its deﬁnitive form.

1 The best general account of the development of the Suprarenal organs is that by Poll (1905).

Only the tailward part of the series of original rudiments completes its development in the way described. The whole series extends through about 25 segments but of these only about the posterior half take part in the formation of the interrenal rod: the anterior ones either atrophy completely or develop into small accessory interrcnals.

The chromophile organs of the Elasmobranch (Swale Vincent, 1897) are small, rounded, segmentally arranged bodies lying ventral to the intercostal arteries-—the anterior few on either side forming a continuous structure which was regarded by the earlier workers as an accessory heart (Duvernoy). These bodies are, as Balfour showed (I878), derivatives of the sympathetic ganglia. In a Scyllium of about 53 mm. the lateral part of the ganglion rudiment begins to show differentiation from the rest, its cells being relatively smaller than those which are destined to become ganglion—cells, and their protoplasm not only staining more deeply with ordinary stains but also developing the characteristic chromic acid reaction. In the Scyllium of 90 mm. the chromophile organ has assumed its deﬁnitive rounded form. Intrusive connective tissue forms a sparse stroma and capsule and in the former a capillary network is present. The series of segmentally arranged chromophile masses undergoes much modiﬁcation in subsequent develop1nent—son1e, particularly at the ends of the series, aborting, others undergoing fusion. The details vary in different genera, the result being a striking variety in the adult arrangements in the various members of the group.

AMPHIBIA.——B1‘a11er (1902) in his work on the renal organs of H3/pogeophis gives a clear account of the development of the suprarenals.

The interrenals appear as in Elasmobranchs in the form of cellular proliferations of the coelomic epithelium, in this case a little external to the root of the mesentery. These proliferations are paired and segmental in their arrangement, and extend from the region of the pronephros to that of the eloaca. The cellular buds become constricted off from the coelomic epithelium and lie above it as rounded masses embedded in the mesenchyme. As the two posterior cardinal veins approach and fuse the interrenal buds become displaced upwards so as to lie between the cardinal vein and the dorsal aorta. As development goes on processes of fusion take place between the rudiments more especially anteriorly where they come to form an unpaired elongated mass lying below the dorsal aorta and for the most part dorsal to the posterior vena cava (role. the fused posterior cardinals) but here and there passing laterally round the vein to its ventral surface or even piercing it—the fusion between the two cardinals having been obstructed at such points. In the posterior half of the organ the several rudiments retain their distinctness and lie on the ventral face of the opisthonephros.

The chromophile bodies develop as in‘ the Elasmobranchs from split off portions of the sympathetic ganglion rudiments. These become shifted in a ventral direction round the dorsal aorta and take up their position in intimate contact with the interrenal bodies, lyiiig in the posterior paired region of the interrenal on its mesial face, elsewhere dorsal or lateral or even completely surrounding the

FIG. l44.—-——I11nstrating origin of the sclerotome as seen in transverse sections through young stages of

A, Amplu'..»..ru.< (for tha :<ulu- n1'vlo'-:n'm-.~'s the slun-4-s h:1\‘e linen a‘x:i_'_,-‘:44-l‘:1t.(=.«l, uml t-he \'vnl1':1l poi-Lion of the ding,-‘1':un is Lulu-n I‘:-om :1 rather _yuun;.-_-mi .~a1.-:;_~,-- llnul the <|m-:-ml): ll, I,rp[4lu.s-ir.-11., stagu -3.1; (1, Polyptcms, .st.=ig«- ‘.23. mt. o-nlt.*ri(- (‘H\'ll)'§ my, 1n_yui-nuu': .\". nutm-Inn-«I; nu, m-phrocmrle; pn, pronephros; 5..-, spinal wml; .-cl, .~u-lemlmm-; sn, subImt'(n:lnmlul me! : Spit‘, 5;aluin-lxmu-¢_w1e_

interrenal. The chrornophile bodies stand out with great; distinctness from the interronal by their ﬁne grained"deeply-staining protoplasm and their larger nuclei.

We thus see that in the Amphibia the originally separate interrenal and chromophile bodies become during the course of development associated together to form a suprarenal complex of the type seen in the higher Vertebrates. Incidentally the unsuitability of the terms medullary and cortical is accentuated, for here when one of the elements comes to surround the other it is the chromophile which does so -—- precisely the opposite to what happens in the Mammalia.

Tum Sc1.uno'roMs.—-In Amp/z.iowus the sclerotome (Fig. 144 A, .90!) arises as a pocket-like diverticulum of the splanehnic mesoderm just ventral to the myotome. It grows inwards and dorsalwards, pushing its way between the notoehord and spinal cord on the one hand and the myotome o11 the other. until it reaches the mid-dorsal line where it meets its fellow of the opposite side. The epithelial walls of the sclcrotome ﬁnally break up into mesenchyme --amoehoid connective tissue cells. The cells derived in this way from the outer wall of the sclerotome apply themselves to the mesial face of the myotome, penetrating in between its muscle cells and forming septa of connective tissue between adjacent myotomes, while those derived from the inner wall go to form packing tissue in the interstices round spinal cord and notochord. Over the spinal cord this packing tissue forms a tough protective roof. During this resolution of the sclerotomes into mesenchymc all trace of the original scgniental character of the sclerotomes disappears.

It is customary -——— although the present writer regards it as questionable whether this is wholly justiﬁed——to regard the mode of origin of the sclerotome seen in the developing Amphioxus as representing the primitive mode of development. Upon this assumption we may describe what takes place in the_ typical Vertebrates as follows. The breaking up of‘ the sclerotomc into niesonchyme tends to take place at earlier and earlier periods of development——the diverticulum stage becoming more and more transient and eventually disappearing completely so that sclerotome formation comes to be represented merely by a very active proliferation of mcsenehyme cells from the splanehnic surface of the mesoderm ventral to the myotome (cf. Fig. 144 O, scl).

It must not be supposed that the whole ‘of the connective tissue in the body is necessarily derived from the sclerotome. On the contrary it would appear that other regions of the mesoderm also give rise to mesenchyme cells. Thus the inner surface of the splanehnic mesoderm of the gut-wall would appear to give rise to the connective tissue of this region, and the whole of the splanchnoeoelie mesoderm of the postanal region apparently becomes resolved into mesenchyme.

On the whole perhaps the safest position to take up is that of regarding the power of. forming mesenchyme as a general property of the mesoderm, and of regarding the sclerotome merely as expressing a localized concentration of this power, rather than as being the representative of some primitive pocket-likesdivertieulum of unknown function.

Literature

Abramowicz. Morph. Jahrb., xlvii, 1913.

Agar. Trans. Roy. Soc. Edin., xlv, 1907.

Balfour. Monograph on the development of Elasmobrancli Fishes. London, 1878.

Balfour and Parker. l’hil. Trans. Roy. Soc., clxxiii, 1882.

Balfour and Sedgwick. Proc. Roy. Soc., xxvii, 1878.

Bles. Proc. Roy. Soc., lxii, 1897.

Bouin. Arch. dc Biol., xvii, 1901.

Boveri. Anat. Anz., vii, 1892.

Brauer. Zool. Jahrb. (Anat.), xvi, 1902.

Braus. Morph. Jahrb., xxvii, 1899.

Budgett. Trans. Zool. Soc. Loml., xvi, 1902.

Dahlgren and Kepner. Animal Histology. New York, 1908.

Ewart. Phil. Trans. Roy. Soc., 179 B, 1888.

Ewart. Phil. ’l‘ran.~x. Roy. Soc., 179 B, 1889.

Ewart. Phil. Trans. Roy. Soc., 183 B, 1892.

Felix. Anat. Ileftv (Arb.), viii, 1897.

Felix. Hertwigs Haudbnch der Entwicklun,<,-slehre, iii, 1904.

Field. Bull. Mus. Comp. Zool. Harvard, xxi, 1891.

Piirbringer. Entwick. der Amphibienniere. Heidelberg, 1877.

(See also Morph. Ja.hrb., iv, 1878.) .

Goodrich. Quart. Journ. Micr. Sci., xxxvii, 1895.

Guitel. Bull. Soc. Sci. et Méd. de l’Ouest, x, 1901, and xi, 1902.

Gregory. Semons Forscliiingsreison. i, 1906.

Hochstetter. Morph. .|abrb., xxix, 1900.

Jungersen. Arb. zoo].-zoot. Inst. Wurzburg, ix, 1889.

Jungersen. Zool. Anzc-iger, xvi and xvii, 1893, 1894.

Kerr, Graham. Proe. Zool. Soc. Lond., 1901.

Kerr, Graham. The Work of J olm. Samuel Bridgett. Cambridge, 1907.

Kerr, Graham. Quart. Journ. Micr. Se.-i., liv, 1910.

Koltzoﬁ‘. Bull. Soc. Imp. Nat. Moscow, xv, 1901.

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

Lebedinaky. Arch. mikr. Anat., xliv, 1895.

Marshall. Vertebrate Embryology. London, 1893.

Marshall and Bles. Studies Biol. Lab. Owens College, ii, 1890.

Mollier. Anat. llefte (Arb.), iii, 1893.

Mliller, E. Anat. Hefte (Arb.), xliii, 1911.

Nuasbaum. Arch. mikr. Ana1.., xxvii, 1886.

Poll. Hertwigs Handbnch der Entwicklungslehre, iii, 1905.

Poole. Proe. Zool. Soc. Lond., 1909.

Rabl, G. Morph. Jahrb., xxiv, 1896.

Rabl, H. Arch. mikr. Anat., lxiv, 1904.

Rosa. Al‘ClllVl0 zoologico, iii, 1906.

Riickert. Arch. f. Anat. u. Entwick., 1888.

Schapitz. Arch. mikr. Ana.t., lxxix, 1912.

Schmalhausen. Zeitsclrr. wiss. Zoo1., C, 1912.

Schreiner. Zeitschr. wiss. Zool., lxxi, 1902.

Sedgwick. Quart. Journ. Micr. Sci., xx, 1880.

Sedgwick. (,),uart. J ourn. Micr. Sci., xxi, 1881.

Semon. Keibels Normentafeln. iii, 1901.

(Also in Zoo]. Anzeiger, xxiv, 1901.)

Semper. Arb. zool.-zoot. Inst. Wnrzburg, ii, 1876.

Vincent, Swale. Trans. Zool. Soc. Lond., xiv, 1897.

Wijhe, van. Ilber die Mesodormsegmente und die Entwicklnng der Nerven des Selachierkopfes. Verhaml. Akad. Wet. Amsterdam, xxii, 1883. (Reprinted, Greningen, 1915).

Wijhe, van. Arch. mikr. Anat., xxxiii, 1889.

Zarnik. Zool. Jahrb. (Ana.t.), xxi, 1904.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

- Currently only Draft Version of Text -

Reference

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

Cite this page: Hill, M.A. (2024, April 19) Embryology Text-Book of Embryology 2-4 (1919). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Text-Book_of_Embryology_2-4_(1919)

What Links Here?