Volume 2 / Part 2 — CHAPTER XXIII. (continued 2)

In the female the groove on the genital prominence gradually disappears, and the prominence remains as the clitoris, which is therefore the homologue of the penis: the two genital folds form the labia majora. The urethra and vagina open independently into the common urinogenital sinus.

General conclusions and Summary.

Pronephros. Sedgwick has pointed out that the pronephros is always present in types with a larval development, and either absent or imperfectly developed in those types which undergo the greater part of their development within the egg. Thus it is practically absent in the embryos of Elasmobranchii and the Amniota, but present in the larvæ of all other forms.

This coincidence, on the principles already laid down in a previous chapter on larval forms, affords a strong presumption that the pronephros is an ancestral organ; and, coupled with the fact that it is the first part of the excretory system to be developed, and often the sole excretory organ for a considerable period, points to the conclusion that the pronephros and its duct—the segmental duct—are the most primitive parts of the Vertebrate excretory system. This conclusion coincides with that arrived at by Gegenbaur and Fürbringer.

The duct of the pronephros is always developed prior to the gland, and there are two types according to which its development may take place. It may either be formed by the closing in of a continuous groove of the somatic peritoneal epithelium (Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of cells derived from the somatic mesoblast, which grows backwards between the epiblast and the mesoblast (Petromyzon, Elasmobranchii, and the Amniota).

It is quite certain that the second of these processes is not a true record of the evolution of the duct, and though it is more possible that the process observable in Amphibia and the Teleostei may afford some indications of the manner in which the duct was established, this cannot be regarded as by any means certain.

The mode of development of the pronephros itself is apparently partly dependent on that of its duct. In Petromyzon, where the duct does not at first communicate with the body cavity, the pronephros is formed as a series of outgrowths from the duct, which meet the peritoneal epithelium and open into the body cavity; but in other instances it is derived from the anterior open end of the groove which gives rise to the segmental duct. The open end of this groove may either remain single (Teleostei, Ganoidei) or be divided into two, three or more apertures (Amphibia). The main part of the gland in either case is formed by convolutions of the tube connected with the peritoneal funnel or funnels. The peritoneal funnels of the pronephros appear to be segmentally arranged.

The pronephros is distinguished from the mesonephros by developmental as well as structural features. The most important of the former is the fact that the glandular tubules of which it is formed are always outgrowths of the segmental duct; while in the mesonephros they are always or almost always[263] formed independently of the duct.

The chief structural peculiarity of the pronephros is the absence from it of Malpighian bodies with the same relations as those in the meso- and metanephros; unless the structures found in Myxine are to be regarded as such. Functionally the place of such Malpighian bodies is taken by the vascular peritoneal ridge spoken of in the previous pages as the glomerulus.

That this body is really related functionally to the pronephros appears to be indicated (1) by its constant occurrence with the pronephros and its position opposite the peritoneal openings of this body; (2) by its atrophy at the same time as the pronephros; (3) by its enclosure together with the pronephridian stoma in a special compartment of the body-cavity in Teleostei and Ganoids, and its partial enclosure in such a compartment in Amphibia.

The pronephros atrophies more or less completely in most types, though it probably persists for life in the Teleostei and Ganoids, and in some members of the former group it perhaps forms the sole adult organ of excretion.

The cause of its atrophy may perhaps be related to the fact that it is situated in the pericardial region of the body-cavity, the dorsal part of which is aborted on the formation of a closed pericardium; and its preservation in Teleostei and Ganoids may on this view be due to the fact that in these types its peritoneal funnel and its glomerulus are early isolated in a special cavity.

Mesonephros. The mesonephros is in all instances composed of a series of tubules (segmental tubes) which are developed independently of the segmental duct. Each tubule is typically formed of (1) a peritoneal funnel opening into (2) a Malpighian body, from which there proceeds (3) a coiled glandular tube, finally opening by (4) a collecting tube into the segmental duct, which constitutes the primitive duct for the mesonephros as well as for the pronephros.

The development of the mesonephridian tubules is subject to considerable variations.

(1) They may be formed as differentiations of the intermediate cell mass, and be from the first provided with a lumen, opening into the body-cavity, and directly derived from the section of the body-cavity present in the intermediate cell mass; the peritoneal funnels often persisting for life (Elasmobranchii).

(2) They may be formed as solid cords either attached to or independent of the peritoneal epithelium, which after first becoming independent of the peritoneal epithelium subsequently send downwards a process, which unites with it and forms a peritoneal funnel, which may or may not persist (Acipenser, Amphibia).

(3) They may be formed as in the last case, but acquire no secondary connection with the peritoneal epithelium (Teleostei, Amniota). In connection with the original attachment to the peritoneal epithelium, a true peritoneal funnel may however be developed (Aves, Lacertilia).

Physiological considerations appear to shew that of these three methods of development the first is the most primitive. The development of the tubes as solid cords can hardly be primary.

A question which has to be answered in reference to the segmental tubes is that of the homology of the secondarily developed peritoneal openings of Amphibia, with the primary openings of the Elasmobranchii. It is on the one hand difficult to understand why, if the openings are homologous in the two types, the original peritoneal attachment should be obliterated in Amphibia, only to be shortly afterwards reacquired. On the other hand it is still more difficult to understand what physiological gain there could be, on the assumption of the non-homology of the openings, in the replacement of the primary opening by a secondary opening exactly similar to it. Considering the great variations in development which occur in undoubtedly homologous parts I incline to the view that the openings in the two types are homologous.

In the majority of the lower Vertebrata the mesonephric tubes have at first a segmental arrangement, and this is no doubt the primitive condition. The coexistence of two, three, or more of them in a single segment in Amphibia, Aves and Mammalia has recently been shewn, by an interesting discovery of Eisig, to have a parallel amongst Chætopods, in the coexistence of several segmental organs in a single segment in some of the Capitellidæ.

In connection with the segmental features of the mesonephros it is perhaps worth recalling the fact that in Elasmobranchii as well as other types there are traces of segmental tubes in some of the postanal segments. In the case of all the segmental tubes a Malpighian body becomes established close to the extremity of the tube adjoining the peritoneal opening, or in an homologous position in tubes without such an opening. The opposite extremity of the tube always becomes attached to the segmental duct.

In many of the segments of the mesonephros, especially in the hinder ones, secondary and tertiary tubes become developed in certain types, which join the collecting canals of the primary tubes, and are provided, like the primary tubes, with Malpighian bodies at their blind extremities.

There can it appears to me be little or no doubt that the secondary tubes in the different types are homodynamous if not homologous. Under these circumstances it is surprising to find in what different ways they take their origin. In Elasmobranchii a bud sprouts out from the Malpighian body of one segment, and joins the collecting tube of the preceding segment, and subsequently, becoming detached from the Malpighian body from which it sprouted, forms a fresh secondary Malpighian body at its blind extremity. Thus the secondary tubes of one segment are formed as buds from the segment behind. In Amphibia (Salamandra) and Aves the secondary tubes develop independently in the mesoblast. These great differences in development are important in reference to the homology of the metanephros or permanent kidney, which is discussed below.

Before leaving the mesonephros it may be worth while putting forward some hypothetical suggestions as to its origin and relation to the pronephros, leaving however the difficult questions as to the homology of the segmental tubes with the segmental organs of Chætopods for subsequent discussion.

It is a peculiarity in the development of the segmental tubes that they at first end blindly, though they subsequently grow till they meet the segmental duct with which they unite directly, without the latter sending out any offshoot to meet them[264]. It is difficult to believe that peritoneal infundibula ending blindly and unprovided with some external orifice can have had an excretory function, and we are therefore rather driven to suppose that the peritoneal infundibula which become the segmental tubes were either from the first provided each with an orifice opening to the exterior, or were united with the segmental duct. If they were from the first provided with external openings we may suppose that they became secondarily attached to the duct of the pronephros (segmental duct), and then lost their external openings, no trace of these structures being left, even in the ontogeny of the system. It would appear to me more probable that the pronephros, with its duct opening into the cloaca, was the only excretory organ of the unsegmented ancestors of the Chordata, and that, on the elongation of the trunk and its subsequent segmentation, a series of metameric segmental tubes became evolved opening into the segmental duct, each tube being in a sort of way serially homologous with the primitive pronephros. With the segmentation of the trunk the latter structure itself may have acquired the more or less definite metameric arrangement of its parts.

Another possible view is that the segmental tubes may be modified derivatives of posterior lateral branches of the pronephros, which may at first have extended for the whole length of the body-cavity. If there is any truth in this hypothesis it is necessary to suppose that, when the unsegmented ancestor of the Chordata became segmented, the posterior branches of the primitive excretory organ became segmentally arranged, and that, in accordance with the change thus gradually introduced in them, the time of their development became deferred, so as to accord to a certain extent with the time of formation of the segments to which they belonged. The change in their mode of development which would be thereby introduced is certainly not greater than that which has taken place in the case of segmental tubes, which, having originally developed on the Elasmobranch type, have come to develop as they do in the posterior part of the mesonephros of Salamandra, Birds, etc.

Genital ducts. So far the origin and development of the excretory organs have been considered without reference to the modifications introduced by the excretory passages coming to serve as generative ducts. Such an unmodified state of the excretory organs is perhaps found permanently in Cyclostomata[265] and transitorily in the embryos of most forms.

At first the generative products seem to have been discharged freely into the body-cavity, and transported to the exterior by the abdominal pores (vide p. 626).

The secondary relations of the excretory ducts to the generative organs seem to have been introduced by an opening connected with the pronephridian extremity of the segmental duct having acquired the function of admitting the generative products into it, and of carrying them outwards; so that primitively the segmental duct must have served as efferent duct both for the generative products and the pronephric secretion (just as the Wolffian duct still does for the testicular products and secretion of the Wolffian body in Elasmobranchii and Amphibia).

The opening by which the generative products entered the segmental duct can hardly have been specially developed for this purpose, but must almost certainly have been one of the peritoneal openings of the pronephros. As a consequence (by a process of natural selection) of the segmental duct having both a generative and a urinary function, a further differentiation took place, by which that duct became split into two—a ventral Müllerian duct and a dorsal Wolffian duct.

The Müllerian duct was probably continuous with one or more of the abdominal openings of the pronephros which served as generative pores. At first the segmental duct was probably split longitudinally into two equal portions, and this mode of splitting is exceptionally retained in some Elasmobranchii; but the generative function of the Müllerian duct gradually impressed itself more and more upon the embryonic development, so that, in the course of time, the Müllerian duct developed less and less at the expense of the Wolffian duct. This process appears partly to have taken place in Elasmobranchii, and still more in Amphibia, the Amphibia offering in this respect a less primitive condition than the Elasmobranchii; while in Aves it has been carried even further, and it seems possible that in some Amniota the Müllerian and segmental ducts may actually develop independently, as they do exceptionally in individual specimens of Salamandra (Fürbringer). The abdominal opening no doubt also became specialised. At first it is quite possible that more than one pronephric abdominal funnel may have served for the entrance of the generative products; this function being, no doubt, eventually restricted to one of them.

Three different types of development of the abdominal opening of the Müllerian duct have been observed.

In Amphibia (Salamandra) the permanent opening of the Müllerian duct is formed independently, some way behind the pronephros.

In Elasmobranchii the original opening of the segmental duct forms the permanent opening of the Müllerian duct, and no true pronephros appears to be formed.

In Birds the anterior of the three openings of the rudimentary pronephros remains as the permanent opening of the Müllerian duct.

These three modes of development very probably represent specialisations of the primitive state along three different lines. In Amphibia the specialisation of the opening appears to have gone so far that it no longer has any relation to the pronephros. It was probably originally one of the posterior openings of this gland.

In Elasmobranchii, on the other hand, the functional opening is formed at a period when we should expect the pronephros to develop. This state is very possibly the result of a differentiation by which the pronephros gradually ceased to become developed, but one of its peritoneal openings remained as the abdominal aperture of the Müllerian duct. Aves, finally, appear to have become differentiated along a third line; since in their ancestors the anterior (?) pore of the head-kidney appears to have become specialised as the permanent opening of the Müllerian duct.

The Müllerian duct is usually formed in a more or less complete manner in both sexes. In Ganoids, where the separation between it and the Wolffian duct is not completed to the cloaca, and in the Dipnoi, it probably serves to carry off the generative products of both sexes. In other cases however only the female products pass out by it, and the partial or complete formation of the Müllerian duct in the male in these cases needs to be explained. This may be done either by supposing the Ganoid arrangement to have been the primitive one in the ancestors of the other forms, or, by supposing characters acquired primitively by the female to have become inherited by both sexes.

It is a question whether the nature of the generative ducts of Teleostei can be explained by comparison with those of Ganoids. The fact that the Müllerian ducts of the Teleostean Ganoid Lepidosteus attach themselves to the generative organs, and thus acquire a resemblance to the generative ducts of Teleostei, affords a powerful argument in favour of the view that the generative ducts of both sexes in the Teleostei are modified Müllerian ducts. Embryology can however alone definitely settle this question.

In the Elasmobranchii, Amphibia, and Amniota the male products are carried off by the Wolffian duct, and they are transported to this duct, not by open peritoneal funnels of the mesonephros, but by a network of ducts which sprout either from a certain number of the Malpighian bodies opposite the testis (Amphibia, Amniota), or from the stalks connecting the Malpighian bodies with the open funnels (Elasmobranchii). After traversing this network the semen passes (except in certain Anura) through a variable number of the segmental tubes directly to the Wolffian duct. The extent of the connection of the testis with the Wolffian body is subject to great variations, but it is usually more or less in the anterior region. Rudiments of the testicular network have in many cases become inherited by the female.

The origin of the connection between the testis and Wolffian body is still very obscure. It would be easy to understand how the testicular products, after falling into the body-cavity, might be taken up by the open extremities of some of the peritoneal funnels, and how such open funnels might have groove-like prolongations along the mesorchium, which might eventually be converted into ducts. Ontogeny does not however altogether favour this view of the origin of the testicular network. It seems to me nevertheless the most probable view which has yet been put forward.

The mode of transportation of the semen by means of the mesonephric tubules is so peculiar as to render it highly improbable that it was twice acquired, it becomes therefore necessary to suppose that the Amphibia and Amniota inherited this mode of transportation of the semen from the same ancestors as the Elasmobranchii. It is remarkable therefore that in the Ganoidei and Dipnoi this arrangement is not found.

Either (1) the arrangement (found in the Ganoidei and Dipnoi) of the Müllerian duct serving for both sexes is the primitive arrangement, and the Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary condition, which has originated at a stage in the evolution of the Vertebrata when some of the segmental tubes had begun to serve as the efferent ducts of the testis, and has resulted in consequence of a degeneration of the latter structures. Although the second alternative is the more easy to reconcile with the affinities of the Ganoid and Elasmobranch types, as indicated by the other features of their organization, I am still inclined to accept the former; and consider that the incomplete splitting of the segmental duct in Ganoidei is a strong argument in favour of this view.

Metanephros. With the employment of the Wolffian duct to transport the semen there seems to be correlated (1) a tendency of the posterior segmental tubes to have a duct of their own, in which the seminal and urinary fluids cannot become mixed, and (2) a tendency on the part of the anterior segmental tubes to lose their excretory function. The posterior segmental tubes, when connected in this way with a more or less specialised duct, have been regarded in the preceding pages as constituting a metanephros.

This differentiation is hardly marked in the Anura, but is well developed in the Urodela and in the Elasmobranchii; and in the latter group has become inherited by both sexes. In the Amniota it culminates, according to the view independently arrived at by Semper and myself, (1) in the formation of a completely distinct metanephros in both sexes, formed however, as shewn by Sedgwick, from the same blastema as the Wolffian body, and (2) in the atrophy in the adult of the whole Wolffian body, except the part uniting the testis and the Wolffian duct.

The homology between the posterior metanephridian section of the Wolffian body, in Elasmobranchii and Urodela, and the kidney of the Amniota, is only in my opinion a general one, i.e. in both cases a common cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more or less similar differentiation of parts.

Fürbringer has urged against Semper’s and my view that no satisfactory proof of it has yet been offered. This proof has however, since Fürbringer wrote his paper, been supplied by Sedgwick’s observations. The development of the kidney in the Amniota is no doubt a direct as opposed to a phylogenetic development; and the substitution of a direct for a phylogenetic development has most probably been rendered possible by the fact that the anterior part of the mesonephros continued all the while to be unaffected and to remain as the main excretory organ during fœtal life.

The most serious difficulty urged by Fürbringer against the homology is the fact that the ureter of the metanephros develops on a type of its own, which is quite distinct from the mode of development of the ureters of the metanephros of the Ichthyopsidan forms. It is however quite possible, though far from certain, that the ureter of Amniota may be a special formation confined to that group, and this fact would in no wise militate against the homology I have been attempting to establish.

Comparison of the Excretory organs of the Chordata and Invertebrata.

The structural characters and development of the various forms of excretory organs described in the preceding pages do not appear to me to be sufficiently distinctive to render it possible to establish homologies between these organs on a satisfactory basis, except in closely related groups.

The excretory organs of the Platyelminthes are in many respects similar to the provisional excretory organ of the trochosphere of Polygordius and the Gephyrea on the one hand, and to the Vertebrate pronephros on the other; and the Platyelminth excretory organ with an anterior opening might be regarded as having given origin to the trochosphere organ, while that with a posterior opening may have done so for the Vertebrate pronephros[266].

Hatschek has compared the provisional trochosphere excretory organ of Polygordius to the Vertebrate pronephros, and the posterior Chætopod segmental tubes to the mesonephric tubes; the latter homology having been already suggested independently by both Semper and myself. With reference to the comparison of the pronephros with the provisional excretory organ of Polygordius there are two serious difficulties:

(1) The pronephric (segmental) duct opens directly into the cloaca, while the duct of the provisional trochosphere excretory organ opens anteriorly, and directly to the exterior.

(2) The pronephros is situated within the segmented region of the trunk, and has a more or less distinct metameric arrangement of its parts; while the provisional trochosphere organ is placed in front of the segmented region of the trunk, and is in no way segmented.

The comparison of the mesonephric tubules with the segmental excretory organs of the Chætopoda, though not impossible, cannot be satisfactorily admitted till some light has been thrown upon the loss of the supposed external openings of the tubes, and the origin of their secondary connection with the segmental duct.

Confining our attention to the Invertebrata it appears to me fairly clear that Hatschek is justified in holding the provisional trochosphere excretory organs of Polygordius, Echiurus and the Mollusca to be homologous. The atrophy of all these larval organs may perhaps be due to the presence of a well-developed trunk region in the adult (absent in the larva), in which excretory organs, probably serially homologous with those present in the anterior part of the larva, became developed. The excretory organs in the trunk were probably more conveniently situated than those in the head, and the atrophy of the latter in the adult state was therefore brought about, while the trunk organs became sufficiently enlarged to serve as the sole excretory organs.

Bibliography of the Excretory Organs.

Invertebrata.

(512) H. Eisig. “Die Segmentalorgane d. Capitelliden.” Mitth. a. d. zool. Stat. z. Neapel, Vol. I. 1879. (513) J. Fraipont. “Recherches s. l'appareil excréteur des Trematodes et d. Cestoïdes.” Archives de Biologie, Vol. I. 1880. (514) B. Hatschek. “Studien üb. Entwick. d. Anneliden.” Arbeit. a. d. zool. Instit. Wien, Vol. I. 1878. (515) B. Hatschek. “Ueber Entwick. von Echiurus,” etc. Arbeit. a. d. zool. Instit. Wien, Vol. III. 1880.

Excretory Organs of Vertebrata.

General.

(516) F. M. Balfour. “On the origin and history of the urinogenital organs of Vertebrates.” Journal of Anat. and Phys., Vol. X. 1876. (517) Max. Fürbringer[267]. “Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten.” Morphol. Jahrbuch, Vol. IV. 1878. (518) H. Meckel. Zur Morphol. d. Harn-u. Geschlechtswerkz. d. Wirbelthiere, etc. Halle, 1848. (519) Joh. Müller. Bildungsgeschichte d. Genitalien, etc. Düsseldorf, 1830. (520) H. Rathke. “Beobachtungen u. Betrachtungen ü. d. Entwicklung d. Geschlechtswerkzeuge bei den Wirbelthieren.” N. Schriften d. naturf. Gesell. in Dantzig, Bd. I. 1825. (521) C. Semper[267]. “Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. übrigen Wirbelthiere.” Arb. a. d. zool.-zoot. Instit. Würzburg, Vol. II. 1875. (522) W. Waldeyer[267]. Eierstock u. Ei. Leipzig, 1870.

Elasmobranchii.

(523) A. Schultz. “Zur Entwick. d. Selachiereies.” Archiv f. mikr. Anat., Vol. XI. 1875.

Vide also Semper (No. 521) and Balfour (No. 292).

Cyclostomata.

(524) J. Müller. “Untersuchungen ü. d. Eingeweide d. Fische.” Abh. d. k. Ak. Wiss. Berlin, 1845. (525) W. Müller. “Ueber d. Persistenz d. Urniere b. Myxine glutinosa.” Jenaische Zeitschrift, Vol. VII. 1873. (526) W. Müller. “Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen.” Jenaische Zeitschrift, Vol. IX. 1875. (527) A. Schneider. Beiträge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879. (528) W. B. Scott. “Beiträge z. Entwick. d. Petromyzonten.” Morphol. Jahrbuch, Vol. VII. 1881.

Teleostei.

(529) J. Hyrtl. “Das uropoetische System d. Knochenfische.” Denkschr. d. k. k. Akad. Wiss. Wien, Vol. II. 1850. (530) A. Rosenberg. Untersuchungen üb. die Entwicklung d. Teleostierniere. Dorpat, 1867.

Vide also Oellacher (No. 72).

Amphibia.

(531) F. H. Bidder. Vergleichend-anatomische u. histologische Untersuchungen ü. die männlichen Geschlechts- und Harnwerkzeuge d. nackten Amphibien. Dorpat, 1846. (532) C. L. Duvernoy. “Fragments s. les Organes genito-urinaires des Reptiles,” etc. Mém. Acad. Sciences. Paris. Vol. XI. 1851, pp. 17-95. (533) M. Fürbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877. (534) F. Leydig. Anatomie d. Amphibien u. Reptilien. Berlin, 1853. (535) F. Leydig. Lehrbuch d. Histologie. Hamm, 1857. (536) F. Meyer. “Anat. d. Urogenitalsystems d. Selachier u. Amphibien.” Sitz. d. naturfor. Gesellsch. Leipzig, 1875. (537) J. W. Spengel. “Das Urogenitalsystem d. Amphibien.” Arb. a. d. zool.-zoot. Instit. Würzburg. Vol. III. 1876. (538) Von Wittich. “Harn- u. Geschlechtswerkzeuge d. Amphibien.” Zeit. f. wiss. Zool., Vol. IV.

Vide also Götte (No. 296).

Amniota.

(539) F. M. Balfour and A. Sedgwick. “On the existence of a head-kidney in the embryo Chick,” etc. Quart. J. of Micr. Science, Vol. XIX. 1878. (540) Banks. On the Wolffian bodies of the fœtus and their remains in the adult. Edinburgh, 1864. (541) Th. Bornhaupt. Untersuchungen üb. die Entwicklung d. Urogenitalsystems beim Hühnchen. Inaug. Diss. Riga, 1867. (542) Max Braun. “Das Urogenitalsystem d. einheimischen Reptilien.” Arbeiten a. d. zool.-zoot. Instit. Würzburg. Vol. IV. 1877. (543) J. Dansky u. J. Kostenitsch. “Ueb. d. Entwick. d. Keimblätter u. d. Wolff’schen Ganges im Hühnerei.” Mém. Acad. Imp. Pétersbourg, VII. Series, Vol. XXVII. 1880. (544) Th. Egli. Beiträge zur Anat. und Entwick. d. Geschlechtsorgane. Inaug. Diss. Zürich, 1876. (545) E. Gasser. Beiträge zur Entwicklungsgeschichte d. Allantois, der Müller’schen Gänge u. des Afters. Frankfurt, 1874. (546) E. Gasser. “Beob. üb. d. Entstehung d. Wolff’schen Ganges bei Embryonen von Hühnern u. Gänsen.” Arch. für mikr. Anat., Vol. XIV. 1877. (547) E. Gasser. “Beiträge z. Entwicklung d. Urogenitalsystems d. Hühnerembryonen.” Sitz. d. Gesell. zur Beförderung d. gesam. Naturwiss. Marburg, 1879. (548) C. Kupffer. “Untersuchung über die Entwicklung des Harn- und Geschlechtssystems.” Archiv für mikr. Anat., Vol. II. 1866. (549) A. Sedgwick. “Development of the kidney in its relation to the Wolffian body in the Chick.” Quart. J. of Micros. Science, Vol. XX. 1880. (550) A. Sedgwick. “On the development of the structure known as the glomerulus of the head-kidney in the Chick.” Quart. J. of Micros. Science, Vol. XX. 1880. (551) A. Sedgwick. “Early development of the Wolffian duct and anterior Wolffian tubules in the Chick; with some remarks on the vertebrate excretory system.” Quart. J. of Micros. Science, Vol. XXI. 1881. (552) M. Watson. “The homology of the sexual organs, illustrated by comparative anatomy and pathology.” Journal of Anat. and Phys., Vol. XIV. 1879. (553) E. H. Weber. Zusätze z. Lehre von Baue u. d. Verrichtungen d. Geschlechtsorgane. Leipzig, 1846. Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297), Kölliker (No. 298).