[79] EMBRYOGRAPHY OF OSSEOUS FISHES. 533 



the liver, and partially enveloping" the hind-gut, in embryos which had 

 not yet absorbed the yelk-sack. The pancreatic tissues of fishes seem 

 to be intimatelj" bound up with the history of the pyloric appendages, 

 and we may therefore expect to know more of them when the develop- 

 ment of the caeca has been worked out. 



21. — Development of the renal organs or corpora 



wolffiana. 



The remarkable researches of Semper, Balfour, Sedgwick, Fiirbringer, 

 Eosenberg, O^llacher, and others, on the early history of the kidneys of 

 vertebrate embryos has within a comparatively recent period thrown a 

 flood of light upon what had previously been a most obscure and poorly 

 understood subject. While it will not be possible for me to add much 

 to the general principles of development of the renal organs, so ably 

 worked out by my predecessors, and in many senses my monitors, I can 

 add here what I have observed in the development of those organs in 

 Oadus, Alosa, Gambusia, and Salmo, bringing out some singular pecu- 

 liarities in the evolution of the mesonephros or wolffian body itself as 

 manifested in the embryos of these different genera. 



In the figures accompanying this memoir I have not represented the 

 segmental ducts, except in Figs. 40, 49, pnp, and in a diagrammatic cross- 

 section. Fig. 33, sd. The details, which I have mainly worked out by 

 means of sections of the embryos of various other genera, I reserve for 

 illustration and description in future special essays upon those types. 

 The development of the renal organs in different genera o^ Tcleosfei 

 differs greatly in detail, as we shall learn further on. The following 

 general description of the development of the segmental ducts or pro- 

 nephros seems to apply to osseous fishes generally: 



At about the time the tail begins to bud out and the muscular somites 

 of the body have been formed, the segmental ducts are folded off from the 

 8i)lanchnopleure or peritoneum as a pair of longitudinal canals on either 

 side of the middle line, as shown at sd, Fig. 33. They lie in close con- 

 tact with the peritoneal wall of the abdomen, and at their anterior ends 

 they oi)en freely into its cavity. They are also usually bent upon them- 

 selves more or less markedly, inwards and backwards, and then forwards 

 again, as shown in Fig. 4G, from below, at prq), and in Fig. 49. These 

 Jire the primitive open funnels or free anterior extremities of the pro- 

 nephric or segmental ducts as we see them in the living cod embryo of 

 a late stage. In the young cod, some days after hatching, their anterior 

 ends are found to lie on either sideof the front end of body, extending 

 forwards to near the auditory vesicles, and as development advances 

 they seem to approximate the latter more closely. Traced ba(;kwards, 

 the segmental ducts pass over the ])eritoneum almost exactly parallel 

 to each other till they converge and join the allantoic or urinary vesicle 

 al, conspicuously shown in Figs. 40, 45, and 49. The exact mode of 

 their union with this vesicle I have not learned in Gadus, but it prob- 



