hall: MESONEPHROS and MiJLLERIAN DUCT IN AMPHIBIA. 49 



with that found by Semon ('92) iu Ichthyophis. (See Figures F to 

 H, page 72.) 



What has been said in regard to this persistent connection applies 

 only to the somites anterior to the sixteenth. Posterior to the sixteenth 

 the blastulae are separated more and more widely from the peritoneum 

 by loose, mesenchymatous tissue (compare Figure 20, a section through 

 somite 16 of a larva 21 mm. long). The nephrostomes of these pos- 

 terior units, as well as those of all secondary units, must therefore join 

 the peritoneum secondarily. Whether those of the posterior primary 

 units join the peritoneum at the point of junction of somatic and splanch- 

 nic layers or not, is impossible to determine. It is certain that these 

 and the nephrostomes of secondary units cannot both open at the line 

 of junction, for both sets of nephrostomes are often seen in one cross- 

 section, opening at some distance from each other (see Fig. 24, Plate 3, 

 njjh'stm. 1 and npli'stm. 2). 



Development of a Mesoneph'ie Unit in Amhlystoma comjjared tvith that 



in Pristiurus. 



In resuming the description of the mesonephros I shall no longer de- 

 scribe larvae at successive stages of development, but instead briefly 

 describe the development of a hlastula into a functional unit/ — briefly, 

 because these changes have been well described, in the main, by Fiir- 

 bringer ('78) and others. 



In order the better to compare the course of development in Amhlys- 

 toma with that in elasraobranchs, it vvill be well to trace the histoi'y of 

 such a unit in Pristiurus as described by Rabl ('96). According to that 

 author the blastula is formed in the following manner. The mesomer, 

 which is in the form of an epithelial tube connecting the lateral plates 

 •with the epimer, is early rendered incomplete by the breaking up of the 

 dorsal portion of the splanchuoderm to form the sclerotome. The gap 

 thus formed is again closed by the growth mesad and ventrad of the 

 dorsal edge of the somatoderm, which has already freed itself from the 

 epimer. There is thus formed a blind pocket opening into the coelom. 

 Of the median wall of this pocket, a portion adjoining the lateral meso- 

 derm is thus formed of splanchuoderm. The rest of the pocket is com- 

 posed of somatoderm. The splanchnic portion is thin ; the somatic 

 portion, especially on the lateral side of the blastula, is thick. The 



1 The development can be followed by examining successive units in the same 

 larva, beginning at tlie caudal end of the series, where they are less differentiated, 

 and progressing cephalad. 



