48 bulletin: museum of comparative zoology. 



The arrangement of the nuclei at the point coel. in Figure 17 seems to 

 indicate that it has arisen as an outfolding of the splanchnic layer of the 

 lateral mesoderm toward the median plane. In the elasmobranchs such 

 an outfolding is very evident (Rabl, '96, Taf. 15, Fig. 8 ; Riicliert, '88, 

 Taf. 15, Fig. 21). In those animals the mesonephric nephrostomes 

 unquestionably mark the morphologically dorsal angle of the body 

 cavity, and they lie just mesad to the duct, while the apparent dorsal 

 angle is much nearer the median plane, and is formed by an outfolding 

 of the splanchnic layer of the lateral mesoderm. 



In Figure 19 the ventral angle of the blastula (directly above the 

 point nph'st)7i.), which is to form the nephrostome, seems to have moved 

 ventrad into a position between the duct and the germ-cell mass. Such 

 a shifting of the fundament of the nephrostome would seem to neces- 

 sitate a severance of its connections with the two layers of the lateral 

 mesoderm and the re-establishment of a connection exclusively with the 

 somatic layer of the lateral mesoderm. Such a condition would exclude 

 a strict parallel between the nephrostome development in Amblystoma 

 and that in elasmobranchs, where, as above stated, the original connec- 

 tion between the lumen of the mesomer and that of lateral plates is 

 retained as the nephrostome, which consequently opens at the point of 

 union of the two layers of the lateral mesoderm. But such an interrup- 

 tion of the connection between the fundament of the nephrostome and 

 the lateral plates in Amblystoma is not the only possible interpretation of 

 the condition shown in Figure 19. The indicated migration of the 

 ventral region of the blastula, uithout loss of continuity between the layers 

 of the mesomer and those of the lateral plate might be brought about by 

 a degeneration of the germ cells of the somatic layer into ordinary epithe- 

 lial cells. Rabl ('96) assumes precisely such a degeneration in elasmo- 

 branchs, where, in early stages, he finds germ cells present in both layers, 

 whereas later they are confined to the splanchnoderm. In Amblystoma 

 the degeneration of the cells in question is rendered probable not only by 

 the fact that all of the germ cells disappear from posterior somites (see 

 Diagram 1), but also by the additional fact that, in the larva from which 

 Figure 19 was taken, there were frequently seen, lying just ventral to the 

 blastula cells, which are far too small to be germ cells, yet resemble 

 them in containing a conspicuous amount of yolk. 



There is probably, then, a persistent connection between the two 

 layers of the blastula and the corresponding layers of the lateral meso- 

 derm, which brings the condition in Amblystoma into close conformity 

 with that in elasmobranchs. and, what is of even greater importance, 



