50 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



pocket then becomes differentiated to form a wider portion distally and 

 a narrower portion proximally. The latter becomes the outer tubule, 

 while from the much-thickened lateral wall of the former the inner 

 tubule is differentiated and grows outward and downward to join the 

 duct. The median wall, or sometimes the dorsal wall, of the distal por- 

 tion becomes invaginated to form the Malpighian body. 



As far as we have progressed in our description of the development of 

 a unit in Amblystoma, it will be seen that there is a rather close simi- 

 larity to the development in Pristiurus. As will be remembered, the 

 blastula in Amblystoma is composed of cells from both splanchnoderm 

 and somatoderm. These layers probably retain their original relative 

 position, the somatoderm giving rise to the thick lateral wall of the blas- 

 tula, the splanchnoderm forming at least a portion of the thinner median 

 wall. In the anterior units, Avhere the blastula retains its connection 

 with the lateral mesoderm, this is undoubtedly the case. 



Later Development of the Mesonepliric Units. 



Turning to the later development of the mesonephric unit, I have 

 figured four stages (Plate 2, Figs. 20-23), all from the posterior half of 

 the sixteenth somite of a larva 21 mm. in length (see Explanation of 

 Plates). The first change in such blastulae as are shown in Figure 19 

 is the shortening of each to form a more nearly spherical mass (Fig. 20), 

 which thus becomes larger in cross-section and more widely separated 

 from its neighbors. Then there is formed a process which grows veutrad 

 to touch, and later to fuse with, the peritoneum (Fig. 21, nph^stm.). In 

 those anterior somites in which contact between blastula and peritoneum 

 is present from the beginning, the same appearance is produced by a 

 slight dorsal migration of the blastula, leaving a cone of cells joining 

 it with the peritoneum. The thinness of the median wall of the 

 blastula (Fig. 21) has become more marked than in previous stages. 

 As above stated, this thinner median wall probably represents the 

 splanchnic layer of the mesomer. The thickened lateral wall (presum- 

 ably all somatic) sends out a conical proliferation of cells, which bends 

 ventrad and presses its point against the dorsal wall of the duct 

 (Fig. 22). 



In the stage represented by Figure 23 the inner tubule has fused 

 w4th the duct, and the dorso-median portion of the blastular wall has 

 bent inward (thl. ms'nph. and cps. Bou\ i.). The mesonephric unit now 

 presents in cross-sections of the body the form of a sigmoid curve. 

 The incurved portion, just mentioned, has sometimes been described as 



