HALL: MESONEPHKOS AND MULLERIAN DUCT IN AMnilBIA. 73 



Amblystoma, ou the contrary, the somatic and splanchnic layers of the 

 mesomer are closely applied to each other during its severance from 

 the epimer, and the lumen becomes apparent only in the well-formed 

 blastula. I have consequently anticipated the conditions somewhat 

 in giving the mesomer a lumen in Figure H. Aside from this the only 

 important difference in the processes in the two animals is the formation 

 of the nephrostome in Amblystoma by a secondary fusion of the outer 

 tubule with the peritoneum. It seems jn'obable, it is true, that some of 

 the anterior nephrostomes in Amblystoma are formed as in Ichthyophis, 

 from a persistent connection with the coelom, but posteriorly (and 

 throughout the entire mesonephros in E,ana) the connection is certainly 

 a secondary one. 



Although, as just stated, the final connection between mesomer and 

 lateral plates (by means of the outer tubule) is a secondary one 

 in Amblystoma and Rana, it seems probable that the tw^o layers of 

 the mesomer retain their relative positions and give rise to the same 

 portions of the mesonephric unit as in Ichthyophis and elasmobranchs. 

 What I believe these portions to be will be found illustrated in Figures 

 94-100, where the shading of the supposed somatoderm is made with 

 continuous lines, that of the splanchnoderm with interrupted lines. 



D. Eecapitulation of the Mesonephric Development. 



Amblystoma. 



1. The mesonephric blastula is derived from a portion of the somite 

 •which is homologous with the mesomer of elasmobranchs, and it contains 

 both splanchnoderm and somatoderm. 



2. Contrary to the condition in elasmobranchs and Ichthyophis, the 

 lumen of the mesomer at the time it is cut off from the epimer is 

 only ])otential ; it first appears as the cavity of the mesonephric 

 blastula. 



3. The more anterior blastulae probably never lose their connection 

 with the two layers of the lateral mesoderm. The outer tubules (at 

 least in the anterior primary units) therefore contain both somatoderm 

 and splanchnoderm. In this connection it was noted that the morpho- 

 logically dorsal angle of the body cavity is not at the upper limit of the 

 mesentery, but lies just mesad to the Wolffian duct. Hence the germ 

 cells (except in very early stages) all lie in the splanchnoderm. 



4. The secondary character of the dysmetamerism of the urodele 

 mesonephros sliow^s itself in the fact that the primary blastulae can 



