118 bulletin: museum of comparative zoology. 



is in the form of a tube whose lumen sets the lumen of the rest of the 

 somite (the epimer) in communication with the general body cavity. 

 This ventral, tubular portion of the somite becomes directly transformed 

 into a mesonephric blastula. In the more posterior segments it becomes 

 cut off from the lateral mesoderm, a slight widening of its lumen takes 

 place (a process designated by Brauer as the formation of the nephro- 

 stome), and it becomes constricted from the epimer. In the median por- 

 tion of the typical region, the widening of the lumen of the mesomer 

 talves place before that structure loses its connection with the lateral 

 plates. In either case the result is the transformation of the lower por- 

 tion of the somite (the mesomer) into a single rounded mesonephric 

 blastula, which is independent of all other organs. Very significant is 

 the fact that in the more anterior portion of the typical region, cei'tain 

 of the mesomers never lose their connection with the lateral mesoderm, so 

 that their splanchnoderm and soraatoderm remain continuous with the 

 corresponding layers of the lateral mesoderm and their lumina con- 

 tinuous with the general body cavity. The outer tubule and nephro- 

 stome of a primary unit may thus be formed, in certain cases, by a 

 simple transformation of structures present from the very beginning 

 of somite differentiation. This, as has already been noted, is typically 

 the case in elasmobranchs. It will be remembered that there was 

 evidence of a similar retention of continuity between mesomer and lat- 

 eral mesoderm in the more anterior portion of the mesonephros of 

 Amblystonia. In that form, however, a continuous lumen was not 

 recognizable. 



The formation of a mesonephric blastula in Hypogeophis is thus 

 similar to that in Amblystonia with the important exception that in the 

 former genus the entire mesomer is transformed directly into a single 

 primary unit, its lumen being not even temporarily obliterated. 



The differentiation of the primary blastula is much as in Amblystoma. 

 As in that form, the inner tubule arises as an ecagination of the lateral 

 wall of the blastula and is forced dorsad by the proximity of the Wolffian 

 duct. If the peritoneal connection has not been retained, it is estab- 

 lished by means of a nephrostomal evagination appearing at the ventro- 

 lateral angle of the blastula. The secondary blastula is then formed by 

 an outbudding of the posterior, median portion of the blastula. It will 

 be seen that the secondary blastula arises from the primary at a later 

 period than in Amblystoma. The secondary blastula resembles that of 

 Rana sylvatica in that it may retain its connection with the primary by 

 means of a cord of cells. Where this occurs, it is seen to be attached to 



