DEVELOPMENT OF SEELERPES BILINEATUS 207 



This view of the ventral extension of the mesoderm in its 

 essential feature, namely, the transformation of the endoderm 

 cells into mesoderm, leading to the ventral extension of the latter, 

 is not very different from the view put forth by Hertwig in 1883, 

 although I find no evidence of archenteric pouches. There is 

 never any cleft among the cells at the point where they are almost 

 entirely separated from the notochord and endoderm cells in fig. 

 41. Very soon they separate completely. 



The peristomal mesoderm can be distinguished earlier than the 

 gastral. It is already apparent ventrally (fig. 33, vm), as the 

 archenteric invagination is nearing completion. It can also be 

 distinguished in cross-sections as polygonal cells in the dorsal 

 lip of the blastopore, although it does not surround the immediate 

 edges of the blastopore until the stages shown in fig. 34. The 

 ventral mesoderm can readily be traced back to the yolk cells 

 'l^dng just beneath the lower corner of the blastocoele, opposite 

 the blastopore. They are carried (or move) along bodily, differ- 

 entiating as they go until at the close of gastrulation they can be 

 seen as a triangular mass forming most of the ventral side of the 

 blastopore (fig. 34, V7n, cf. figs. 28 to 33). Later on this mass 

 thins out. Part of its uppermost cells come to lie in the posterior 

 wall of the gut. 



Brauer's ('97) results on Hypogeophis show clearly that the 

 origin and formation of the mesoderm in the Gymnophionians 

 is much specialized. It is formed by an invagination of animkl 

 cells whose sole destiny is the formation of mesoderm and noto- 

 chord. In the Amphibians proper, there are considerable dif- 

 ferences of opinion as to the source of the mesoderm and the 

 mode of its formation. 



Unless one has material other than one's own at hand, it is 

 impossible to deny views based on the study of such material. 

 I have had the opportunity to examine the mesoderm formation 

 in only one other Urodele. In cross-sections of eggs of Ambly- 

 stoma, conditions quite similar to those shown in figs. 36-41 

 were found. I conclude, therefore, that the formation of the 

 mesoderm in Amblystoma is the same as in Spelerpes, although 

 the degree of invagination at the dorsal lip has not been ascer- 



