No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 379 



A certain amount of this thinning may be due to a migration 

 of the cells toward the equator, as held by some authors, yet I 

 am satisfied that this is not the full explanation, since if the 

 cells passed around the sides they should be recognizable from 

 their smaller size. Moreover, it is impossible to interpret such 

 appearances as shown in Fig. 5, except on one of the following 

 hypotheses: Either these cells are wedging themselves in 

 between the cells of the external layer, or they are being 

 pressed out from between them. The former seems the more 

 probable, since one must, at this time, think of an extension of 

 the superficial layer rather than a reduction. Besides these 

 cells possess coarser granules than the cells of the external 

 layer, which accords perfectly with the former view, but not the 

 latter. 



Regarding the formation and extension of the mesoblast I 

 have found no two authors in complete agreement. 



Calberla ('78) states that the mesoderm arises through the 

 division of the primary entoderm into mesoderm and secondary 

 entoderm. 



According to Scott ('81) the mesoblast is derived from two 

 sources : One portion arises from the invaginated blastoderm, 

 and a second portion is derived from the outermost layer of 

 yolk cells. The latter completes the mesoblast on the ventral 

 side of the body. 



Nuel ('81) holds that a layer of hypoblast, lying between the 

 layer forming the roof of the alimentary canal and the epiblast, 

 gives rise to the mesoblast. 



Shipley ('88) describes the mesoblast as arising from two 

 bands of yolk cells which lie in the angles formed by the 

 mesenteron and the epiblast. The differentiation begins in 

 front and is continued backward. 



Kupffer ('90) agrees with Shipley that the mesoderm ex- 

 tends from before backward, and when the region of the tclco- 

 blast is reached its growth is supplemented by cells derived 

 from this group. 



Hatta (-92) believes the mesoderm to have a double origin. 

 The part arising first is the "peristomial," which is found 

 around the entire rim of the blastopore, save a point at the 



