508 



BERTRAM G. SMITH 



folds are conspicuous at an earlier stage in living than in preserved 

 material. In embryos later than the one figured, transverse 

 furrows in the neural plate appear as described in preserved 

 material. 



During Stage 13 the blastopore nearly closes, then makes 

 little advance in this respect during the next two stages. Varia- 

 tions in the degree of reduction of the blastopore during these 

 three stages are so great that this structure cannot be used as 

 a character for classifying embryos into stages. 



As shown in figures 160 to 162, during Stage 13 the blastopore 

 changes from a diamond shape to that of an anchor; the forward- 



160 



161 



162 



Figs. 160 to 162 A series of embryos of Cryptobranchus allegheniensis in Stage 

 13, showing changes in the size and form of the late blastopore. Camera drawings 

 from preserved material. X 5. 



projecting part is derived through an exaggeration of the notch 

 previously noted in the dorsal lip of the blastopore. The lappets 

 lying on each side of this median notch of the blastopore are 

 continuous with the neural folds; through their apposition the 

 dorsal part of the yolk plug becomes closed over. Thus the 

 extreme posterior end of the embryo is undoubtedly formed by 

 a process of concrescence. As shown in later stages, the ventral 

 part of the blastopore becomes reduced to a transverse slit (figs. 

 177 and 178) ; during this process the yolk plug usually becomes 

 entirely withdrawn into the egg, but a small mass of yolk some- 

 times persists at the surface. The late history of the blastopore 

 is much the same in Cryptobranchus japonicus, as described by 

 Ishikawa ('08). 



