498 



BERTRAM G. SMITH 



138 



Figs. 138 and 139 Posterior views of embryos of Cryptobranchus allegheniensis 

 in Stage 12, showing the condition of the blastojior^f and the cleavage furrows of 

 the yolk plug. Camera drawings from preserved material. The embryos are 

 not accurately oriented with respect to the vertical axis determined by gravity. 

 X 7. 



Fig. 138 Showing condition shortly after the appearance of the neural groove. 



Fig. 139 A little later than the preceding. 



138 to 142; cf. figs. 113 to 115). Toward the close of this stage, 

 the smaller cells become completely overgrown by the ventral 

 and lateral lips of the blastopore, leaving only the larger ones 

 exposed (fig. 144) ; evidently overgrowth is now proceeding more 

 rapidly at the ventral than at the dorsal lip of the blastopore. 

 At the close of the stage the greatly reduced yolk plug lies entirely 

 on the postero-dorsal side of the lower pole of the vertical axis. 

 During the earlier part of the stage under consideration the 

 closing fenestra often persists as a pit or small tract of distinct 



Figs. 140 to 14.5 Dorsal views of embryos of Cryptobranchus allegheniensis 

 in Stage 12, showing a series of stages in the development of the neural groove. 

 Camera drawings from preserved material. The embryos are not oriented with 

 respect to the vertical axis determined by gravity. X 7. 



Fig. 140 Showing earliest appearance of the neural groove. 



Fig. 141 Slightly later than the preceding. 



Fig. 142 Slightly later than the preceding, showing segmented neural groove. 



Fig. 143 Slightly later, segmented neural groove. See also figure 225 from 

 the same embryo. 



Fig. 144 Late neural groove. 



Fig. 14.5 Showing the condition of the neural groove at the time of the first 

 faint indications of the neural folds. 



