504 



BERTRAM G. SMITH 



bip. 



yp- 



bIp' 



49 



50 



I'ig. 149 Diagram of an egg of Cryptobranchus allegheniensis at the close of 

 Stage 12, showing the amount of rotation and the position of the yolk plug; m, 

 morphological axis; v, vertical axis determined by gravity; y.p., yolk plug. The 

 cross indicates the position of the anterior end of the neural groove. 



Fig. 150 Combination of figure 144 with some features of figure 137, showing 

 successive positions of the blastopore; hip and blp' respectively indicate early 

 and late positions of the dorsal lip of the blastopore. The black band at the right 

 of the figure indicates the amount of overgrowth (55 degrees) of the dorsal lip of 

 the blastopore; the dotted line indicates the amount of overgrowth (55 degrees) 

 of the ventral lip. 



that is to say, the distances are approximately equal (fig. 150). 

 By far the greater amount of overgrowth of the dorsal lip occurred 

 during the preceding stage, hence it is clear that during the pres- 

 ent stage overgrowth is taking place much more rapidly at the 

 ventral than at the dorsal lip of the blastopore. 



Stage 13: {figs. 151 to 164 and 226 to 228). The most conspic- 

 uous changes during this stage are those concerned with the 

 formation of. the neural folds and the segmentation of the neural 

 plate. The neural folds begin to form about one and one-half 

 days after the appearance of the neural groove. During the 

 progress of this stage the neural groove becomes most conspic- 

 uous in an anterior and a posterior portion, separated by a middle 

 region in which it is comparatively faint (see especially figs. 151 

 to 155). 



The early stages in the formation of the neural folds are shown 

 in figures 151 to 158 and need no further description, save to men- 



