EMBRYOLOGY OF CRYPTOBRANCHUS 



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at intervals along a narrow zone several cells in width; hence 

 from its very beginning the process is not a splitting-apart of 

 cells, but invagination. The groove soon becomes continuous 

 and deepens by the inturning of cells along both margins. 



After the groove has reached a length of three millimeters or 

 more, the process of invagination becomes accompanied by one 

 of overgrowth or epiboly: the dorsal lip grows slowly down over 



Fig. 115 Lower hemisphere of a gastrula of Cryptobranchus allegheniensis, 

 in a slightly later stage than the preceding, showing the lining-up of the cells within 

 the horns of the blastopore. Freehand drawing from a photograph of preserved 

 material. X 10. 



the cells transitional between micromeres and macromeres (figs. 

 113 to 115). As shown in figure 115, the transitional cells just 

 within the horns of the blastopore are elongated as if compressed; 

 here the cells line up and lengthen out at right angles to a line 

 connecting the horns of the blastopore. 



After the blastoporic groove has attained the form of a semi- 

 circle (fig. 133), a zone of rather abrupt demarcation between 

 micromeres and transitional cells completes the circle begun by 

 the crescentic blastopore; this zone marks the site of the ventral 

 lip of the blastopore. A little later, the blastoporic groove ex- 

 tends rapidly along this line of demarcation, becoming an almost 



