252 



BERTRAM G. SMITH. 



turning on all sides of the yolk plug and particularly on the dorsal 

 side. In addition this figure shows that convergence involves a 

 zone of cells occupying the ventral as well as the lateral margin 

 of the yolk plug. This figure in connection with Fig. 10 shows 

 that the movement of the ventral margin occurs considerably 

 later than the movement of the lateral margin a result correlated 

 with the delayed development of the ventral lip of the blastopore. 



17 18 



FIG. 17. Lower hemisphere of an early gastrula of Cryptobranchus allegheniensis, 

 showing cell outlines. X 7. 



FIG. 1 8. Posterior view of an embryo of Cryptobranchus allegheniensis in the 

 neural groove stage, showing cell outlines in the yolk plug. X 7. 



Figs. 17 and 18 show that the cells occupying the lateral and 

 ventral margins of the yolk plug, which figure so conspicuously 

 in these movements, are readily distinguishable from the larger 

 yolk cells on the one hand, and the smaller cells of the lips of the 

 blastopore on the other; these marginal cells are arranged in the 

 form of a crescent. We have shown that the marginal cells are 

 the descendents of cells that formed a band in the equatorial 

 region of the late blastula, and it may be here stated that these 

 equatorial cells were of a transitional character, intermediate 

 in size between micromeres and macromeres. 



B. Confluence. The experimental study of the movement of 

 material in the region of the dorsal lip of the blastopore aims to 

 determine (a) whether movements other than overgrowth and 

 in-turning occur in the dorsal lip of the blastopore, and (6) if 



