384 BERTRAM G. SMITH 



micromeres and transitional cells approach nearer the vegetal 

 pole (figs. 26 and 27). In most eggs the transition from small to 

 large cells is more gradual on the side where the more rapid 

 multiplication of cells occurs; on the opposite side it is character- 

 ized by a rather abrupt line of demarcation. These features im- 

 pose a phase of excentricity and bilateral symmetry upon the 

 previously existing biradial symmetry of the cleavage pattern 

 of the lower hemisphere. A meridian drawn through the vegetal 



Figs. 26 and 27 Surface views of the lower hemispheres of two eggs of Crypto- 

 branchus allegheniensis in late blastula stages (stage 10, early and late phases, 

 respectively). In each egg the lower pole as determined by gravity lies at the 

 center of the figure; the vegetal pole, at the intersection of the first two cleavage 

 furrows, is slightly above this point and excentrically situated within the macro- 

 meres. The upper part of each figure represents the side on which the blasto- 

 pore is to appear. The figures were drawn with the aid of a camera lucida. X 7. 



pole and the center of the area occupied by the macromeres 

 defines the axis of excentricity and bilateral symmetry in the 

 cleavage pattern. 



1. Relation to the median plane of the gastrula. In many eggs 

 preserved in the early gastrula stage the cleavage pattern of the 

 lower hemisphere is well-defined; it is sometimes possible to 

 identify first and second cleavage furrows and thus to locate 

 the vegetal pole at their intersection (figs. 28 and 29). The ex- 

 centric position of the vegetal pole within the region of macro- 

 meres and the bilateral phase of the cleavage persist into the 



