EMBRYOLOGY OF CRYPTOBRANCHUS 129 



collected beneath it, and is now beginning to mingle with the sur- 

 rounding yolk. 



A germinal disc or blastodisc is evident in surface views of living 

 material as a circular area, lighter in color than the rest of the egg, 

 about 60° in diameter and situated on the more exposed side of 

 the egg. In meridional sections it is shown to consist of two layers : 

 a thin peripheral layer of cytoplasm, the cytodisc ; underlying this 

 a thick lenticular mass of mingled fine yolk particles and cyto- 

 plasm, the yolk disc. The germinal vesicle lies at the center of 

 the yolk disc. 



The yolk disc is continuous with a thin peripheral layer of fine 

 yolk granules, mixed with cytoplasm, which lies in contact with 

 the zona radiata everywhere except in the region of the cytodisc. 



The remainder of the egg is filled with coarse yolk granules 

 mingled with fine yolk granules and a small amount of cytoplasm. 



The nucleoli are grouped at the center of the germinal vesicle, 

 and amongst them are numerous chromatin granules. In some 

 eggs chromosomes are found at the center of the group of nucleoli, 

 in others the chromosomes have disappeared. 



A point on the surface overlying the center of the germinal 

 vesicle marks the animal pole. The general arrangement of 

 materials is radially symmetrical about the axis of polarity, with 

 differentiation proceeding in the direction of this axis. 



B. MATURATION 



1. The germinal vesicle at the surface 



Meridional sections through ovocytes with the germinal vesicle 

 at the surface show little change in the details of structure from 

 the condition previously described. The germinal vesicle is 

 usually somewhat flattened against the periphery, and a portion 

 of its surface is in direct contact with the zona radiata. Masses of 

 a wavy fibrous material are occasionally found in the nuclear 

 sap. A few fragments of chromosomes are present in some eggs; 

 in others no chromosomes have been found. The nucleoli and 

 chromatin granules persist at the center of the germinal vesicle. 



