EMBRYOLOGY OF CRYPTOBRANCHUS 131 



tunity for the contribution of important nuclear material to the 

 cytoplasm. 



The germinal disc or blastodisc no longer shows a division into 

 two laj^ers ; the material of the cytodisc is intimately mingled with 

 that of the yolk disc. The cone of cytoplasm following the germi- 

 nal vesicle in its migration is likewise more or less thoroughly 

 incorporated into the blastodisc. 



The end result of the migration of the germinal vesicle to the 

 surface and its disintegration in that situation is now apparent. 

 All the material of the nucleus and a considerable amount of cyto- 

 plasm have been brought from the interior of the egg to the vicin- 

 ity of the animal pole, fragmented, and the debris more or less 

 scattered throughout the blastodisc. Out of this complex there 

 soon emerges close to the surface at the animal pole the recon- 

 structed nucleus in the form of the first polar spindle. One function 

 of migration is doubtless to get this nuclear material to the periph- 

 ery where a part of it may be disposed of in the maturation divi- 

 sions. A further adaptation is found in the fact that, later, the 

 egg-nucleus or female pronucleus is left in the center of the forma- 

 tive material of the blastodisc. A third end attained by migra- 

 tion is that the formative material of the blastodisc is added to by 

 cytoplasm following the germinal vesicle, and also by substances 

 derived from the germinal vesicle itself. 



The zona radiata has become reduced in thickness, has lost its 

 striation and no longer shows a distinct limiting inner surface — 

 its inner margin is irregular or blends with the peripheral cyto- 

 plasm of the egg. When the egg is shrunken away from the zona 

 pellucida the zona radiata usually remains organically connected 

 with the egg. The character of the zona radiata has changed so 

 radically that it will no longer be referred to by this name; it has 

 become a simple cell wall to the egg, and as such takes part in the 

 later process of cleavage. 



The zona pellucida persists unchanged as the so-called vitelline 

 membrane of the egg at the time of fertilization and during the early 

 stages of embryonic development. 



As in other amphibian eggs, only these two membranes, the 

 zona pellucida and the cell wall formed from the zona radiata, 



JOURNAL OF MORPHOLOGY, VOL. 23, NO. 1 



