EMBRYOLOGY OF CRYPTOBRANCHUS 121 



which the germinal vesicle is moving, lies in general on the more 

 exposed side of the egg, within the stalk of the follicle. The ani- 

 mal pole thus lies in the opposite direction from that assumed in 

 the ovarian egg of the hen (Lilhe, '08, p. 29). According to King 

 ('02) in the great majority of cases the egg of Bufo is attached in 

 the equatorial region by the stalk of the follicle. 



From a comparison of this stage wdth the preceding one (the 

 ovocytes of a spent female) , it is evident that yolk-formation pro- 

 ceeds concentrically about a centrally situated germinal vesicle 

 until the egg is nearly or quite filled with yolk, and that axial 

 differentiation in the arrangement of yolk particles does not 

 appear until a very late stage of ovogenesis, two or three months 

 before maturation. It is further apparent that the germinal vesi- 

 cle attains its final position, not through unequal growth of the 

 cytoplasm or excessive accumulation of yolk on the other side of 

 the egg, but by a process of migration. 



In the ovocytes of adults taken July 20, the germinal vesicle 

 has migrated further toward the animal pole; it lies about one- 

 third of the distance from the surface to the center of the egg. 

 Both nucleoh and chromosomes are now aggregated at the center 

 of the germinal vesicle. The yolk-cup persists, and there is an 

 increase in the extent of the germinal disc. In some eggs a small 

 cone-shaped mass of dense cytoplasm, with the apex of the cone 

 pointing inward, lies immediately beneath the germinal vesicle. 



In the ovary of an adult female killed August 17, the egg (fig. 

 24) has nearly reached its maximum size before fertilization; 

 a meridional section cut in paraffin has a diameter of about 6 

 mm. (It should be noted that a yolk-laden egg does not shrink in 

 paraffin to the same extent as ordinary tissues). The germinal 

 vesicle hes only a short distance from the surface, and is bounded 

 on the side toward the center of the egg by a large cone-shaped 

 mass of cytoplasm. The apex of this cone is continuous with a 

 slender meshwork of less dense but yolk-free cytoplasm extending 

 half-way to the center of the egg. Owing to a slight obliquity of 

 the slender cytoplasmic mass, it has not been found complete in 

 any one section; in fig. 24 it has been added, from adjacent sec- 

 tions, to the one chosen for the remainder of the drawing. 



