EMBRYOLOGY OF CRYPTOBRANCHUS 113 



with the exception that there is a marked difference in the appear- 

 ance of the nuclei of the folhcle proper: in Necturus these nuclei 

 are more numerous, and in form are spherical or even elongated in 

 a radial direction, instead of being flattened in the direction of the 

 circumference of the egg as in Cryptobranchus. The follicle 

 of Necturus more closely resembles that of the selachian egg in an 

 early stage (see Hertwig's Handbuch, '06, figs. 105 and 195). The 

 zona pellucida and zona radiata are much alike in the two urodeles; 

 the striations of the latter membrane are rather more distinct 

 in Necturus. 



2. Tine establishment of polarity, and the progress of axial 



differentiation ^ 



As already noted in the surface study of the ovarian egg, the ovo- 

 cyte ready for maturation shows its telolecithal character in the 

 presence of a superficial germinal area, in the center of which lies 

 the germinal vesicle, while the remainder of the egg is heavily 

 laden with yolk. It is the purpose of the present section to trace 

 the changes by which this axial differentiation is brought about. 



In the ovary of a 9 cm. larva, vitelline bodies (see King, '08) 

 are recognizable in the cytoplasm of the ovocytes in all stages 

 present, but are not very numerous nor conspicuous even in the 

 most advanced ovocytes of such an ovary (see figs. 13 to 17). 

 In the largest ovocytes, the germinal vesicle is usually somewhat 

 excentrically situated, but with no constancy in the direction of 

 excentricity. Faintly-staining nucleoli are distributed quite pro- 

 miscuously thoughout the germinal vesicle, in the later stages with 

 a slight tendency toward forming a ring at the periphery. 



In the most advanced ovocytes of a 26 cm. female (see fig. 18) 

 there is less excentricity in the position of the germinal vesicle; 

 the nucleoli are most numerous at the periphery. There is an 

 increase in the number and size of the vitelline bodies, which are 

 more numerous on the side toward the central cavity of the ovary. 

 After fixation in Zenker's fluid, both nucleoli and vitelline bodies 

 take the nuclear stain, though faintly. In the ovary of a 27 cm. 



