THE EARLY DEVELOPMENT OF THE MAMMAL 373 



diameter, although these limits are occasionally exceeded; for 

 example, the ovum of the mouse measures about 0.06 mm., 

 deer 0.07-0 . 10 mm., guinea-pig 0.09 mm., dog about 0.18 mm., 

 human 0.22-0.32 mm., cat 0.135-0.15 mm., rabbit 0.1 1-0. 12 mm. 

 The cytoplasm of the ovum ordinarily exhibits two general 

 regions, a clear exoplasm or cortical layer surrounding an opaque 



FIG. 144. Fully grown human oocyte just removed from the ovary. Out- 

 side the oocyte are the clear zona pellucida and the follicular epithelium (corona 

 radiata). The central part of the oocyte contains deutoplasmic bodies and the 

 eccentric nucleus (germinal vesicle). Superficially there is a well-marked exo- 

 plasm, or cortical layer. From Waldeyer (Hertwig's Handbuch, etc.). 



endoplasm containing small granules of deutoplasmic material 

 (Figs. 144, 146). The nucleus or germinal vesicle of the fully 

 formed oocyte is relatively large, usually 0.025-0.040 mm. in 

 diameter; it is spherical, possesses a definite nuclear membrane, 

 a large nucleolus (karyosome), and has a slightly eccentric 

 position. 



