90 GENERAL EMBRYOLOGY 



and deposition of the yolk 'and certain other substances within 

 the egg cytoplasm. Sometimes the cytoplasm appears homo- 

 geneous, in other cases it shows considerable differentiation. 

 Often the peripheral layer of the cytoplasm is more vacuolated 

 and less granular than the central portion; the former is then 

 spoken of as exoplasm or as the cortical layer, the latter as 

 endoplasm (Figs. 42, 43, 45, 46). There may also be various 

 materials in the cytoplasm which have been laid down during 

 the formation of the egg, under the influence of the nucleus, or 

 yolk-nucleus, and deposited in different regions (Figs. 42-48). 



FIG. 46. Section through the ovarian egg (oocyte) of Amphioxus. After 

 Sobotta. X 525. c, vacuolated cortical layer (exoplasm) ; e, endoplasm con- 

 taining deutoplasmic bodies; v, vitelline membrane; /, first polar body; //, 

 second polar spindle. 



The extent of the cytoplasmic differentiation varies greatly 

 in eggs of different species. In many forms it can hardly be 

 demonstrated in the egg at the time it is fully formed; in such 

 eggs this differentiation appears later, during or after the proc- 

 esses of fertilization, or even still later, during cleavage. We 

 shall have to return to this subj ect in connection with the sub- 

 ject of cleavage, after we have described the fertilization proc- 

 ess. But there are two or three fundamental aspects of this 

 fact that should be mentioned here. There are quite commonly 

 three, sometimes more, distinct forms of cytoplasmic material 

 arranged as definite regions of the ovum, occasionally as zones, 

 or layers, or as localized masses (Figs. 42, 45). These may be 

 distinguished by the more or less vacuolated character of the 



