STRUCTURE OF THE OVUM 255 



consists of a more or less globular mass of protoplasm, 

 generally exhibiting a reticular structure and enclosing a 

 larger or smaller quantity of yolk-granules. Surrounding the 

 cell-body is usually a cell-wall or cuticle, often of considerable 

 thickness and known as the vitelline membrane. The nucleus 

 is large and has the usual constituents (p. 62), nuclear mem- 

 brane, achromatin, and chromatin. As a rule the chromatin 

 or a portion of it is aggregated in the form of a very definite 



FIG. 6 1. Ovum of a Sea-urchin (Toxopneustes lividtts], showing the 

 radially-striated cell-wall (vitelline membrane), the protoplasm contain- 

 ing yolk granules (vitellus), the large nucleus (germinal vesicle) with its 

 protoplasmic network, and a large nucleolus (germinal spot). (From 

 Balfour after Hertwig.) 



nucleolus, which is often known as the germinal spot, the 

 entire nucleus being called the germinal vesicle. 



Such a fully-formed ovum is, however, incapable of being 

 fertilized or of developing into an embryo : before it is ripe for 

 conjugation with a sperm or able to undergo the first stages 

 of yolk division it has to go through a process known as the 

 maturation of the egg. 



