xxiv STRUCTURE OF THE OVUM 259 



larger or smaller quantity of yolk-granules. Surrounding 

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

 siderable thickness and known as the vitelline membrane. 

 The nucleus is large and has the usual constituents (p. 63) 

 nuclear membrane, nuclear matrix, and chromatin. As a 

 rule there is a very definite 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 



FIG. 61. Ovum of a Sea-urchin (Toxopnenstes lividus], showing the 

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

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

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

 Balfour after Hertwig. ) 



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. 



Maturation consists essentially in a twice-repeated process 

 of cell-division. The nucleus (Fig. 62, A, nu) loses its mem- 

 brane, travels to the surface of the egg, and takes on the 



S 2 



