STRUCTURE OF THE OVUM 257 



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 con- 

 siderable thickness and known as the vitelline membrane : 

 frequently it is perforated at one pole by an aperture, the 

 micropyle (fig. 62, microp). The nucleus is large and has 



FIG. 61. Ovum of a Sea-urchin (Toxopneiistes lividits\ showing the 

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

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

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

 Balfour after Hertwig.) 



the usual constituents (p. 63) nuclear membrane, nuclear 

 sap, and chromatin. As a rule there is a very definite nucle- 

 olus, 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 segmentation it has to go through a process known 

 as the maturation of the egg. 



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