8 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



The origin and growth of the ovum within the ovary (oogenesis) are described on 

 pp. 214-217. We may state here that each growing ovum is at first surrounded 



Fig. 1. — Human ovum examined fresh in the liquor follicuU (Waldeyer). X 415. The zona pellu- 

 cida is seen as a thick, clear girdle surrounded by the cells of the corona radiata. The nearly mature 

 egg itself shows a central granular deutoplasmic area and a peripheral clear layer, and encloses the 

 nucleus in which is seen the nucleolus. At the right is a spermatozoon correspondingly enlarged. 



Vitelline ■ , 

 membraneY, 



Nudcwif 



Zona 

 peUucida 



Cytoplasm 

 of ovum 



Fig. 2.— Ovum of monkey. X 430. 



by small nutritive cells known as follicle cells. These increase in number during 

 the growth of the ovum until several layers surround it (Fig. 229). A cavity 

 appearing between these cells becomes filled with fluid and thus forms a sac, 



