MATURATION OF THE OVUM. 



59 



complementary, but the spermatozoon has a longer history 

 behind it. The homologue of the ovum is the mother 

 sperm cell or spermatogonium. This segments much as the 

 ovum does, but the cells into which it divides have little 

 coherence. They go apart, and become spermatozoa. There 

 is a striking resemblance between the different ways in which 

 a mother sperm cell divides and the various kinds of segmen- 

 tation in ova. In most cases the spermatogonium divides into 

 spermatocytes, which usually divide again into spermatides 

 or young spermatozoa. 



Maturation of the Ovum. When the egg cell attains its 

 definite size or limit of growth, it bursts from the ovary or 



C D 



FIG. 9. Diagram of maturation and fertilisation. 

 (From " Evolution of Sex.") 



A. Primitive sex. cell, supposed to be amoeboid. 



B. Ovum ; C. formation of first polar body (i. /..); D. formation 

 of second polar body (2. /.). 



Bi. Mother sperm cell; Ci. the same divided (sperm-morula or 

 polyplast, or spermatogonium). 



Di. Ball of immature spermatozoa or spermatides ; sp. liberated 

 spermatozoa. 



E. Process of fertilisation ; F. approach of male and female nuclei 

 within the ovum. 



from its place of formation, and in favourable conditions 

 meets either within or outside the body with a spermatozoon 

 from another animal. Before this union between ovum and 

 spermatozoon is effected, generally indeed before it has 

 begun, the nucleus or germinal vesicle of the ovum moves 

 to the periphery and divides twice. This division results in 

 the formation and extrusion of two minute cells or polar 



