MATURATION OF OVUM. 61 



spermatogonium divides into spermatocytes, which usually 

 divide again into spermatides or young spermatozoa. 



Maturation of ovum. When the egg-cell attains its 

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

 from its place of formation, and in favourable conditions 

 meets either within or outside the body with a spermatozoon 

 from another animal. Before the 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 



FiG. 29. Diagram of maturation and fertilisation. 

 (From " Evolution of Sex.") 



A. Primitive sex cell, supposed to be amoeboid 



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



of second polar body (2. /..). 



B ' . Mother sperm cell ; C'. the same divided (sperm-morula). 

 D '. Ball of immature spermatozoa : .$/., liberated spermatozoa. 

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



within the ovum. 



bodies, the first containing half, the second a quarter of 

 the nuclear material which composed the germinal vesicle. 

 The second division follows the first without the inter- 

 vention of the "resting stage "which usually succeeds a 

 nuclear division. Moreover, there is this important 

 peculiarity that at some point in the history of matura- 

 tion the number of chromosomes is reduced to half the 

 number characteristic of the cells of the body, whereas in 

 ordinary karyokinesis the daughter nuclei have always as 

 many nuclear rods as the original cell. The extruded 

 polar bodies come to nothing, though they may linger for 

 a time in the precincts of the ovum, and may even divide. 



