MATURATION OF OVUM. 



61 



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

 The second division follows the first without the inter- 

 vention of the "resting stage" which usually succeeds a 

 nuclear division. In most cases the division which forms 

 the first polar body is a reducing or meiotic division, the 

 number of chromosomes being reduced to half the number 

 characteristic of the. cells of the body. The extrusion of 

 polar globules and the associated reduction is almost 

 universal in the history of ova, but in most parthenogenetic 

 ova only one polar body is formed, and there is no reduc- 

 tion in the number of chromosomes. In some other cases 



BCD 



FlG. 29. 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. p.b.) \ D. formation 



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



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

 D' '. Ball of immature spermatozoa : sfi. t liberated spermatozoa. 

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



within the ovum. 



the parthenogenetic ovum passes through the meiotic 

 phase and forms two polar bodies. The second of these, 

 however, is not liberated, but remains within the ovum and 

 re-uniting with the reduced nucleus restores the normal 

 number of chromosomes. 



Reducing or Meiotic Division. In each kind of animal 

 there is a definite number of chromosomes, say n, in each 

 of the body-cells. In the ripe germ-cells, however, there is 

 half the normal number, J, so that when spermatozoon and 

 ovum unite in fertilisation the normal number is restored. 



In the history of the germ-cells, therefore, in one way or 

 another, at one stage or another, the number of chromo- 



