436 



HEREDITY AND DEVELOPMENT 



each of the two nuclei which unite in fertilisation contains one- 

 half of the number of chromosomes characteristic of the somatic 

 cells, though the nuclei of the earlier stages of the germ-cells have 

 the same number as the somatic cells, it has been plain that a re- 

 ducing process must occur at some stage, and there is now general 

 agreement that the reduction takes place in the last two cell- 

 divisions by which the definitive germ-cells arisenamely, when 

 the ovarian ovum gives rise to the mature ovum and two or three 



FIG. 44. Diagram of maturation and fertilisation. (From Evolution 

 of Sex.) 



The upper line shows development of spermatozoa. The lower line shows maturation of 

 the ovum. The middle line to the right shows fertilisation, a, an amoeboid primitive sex- 

 cell ; A, ovum, with nucleus or germinal vesicle (n) ; B, ovum, liberating first polar body (/>') ; 

 C, extrusion of second polar body (p y ) ; i, a mother-sperm-cell or spermatogonium ; 2, 3, balls 

 of immature spermatozoa, resulting from the division of (i) ; sp., mature spermatozoa ; D, 

 the entrance of a spermatozoon into the ovum ; E, the male and female nuclei sp.n and n% 

 approach one another. 



polar bodies, and when a spermatocyte divides into four sper- 

 matids or young spermatozoa. The parallelism in the two cases 

 is very striking, but as O. Hertwig says, " while in the latter case 

 the products of the division are all used as functional sperma- 

 tozoa, in the former case one of the products of the egg-mother- 

 cell becomes the egg, appropriating to itself the entire mass of 

 the yolk at the cost of the others, which persist in rudimentary 

 form as polar bodies." The hypothesis of Minot, adopted also by 



