18 THE GENITAL GLANDS 



in number. Each chromosome sphts into two, form- 

 ing forty-eight ; of these twenty-four pass to one 

 daughter nucleus and twenty-four to the other. 

 Finally, the cell protoplasm cleaves, and the nucleus 

 returns to its resting condition. This process is 

 called homotype (i.e. normal) mitosis. 



Before it meets a spermatozoon, the nucleus of 

 the ovum divides twice, extruding the two polar 

 bodies. At the second of these divisions,* half the 

 chromosomes — that is, in man, twelve — are thrown 

 out, and the centrosome with them. This is to 

 prevent parthenogenesis — the development of an 

 ovum into a foetus without a male element. In 

 bees and wasps, where parthenogenesis occurs, this 

 second or heterotype mitosis does not take place. 



In the formation of the spermatozoon, also, a 

 cell with twenty-four chromosomes divides into two 

 spermatozoa with twelve each ; the head is the 

 nucleus, the neck the centrosome, and the tail is the 

 cell body. Thus the foetus starts life with twenty- 

 four chromosomes, twelve from each parent. In 

 these, according to Weissmann, is bound up its 

 heredity, including the impulse to assume the general 

 shape of mankind, the viscera with their proper 

 anatomy and functions, and some resemblance to 

 the facial appearance and even the tone of voice and 

 character of the parents. How all this is crowded 

 into such microscopical objects is the greatest marvel 

 in biology. 



The spermatozoon probably brings in some 



♦ Some English text-books incorrectly say the first. 



