REPRODUCTION 41 



cytoplasm, which is now called the yolk. During all the 

 changes described above, the number of chromosomes 

 in the ova remains the same as in the body-cells. When 

 the egg undergoes the final preparation for fertilization, 

 as we have seen, the number of chromosomes is reduced 

 by one-half. 



41. Reduction in the male (spermatogenesis). The 

 changes which bring about the reduction of the chromatin 

 in the male germ-cell are almost exactly similar to those 

 which have been described in the development of the 

 ovarian egg. The spermatozoa originate in the primor- 

 dial germ-cells. In their earlier development by cell 

 division, numerous spermatogonia are formed which for 

 a time continue to divide with the typical number of 

 chromosomes found in the soma-cells. In time the 

 spermatogonia cease to divide further and become larger. 

 At this stage they are physiologically equivalent in func- 

 tion to the oocyte of the female and are known as sperma- 

 tocytes. There now occur two divisions resulting in four 

 cells, each having but one-half the typical number of 

 chromosomes. Unlike the reducing process in the female 

 which results in only one perfect ovum and three abortive 

 cells, all four sperm-cells are functionally perfect. 



42. The period of the oestrum or heat. In all the 

 domestic mammals, the ripening of the first egg is asso- 

 ciated with the first appearance of heat. It is'the first 

 evidence of puberty and is accompanied by the rapid 

 development of all the secondary sexual characters dis- 

 tinctive of sex. It is by no means certain that the stimu- 

 lus which causes the heat or oestrum in animals originates 

 in the ovary. It is possible that the stimulus to egg forma- 

 tion is to be found in the influences set in motion by the 

 oestrum itself. The production of ova and the heat 



