702 A MANUAL OF VETERINARY PHYSIOLOGY 



and it is finally only left with half its original number ; but its 

 cytoplasm has undergone no practical loss. The polar bodies 

 are regarded as abortive ova ; they die, and are not concerned 

 in the subsequent changes. The important point is that the 

 mature ovum is left with only half the number of chromosomes 

 normal to the cells of the species. One further change in the 

 process of maturation has also taken place, and that is the 

 centrosome of the cell is lost. These remarkable changes pave 

 the way to an understanding of what follows, should impregna- 

 tion of the ovum occur. 



An ovum without a centrosome and with only half the normal 

 number of chromosomes has, in order to be fertilised, to be 

 furnished with a centrosome and its full number of chromosomes. 

 It is the function of the spermatozoon to supply these. 



Maturation of the Spermatozoa. — The cells of the immature 

 spermatozoa also undergo a process of ripening. The immature 

 spermatozoon is known as a spermatocyte. As the result of cell 

 division it forms two secondary spermatocytes ; both of these 

 undergo a further division and form four spermatids. The four 

 spermatids furnish four mature spermatozoa. There are no 

 abortive spermatozoa formed, as there are abortive ova. At 

 each cell division in the above stages, the number of chromosomes 

 contained in the nucleus in the head of the spermatozoon undergo 

 a reduction, until the mature spermatozoon, in the same way as 

 the mature ovum, only possesses half the number of chromo- 

 somes. When the spermatozoon impregnates the ovum, it 

 brings to it the important centrosome and a nucleus. This 

 nucleus provides the ovum with the chromosomes of which it was 

 short. In the ass the number of chromosomes is twenty-four, 

 and Robertson* explains the infertility of mules on the ground 

 that there is an odd number of chromosomes in their cells — 

 i.e., twelve derived from the ass and thirteen derived from the 

 horse, or a total of twenty-five. 



In the present light of knowledge the interest attached to 

 chromosomes is considerable. The chromosomes, according to 

 modern thought, convey the hereditary characters of the trans- 

 mitters, so that the resulting embryo has its characteristics im- 

 planted at the time of conception, in theory half being derived 

 from each parent. 



Ovulation is the process of egg extrusion. In some animals, 

 as the rabbit and ferret, it can only occur as the result of coition, 

 the presence of spermatozoa in the uterus being essential to 

 the act. In others — and they represent the majority — such as 

 the mare, donkey, cow, sheep, pig, and dog, ovulation occurs 

 during oestrus, but the act of copulation is not necessary to 



* Op. cit. 



