28 



Journal of Agriculture. 



[i I Jan., 1909. 



ovum its tail drops off but the head and neck parts enter and blend with 

 the nucleus of the oxuiu. When fertilization occurs the ovum at onc<- 

 begins to change. It divides into two, then into four, and so on, each 

 daughter cell growing at the ex^^ense of nutriment derived from the mucous 

 membrane of the tube. In this process of division it is interesting to note 

 that each daughter cell has the same number of chromosomes as the original 

 fertilized cell ; for by a process of division, like the splitting of a cane 

 longitudinal Iv, each of the chromosomes of the mother cell divides into two, 

 making a complete set for each cell, and each of the daughter chromo- 

 somes caji divide longitudinally and so on. Thus it is that in every cell of 

 the adult body the chromosomes are derived half from the mother and half 

 from the father. The mass of cells to which the ovum has given rise passes 

 down the FallojHan tube into the uterus and there lodges becoming firmly 

 adherent to the uterme wall. Its further development will be considered 

 later. In the mammal an ovum is utterly incapable of developing unless 

 fertilized bv a spermatozoon, and the spermatozoon must emanate from a 

 male of its own species or at least of species \-ery closely akin. Thus sue- 

 x:essful pairing can occur between hare and rabbit, but not between hare 

 or rabbit and dog. 



Fig. 63, Section of tlie Ovary of a Cat. — i, outer capsule ; 5, cells from wiiich ova 

 arise ; 6, similar cells enlarging and sinking deeper, and ultimately forming follicles ; 

 7 and 8, unripe follicles; 9, ripe follicle, about to l)urst and siied ovum ; 10, corpus 

 luteum. (After Scliion.) 



In the tt-malc, as in the male, secondary srxual characters are dependent 

 on hormones emanating from the active ovaries. If the ovaries are 

 removed before puberty these secondary sexual characters will not appear 

 but they can be induced by grafting into som.e organ of the body (sav the 

 kidney) the ovary of a female of the same species. The ovary can also 

 through its hormones produce a number of significant changes in the body. 

 Thus the sexual cycle in the female, the growth of the mammary gland, 

 the regulation of the amount of fat in the l)ody, the changes in the uterus 

 during pregnancy, and even the continued existence of the uterus itself 

 are due to chem.ical messengers arising from this important organ. 



The ova in a mamma 1 are microscopic in size, being al)OUt ^l ^- inch 

 in man, whereas in the bird the ovum is enormously swollen bv a supplv 

 of food for the embrvo called the volk. 



