8i8 



PHYLUM CHORDATA : CLASS MAMMALIA 



When mature, the ovum protrudes on the surface of 

 the ovary, and is hberated by the bursting of the Graafian 



folUcle. Ovulation may occur 

 spontaneously — as in man, 

 monkeys, horse, cattle, pig, 

 dogs ; or after sexual union — 

 as in rabbit, guinea-pig, mouse, 

 and cat. An ingrowth of epi- 

 thelial cells surrounding the 

 foUicle develops into a gland- 

 ular body called the corpus 

 luteum. Its secretion is be- 

 lieved to be very important — 

 influencing the preparation of 

 the uterus, the early nutrition 

 of the embryo, and the multi- 

 plication of cells in the milk 

 glands. It seems that the ovary, 

 besides producing ova, is a 

 gland whose internal secretion, 

 passing into the blood, induces, 

 directly or indirectly, the phe- 

 nomena of heat and menstrua- 

 tion, and influences the uterus 

 during pregnancy. 



The spermatozoa are formed 

 from germinal epithelium in 

 the testes. The primitive 

 male cells or spermatogonia 

 give rise by division to 

 daughter cells or spermato- 

 cytes, which, with or without 

 further division, form sperma- 

 tozoa. 



The homologue of the 

 ovum is the spermatogonium 

 or mother sperm cell, but 

 the physiological equivalent of 



the ovum is the spermatozoon. 



The ovum, having burst from the ovary, is immediately 



caught by the fimbriated mouth of the Fallopian tube, and 



Fig. 491. — Segmentation of 

 rabbit's ovum. — After Van 

 Beneden. 



ex., External cells (ectoderm or 

 epiblast); i.e., internal cells 

 (endoderni or hypoblast) ; b.v., 

 blastodermic vesicle. 



