DEVELOPMENT AND PLACENTATION. 701 



In the middle of each clump or Graafian follicle, a cavity 

 is formed containing fluid, and into this cavity the follicle 

 cells immediately surrounding the ovum project, forming 

 what is called the discus 

 proligerus (see Fig. 237, 

 p. 494). 



When mature, the ovum 

 protrudes on the surface of 

 the ovary, and is liberated by 

 the bursting of the Graafian 

 follicle. An ingrowth of epi- 

 thelial cells surrounding the 

 follicle develops into what is 

 called the corpus luteum. 



The spermatozoa are formed 

 from germinal epithelium in 

 the testes. The primitive male 

 cells or spermatogonia give rise 

 by division to daughter cells 

 or spermatocytes, which, with 

 or without further division, 

 form spermatozoa. 



The homologue of the 

 ovum is the spermatogonium 

 or mother sperm cell, but 

 the physiological equivalent 

 of the ovum is the spermato- 

 zoon. 



The ovum, having burst 

 from the ovary, is immediately 

 caught by the fimbriated mouth 

 of the Fallopian tube, and 

 begins to pass down the ovi- 

 duct. There it is met by 

 ascending spermatozoa, re- 

 ceived by the female as the 

 result of sexual union, and 

 is fertilised. One of the 



spermatozoa enters the ovum, and sperm nucleus 

 unites with ovum nucleus in an intimate and orderly 

 manner. 



FlG. 344. Segmentation of rabbit's 

 ovum. After Van Beneden. 



e.c., External cells (epiblast?); i.e., in- 

 ternal cells (hypoblast ?) ; b.v., blasto- 

 dermic vesicle. 



