666 



MAMMALIA. 



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. 198, p. 467.) 



When mature, the ovum protrudes on the surface of the 



ovary, and is liberated by the 

 bursting of the Graafian fol- 

 licle. Some blood, which fills 

 up the empty follicle, degener- 

 ates 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 spermatozoon. 



No one has succeeded in 

 satisfactorily observing an ex- 

 trusion of polar bodies in the 

 maturation of the mammalian 

 ovum, but analogous processes 

 occur at an early stage. 



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. 



Fig. 290. — Segmentation of rabbit's 

 ovum. — After Van Beneden. 



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

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

 dermic vesicle. 



