THE EXCRETORY AND REPRODUCTIVE SYSTEMS. 179 



follicle is formed about them. This is lined with several 

 layers of epithelial cells forming the membrana granulosa. 

 At one side of the follicle the membrana granulosa 

 thickens and envelops the ovum, forming the discus 

 proligerus. The cavity of the follicle is filled with liquor 

 folliculi. When the ovum becomes almost mature, the 

 walls of the follicle have grown peripherad, so as to cause 

 a pin-head protuberance on the surface of the ovary 

 clearly visible to the naked eye. In a fresh specimen this 

 protuberance may be picked open and the ovum expelled 



9* 





Mp:'- f j* 



' :- * ' '" 



nii '- '/' 



^v _> r 



;. 



FIG. 86. SECTION OF OVARY. X 40. 



sr, Surface of the ovary; gr, Graafian follicle with mature ovum; o, ovum; 

 c, cavity of the follicle; un, undeveloped ova; strm, stroma or con- 

 nective elements of the ovary; m, membrana granulosa; d, discus 

 proligerus. 



on a slide for examination with the microscope. The 

 ovum is scarcely visible to the unaided eye. The human 

 ovum is still smaller. 



The ova of all mammals, except the Ornithodelphia 

 (Monotremata), are very small. When the ovum becomes 

 mature, the protruding wall of the follicle bursts, per- 

 mitting its contents to be received by the expanded end 

 of the Fallopian tube, whence it passes to the uterus. 

 If the male element, the spermatozoon, enters the ovum, 

 the latter is soon enveloped by a growth of mucous mem- 

 brane and retained in the uterus for development. Ripe 



