§ 3 



OVO-FOLLICULAR SYSTEM 



95 



The developing ovum and the follicle form, in all ver- 

 tebrates, a topographical and functional whole. In all ver- 

 tebrates the follicular wall consists of the same tissues — the 

 granulosa and the theca. Only the placental mammals 

 possess ova poor in yolk and with an antrum in their follicle 

 (folliculus cavus), whilst all other vertebrates, together with 

 a-placental mammals, possess ova rich in yolk, and a follicle 

 without a cavity (folliculus solidus) (Fig. 69). 



Fig. 69. Showing the difference between the folliculus solidus and the 

 folliculus cavus. 



The follicle has the double function of nourishing the 

 growing egg and producing a glandular system which will 

 serve for the subsequent care of the mature ovum. This ovo- 

 follicular system is labile, so that the action of a single factor, 

 generally from outside through stimuli from the anterior 

 pituitary, causes profound modifications in the system. Its 

 response to gonadotrophic pituitary stimuli is quite a different 

 one from that of other glandular systems, whereas other res- 

 ponding organs such as the thyroid, the pancreas, the adrenal, 

 etc. already exist in the form of completely developed func- 

 tioning organs, being merely either excited, directed or 

 checked by hormonal stimuli. It is quite different in the case 

 of the glandular follicle of the ovary. Here, a gonadotrophic 



