68 



VERTEBRATE REPRODUCTIVE CYCLES 



the breeding season, and it is thought that at this early 

 stage it is the folhcle-stimulating-hormone which is 

 produced in the greatest quantities. As a resuh a few of 

 the half-developed follicles surrounding the primary 

 oocytes are stimulated to rapid growth. This growth is 

 the result of two processes: first the secretion of liquid 

 into the follicular cavity, or antrum, so as to cause the 

 mechanical enlargement of the follicle in a manner 

 resembling the blowing up of a balloon; and second, the 



1 2 3 ^ 5 6 7 8 9 10 11 12 13 14 

 Days 



Fig. II. — Diagram illustrating the changes in follicle 

 size which occur during a typical oestrous cycle. The. 

 length of such a cycle may vary from about 5 days in 

 the case of the mouse to about 3 weeks in the case of 

 the pig and the horse. 



action of ©estradiol (contained in the follicular fluid) 

 which induces active cell division in the follicle wall. 

 For a time these two processes keep pace, but ultimately 

 the mechanical process of distension proceeds faster 

 than that of growth by cell division. At this point, in 

 some way not yet understood, ^^^ the anterior pituitary 

 hormones cause the follicle to burst to release the follicu- 

 lar fluid together with the egg, which at this time is 

 still a primary oocyte. 



