HORMONES AND REPRODUCTION 67 



atretica. Thus the pre-ovulation corpus luteum may 

 exert its most important function in the lower verte- 

 brates, while the post-ovulation corpus luteum is most 

 important in mammals. Where exactly in this sequence 

 the production of oviductin ceases and that of progester- 

 one begins has not yet been investigated, and of course 

 the two may overlap. The chemical relationships of 

 these two substances are still unknown, but their 

 biological relationship appears to be close since both 

 seem to exert their main influence on the final stages of 

 oviduct development. 



A final word may be said regarding the fates of these 

 various substances within the body. Oestradiol, secreted 

 by the ovary or introduced by injection, quickly dis- 

 appears. Probably it is partly oxidized to the less active 

 oestrone and oestriol which are excreted by the kidneys.^* 

 Oestradiol itself is rarely found in the urine, ^^ and per- 

 haps the greater part of it is destroyed in the liver, as 

 also is testosterone. As regards progesterone little 

 appears to be known, but it is certainly destroyed quickly 

 and some is excreted in the urine in the relatively inert 

 form of pregnanediol. 



8. THE OESTROUS CYCLE 



This brief survey of the hormones affecting vertebrate 

 reproduction is now complete, but it is still necessary to 

 fit together the facts that have been given to indicate the 

 sequence of physiological events within the reproductive 

 cycles. The two main cycles to be considered are the 

 yearly seasonal cycle which is common to most verte- 

 brates and the shorter oestrous cycle which is peculiar 

 to the female mammal. It is proposed to consider this 

 oestrous cycle first. 



The typical oestrous cycle begins with the active 

 secretion of the gonadotropic hormones of the anterior 

 pituitary. This secretion commences either because the 

 animal has grown to maturity or because of the onset of 



