388 THE PHYSIOLOGY OF REPRODUCTION 



the body came into existence, not by the production on the part of 

 certain tissues of special substances acting as chemical messengers, 

 but by the acquisition of a specific sensibility on the part of other 

 functionally related tissues. It is no doubt possible that the chemical 

 co-ordination of the ovarian and uterine activities arose partly in this 

 way ; but, on the other hand, the definite character of certain of the 

 cyclical changes which take place in the ovary, and particularly those 

 which relate to the formation of the corpus luteum, points to the 

 conclusion that the secretory function of the ovary has been perfected, 

 or at any rate has undergone great development in the phylogenetic 

 history of the Mammalia, though it no doubt existed previously in 

 a minor degree. 



As to whether the ovary elaborates more than one specific 

 substance acting as a chemical excitant, there is some evidence, 1 

 and the composition of the corpus luteum (which is different from 

 that of the rest of the ovary) indicates that the formation of this 

 structure is accompanied by a change in the nature of the ovarian 

 secretion. Lastly, it is possible that the influence of the ovary upon 

 the metabolism is due partly to this organ being excretory as well 

 as secretory in function, but there is no evidence that this is actually 

 the case. 



The fact that the testis is an organ of internal secretion seems 

 also to be definitely proved. This secretion is probably formed 

 throughout the entire reproductive period of an animal's life; but, 

 in those animals which experience a periodic rut, it is no doubt at 

 this season that the testicular hormone is produced in greatest 

 abundance. The development of the prostate and the secondary 

 sexual characters, not to mention the growth of the testes them- 

 selves, is convincing evidence that this is so. The hormone is 

 apparently produced by the interstitial cells (at least in Mammals). 



THE INFLUENCE OF THE REPRODUCTIVE ORGANS AND THE EFFECTS 

 OF CASTRATION UPON THE GENERAL METABOLISM 



In view of the facts referred' to above it is almost self-evident 

 that castration must exercise some influence upon the general 

 metabolism of the body, since it produces such marked effects upon 

 the primary and secondary sexual characters. Moreover, it is 

 commonly believed that the removal of the reproductive glands 

 causes a tendency towards obesity both in man and animals, but 

 it is not quite clear whether this occurs as a direct or an indirect 

 consequence of castration. However this may be it seems certain 

 that sows which have been spayed fatten faster and thrive better 



1 See below, p. 621, Chapter XIII. 



