352 THE PHYSIOLOGY OF REPRODUCTION 



does not supervene, the luteal cells begin to degenerate at a 

 much earlier period and without attaining their full development. 

 The pronounced hypertrophy of the follicular epithelial and 

 interstitial cells, which takes place at the beginning of pregnancy, 

 is directly correlated with a nearly simultaneous hypertrophy 

 on the part of the uterine wall. The corpus luteum, therefore, 

 is to be regarded as an essential factor in maintaining the raised 

 nutrition of the uterus during the earlier stages of the period of 

 gestation. When the later part of this period is reached, the 

 ovarian secretion has probably been already formed in sufficient 

 quantity to prevent the uterus from lapsing into the normal 

 condition until the end of pregnancy. It is to be noted, how- 

 ever, that fibrous degeneration has been described in the 

 maternal placenta in the later stages of its existence. 



Thus the ovaries pass through a series of cyclical changes 

 which are directly correlated with those undergone by the 

 uterus. Moreover, the uterus atrophies after ovariotomy. 



It seems probable that this close co-ordination between the 

 ovarian and uterine functions arose very gradually in evolu- 

 tionary history, and it may be that in the aplacental mammals 

 we have in existence at the present day an intermediate stage 

 in the development of this relation. Starling 1 has suggested 

 that the internal secretions, or hormones generally, arose at first 

 as products of ordinary metabolic activity in certain particular 

 tissues, and that the evolution of the various cases of chemical 

 correlation between different organs in 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 



1 Starling, " The Chemical Co-ordination of the Activities of the Body," 

 Science Progress, vol. i. (April) 1907. 



