THE PERIODICITY OF (ESTRUS 151 



that only one process can proceed at a time. Hammond (264) 

 has tentatively accepted this hypothesis to explain the 

 maturation of immature ovaries grafted into the mature animal, 

 and the mechanism of compensatory hypertrophy of the ovary. 

 On this view, the hypothetical substance in question, to which 

 the name 'generative ferment' was given, is used in the young 

 animal for growth, none being available for the reproductive 

 processes, which are thus in abeyance. When body growth 

 stops, however, the substance can be utilized by the repro- 

 ductive organs, and their activity begins. Similarly, the sub- 

 stance may be regarded as essential for follicular maturation, 

 and the amount present at any one time is only sufficient to 

 ripen a certain number of follicles. When one ovary is removed 

 the entire supply is available for the use of the other, which is 

 therefore able to mature twice the ordinary number of follicles. 

 A similar view has been adopted by Lipschlitz (369) and others, 

 who, how^ever, have used the more reasonable term ' X-sub- 

 stances' to denote the hypothetical factor. An extension of this 

 hypothesis has been put forward to explain the absence of 

 ovarian activity during pregnancy, the use of the substance for 

 uterine and foetal growth preventing its use by the ovary. 

 Though purely speculative, this theory of the somatic control of 

 the ovary appears to have received some vindication from recent 

 work on the effect of anterior pituitary substances on the ovary. 



