THE TESTICLE AND THE OVARY 387 



the prooestrous processes. It is at these periods also, in typical cases, 

 that the follicles become mature. After ovulation, which occurs 

 during cestrus, the secretory cells of the ovary show still greater 

 activity, and become converted by a process of simple hypertrophy 

 into the luteal cells of the corpus luteum. If the ovum is fertilised, 

 these cells continue to increase in size until nearly mid-pregnancy 

 (or, in some animals, a somewhat earlier period), but later they 

 undergo degeneration. If pregnancy does not supervene, the luteal 

 cells in polyoestrous animals under normal conditions begin to 

 degenerate at a much earlier period and without attaining their full 

 development. In monoestrous animals the corpus luteum persists for 

 an approximately equal time under conditions of pregnancy or 

 pseudo-pregnancy. The pronounced hypertrophy of the follicular 

 epithelial and interstitial cells, which takes place at the beginning 

 of pregnancy, is directly correlated witli a nearly simultaneous 

 hypertrophy on the part of the uterus and of the mammary glands. 

 The corpus luteum, therefore, is to be regarded as an essential factor 

 in maintaining the raised nutrition of the uterus during a part at 

 least of the period of gestation ; it is also responsible for stimulating 

 the growth of the mammary tissue preparatory to the secretion of 

 milk. When the later part of gestation 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, however, that fibrous degenera- 

 tion 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 and 

 mammary glands. The ovarian changes are always the cause; the 

 uterine and mammary changes are the effects. Moreover, the uterus 

 atrophies after ovariotomy. Whether or not the corpus luteum 

 (which may persist for a time even after parturition) is a factor in 

 actual milk secretion is an open question, but the influence of luteal 

 extract on this process is suggestive of a connection. 



It seems probable that this close co-ordination between the 

 ovarian and uterine functions arose very gradually in evolutionary 

 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 l 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 



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

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



