880 REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



The, Corpus Luteum. Special interest attaches to the function 

 of the corpus luteum which is formed in the Graafian follicle after 

 the exit of the ovum. We have already seen that this structure 

 attains a great size if pregnancy ensues. The luteal cells are formed 

 from the lining cells of the follicle and not from the interstitial cells 

 just alluded to. It has been generally assumed that the corpus luteum 

 forms a hormone or hormones, and the functions which have been 

 assigned to these chemical messengers are four in number. 



1. To affect the uterus and bring about there the cestrous or 

 menstrual cycle. This view may be dismissed at once ; the hypothesis 

 is negatived, first, because it does not explain the occurrence of the 

 first oestrous cycle in the young animal, and secondly, because 

 injection of luteal extracts does not induce uterine changes. The 

 cells concerned in that function are, as already stated, the interstitial 

 ovarian cells. 



2. To inhibit the ripening of Graafian follicles during pregnancy 

 (Loeb). This may be an effect, but it is hardly a function. 



3. To assist in some way the fixation and early development of 

 the impregnated ovum in the uterus. This view is much favoured 

 by most authorities, and the principal evidence in its favour is that 

 removal of the ovaries, or even destruction of the corpus luteum 

 shortly after pregnancy has begun, brings that process to an end ; 

 the same operation later in pregnancy has no effect upon the 

 development of the foetus or the subsequent act of parturition. 



4. To stimulate the growth of the mammary glands during the 

 early months of pregnancy. Prolonged search for the secretory 

 nerves of the milk glands have failed to reveal their presence. 

 Cases have occurred in which severe injuries to the nervous 

 system preclude any possible nervous connection between the 

 generative organs and the mammary glands; nevertheless, the 

 latter grow and ultimately produce milk. Such considerations 

 point to the conclusion that the correlation must be chemical 

 rather than nervous. A few years ago Starling and Miss 

 Lane Claypon thought that the hormone in question must be 

 secreted by the foetus, because the injection of foetal extracts in 

 rabbits led to a hypertrophy of the mammary tissue. The hyper- 

 trophy, however, is imperfect, and doubt was felt in many quarters 

 whether this could be the whole explanation, because mammary 

 evolution occurs in many virgin animals in which ovulation is not 

 followed by pregnancy ; moreover, in many cases enlargement of the 

 breasts occurs at each menstrual epoch. The view is now advanced 

 that a hormone is secreted by the corpus luteum, which is an inciting 

 cause of the early growth of mammary tissue. The close corre- 

 spondence between the stages in the development of the corpus 

 luteum and of the mammary glands has been clearly brought out 



