INTERNAL SECRETIONS ' 113 



generally possible to explain the results as being in part due to 

 suggestion, and it is noteworthy that the benefit obtained is rarely, 

 if ever, maintained for any prolonged period. Nevertheless, 

 it must be admitted that there is now a considerable accumulation 

 of evidence that both vasectomy and testicular transplantation 

 may be followed by general improvement, and this may be 

 interpreted as a rejuvenation. 



General Conclusions 



The testis is an organ producing an internal secretion which 

 is formed throughout the whole reproductive period of Hfe, and 

 probably earlier, though to a much less extent, and possibly 

 even in the embryo. In those animals which experience rut it is 

 at this season that the testicular hormone is produced in greatest 

 abundance. The periodic development of the prostate and 

 other accessory glands as well as the secondary sexual characters, 

 not to mention the testes themselves, is convincing evidence 

 that this is so. The hormone is apparently produced by the 

 interstitial cells (cells of Leydig), at least in mammals ; 

 in birds and in the lower vertebrates the evidence is much 

 less clear. 



The ovary is also an organ producing internal secretions 

 which vary in character and amount at different stages of the 

 oestrous cycle. There is some evidence (obtained chiefly from 

 transplantation experiments) that the interstitial cells are respon- 

 sible for the maintenance of the normal nutrition of the uterus 

 (which lapses after ovariotomy) and the secondary sexual 

 characters. The follicular epitheUal cells (more especially those 

 of the mature follicle) probably secrete a hormone which brings 

 about the phenomena of menstruation and heat. The part 

 played by the corpus luteum is definitely known. It is respon- 

 sible for the growth and development of the uterus and mammary 

 glands during pregnancy and pseudo-pregnancy (a condition 

 which occurs in monoestrous animals if ovulation is not followed 

 by fertilisation, in the rabbit under certain experimental con- 

 ditions, and in an abbreviated form in poly oestrous animals). 

 Moreover, there is some evidence that as long as the corpus 

 luteum dominates the ovarian metabolism, the normal ovarian 



