THE GONADOTROPIC HORMONES 



with newly named (but not necessarily newly described) 

 hormones, so that its total would be five or more instead of 

 two. To recall only the example of the number of "corpus 

 luteum hormones" once thought to exist before the pure 

 substance had been isolated is to recognize the continued 

 need for caution in discussing the number of gonadotropic 

 hormones secreted by the pars glandularis. At present, there 

 is probably nothing that will further rational interpretation 

 and real advancement in this field as much as the isolation 

 of a gonadotropic hormone of the anterior pituitary in a 

 satisfactorily pure state. 



Much of the recent data requires no reconsideration here. 

 Descriptions of the effects of destruction or removal of the 

 pituitary body supplement former reports as to the atrophic 

 changes which dramatically follow in the ovaries, testes, and 

 secondary sexual organs. Other experiments have increased 

 our knowledge of the effects of pituitary tissue or extracts 

 on the sexual organs of many different animals. It is known 

 more accurately that even a short period of pituitary defi- 

 ciency may markedly lessen the sensitivity of the gonads 

 toward gonadotropic hormone. Also it is of interest that the 

 persistence of corpora lutea in hypophysectomized rats ap- 

 pears to be due to the lack of an anterior pituitary hormone. 



Physiological evidence of the nervous control of the secre- 

 tion of gonadotropic hormones has been greatly strengthened 

 in the past few years. In birds, and at least in some mam- 

 mals, photic stimuli may elicit the secretion of gonadotropic 

 hormones, which in turn stir the gonads into activity. Prob- 

 ably "light" reflexly stimulates the secretory nerves of the 

 pars glandularis by impulses arising in the optic nerves, 

 whence they are guided down fibers of the stalk by one or 

 more groups of neurons in the diencephalon. In animals 

 without precise seasonal variations in sexual, and hence pitui- 

 tary, activity photic stimuli do not play such an evident role. 

 However, diffuse or sometimes sharply localized stimula- 

 tion of the brain may be followed by the release of gonado- 



[121] 



