56 VERTEBRATE REPRODUCTIVE CYCLES 



the production of spermatozoa,'^^ and similarly in the 

 annually breeding ground squirrel it can cause the pre- 

 cocious development of spermatozoa in young animals 

 and their unseasonal development in adults.^^O' i»i 

 Among fish it has been used to cause the precocious 

 maturation of the testes of the millions fish,®' and their 

 unseasonal development in the minnow.^^' 



It has also been shown that quantities of F.S.H. 

 which are beneath the threshold required to promote 

 testis growth, produce a strong reaction if given together 

 with I.C.S-.H. or with testosterone. ^^ Ji ^igy therefore 

 be suggested that the influence of the anterior pituitary 

 is exercised partly by the direct action of F.S.H. and 

 partly by the indirect action of I.C.S.H. which causes 

 the production of male hormone. Thus in normal 

 circumstances spermatogenesis may be the result of the 

 action of the F.S.H. combined with the mitogenic 

 stimulus of the testosterone. 



Male hormone also has the action of checking the 

 secretion by the anterior pituitary of its gonadotropic 

 hormones, and in this way a balance is maintained 

 between the two glands. ^3 If a lack of testosterone 

 develops, the anterior pituitary is able to secrete maxi- 

 mum quantities of its gonadotropins. As a consequence 

 the testis production of testosterone rises, and as a 

 further consequence the secretion of gonadotropins 

 diminishes. One way in which this mechanism can be 

 demonstrated is by castration. When the testes are lost 

 the gonadotropic output of the anterior pituitary rises 

 unchecked to a maximum, and abnormally large 

 quantities of these hormones are excreted with the 

 urine where they can be assayed, ^s 



This introduces a final point, the question of the 

 fate of the testis hormones. These substances are not 

 stored, and are quickly inactivated by destruction in 

 the body and by excretion in the urine. It is now known 

 that the main site of destruction is in the liver,3o, 78 

 and that even the excreted fraction is first converted into 



