§ 4.232 MESODERMAL ENDOCRINE GLANDS OF VHRTK BRA TA 149 



Other hormones from the same source which have marked 

 morphogenetic effects upon the growth of their target organs. 

 Only those causing secretion of endocrine glands need be treated 

 in the present section; ahhough all the gonadotrophic hormones 

 play an important role in relating breeding cycles to seasonal 

 stimuli from the environment (§ 4.232 and Part II, § 4). The 

 gonadial hormones themselves are mainly morphogenetic in action 

 (§ 1.53); but they are peculiar in having some subsidiary kinetic 

 effects (§§ 3.12, 4.12 and 4.324). In their latter capacity they pro- 

 vide the only examples of kinetic hormones which are controlled 

 by endocrinokinetic hormones. 



Information regarding the gonadotrophic hormones of the cold- 

 blooded vertebrates is scanty, compared with that concerning 

 birds and mammals; but in general they appear to have similar 

 effects upon the secretion of the gonadial hormones (Pickford and 

 Atz, 1957). Further reference will be made to them in relation to 

 the latter (Part II, § 4). 



Interstitial cells in the testis stimulated by ICSH 



In birds and mammals, and probably in most other vertebrates, 

 the release of testosterone, or other androgenic hormones, from 

 the testis is brought about by an endocrinokinetic interstitial- 

 cell-stimulating HORMONE, ICSH, secreted from the adeno- 

 hypophysis. The control of its secretion, like that of TSH, comes 

 from the hypothalamus of the brain and can reflect the effects of 

 environmental changes, transmitted to the brain either directly or 

 by the eyes. The secretion of ICSH, by its release of testosterone, 

 fires off the whole chain of events associated with the breeding 

 cycle in the male. In birds these can include migration, courtship 

 and nest-building and the appearance of secondary sexual 

 characters of plumage and wattles, as well as the essential develop- 

 ment of the genital ducts. In mammals corresponding changes in 

 behaviour and structure are brought about by similar hormones; 

 in those with a limited breeding season there is a slow feed-back 

 mechanism, whereby the accumulation of testosterone inhibits 

 further ICSH secretion after a few months. In those species with 

 continuous breeding this inhibition is not effective, at least until 

 extreme old age. 



