§ 4.232 MESODERMAL ENDOCRINE GLANDS OF VERTEBRATA 151 



adenohypophysis. Incidentally, LH was for many years confused 

 with ICSH ; but there is now good reason to beheve that they are 

 distinct and that while ICSH occurs in both sexes, LH is normally 

 present in the female only. Its occurrence throughout life can he 

 completely inhibited if a testis, either natural or implanted, is 

 present during the development of an animal of either sex (Witschi 

 1955). 



The action of LSH, by inducing the secretion of progesterone, 

 initiates the alternate, or dioestrus, phase of the female cycle, a 

 phase which makes pregnancy possible and for which there is no 

 hormonal counterpart in the male. 



This may not have been the primary function of progesterone, 

 for it occurs in association with viviparous development in many 

 vertebrates besides mammals ; it is probably under the same endo- 

 crinokinetic control throughout (Matthews, 1955). 



Interaction of gonadial and endocrinokinetic hormones in repro- 

 duction 



In the sexual reproduction of all vertebrates there is need for 

 the gametes to ripen simultaneously in males and females of the 

 same species, and for the young of most land forms to be pro- 

 duced at a suitable season of the year for their early growth. The 

 endocrinokinetic hormones play a vital role in bringing this 

 about. 



Mammalia. In placental mammals the viviparous development 

 of the young embryo in the maternal uterus and its subsequent 

 nourishment from the mammary glands increase the complexity 

 of the situation, so that several more hormones are needed to 

 control reproduction in the female than in the male. 



In the male, the secretion of testosterone is induced by ICSH, 

 and maintains the reproductive ducts and the accessory organs in 

 a steady, active state, as well as developing secondary sexual 

 characters, such as antlers. At the same time the morphogenetic 

 hormone, FSH, ensures the continuous ripening of sperm. In 

 species with a limited breeding season, like deer, the onset and 

 ending of these hormone-controlled changes appears to be related 

 by the brain to environmental factors, such as temperature and 

 length of daylight. 



