54 VERTEBRATE REPRODUCTIVE CYCLES 



organs of the former or of the comb and wattles of the 

 latter. There are many known substances which can 

 stimulate such growth, although several of them are syn- 

 thetic and are probably not found in nature. The actual 

 substance produced in the mammalian testis is thought 

 to be testosterone (Fig. 5). Other androgenic hormones 



OH 



o^^^V^^X/' 



Fig. 5. — The structure of a molecule 

 of testosterone. 



have a similar molecular structure, although curiously 



enough it is sometimes found that the isomer of some 



particularly potent substance is itself biologically in- 

 active. ^^ 



It has also been discovered that the normal testis 

 secretes female hormones, also known as ©estrogens. 

 The reason for this is still obscure, but evidently the 

 only difference between a male and a female in the 

 formation of these hormones is one of degree. A male 

 typically produces more androgenic and less oestro- 

 genic hormone, while a female does the reverse. It is 

 also important to remember that the effects of the male 

 and female hormones are organ specific rather than sex 

 specific, so that the chief distinction between a male and 

 a female lies not in their hormones but in their possession 

 of different tissues and- organs which are competent to 

 react to these hormones. ^^ In a male the hormone 



