326 AN INTRODUCTION TO ZOOLOGY 



for in addition to their primary function, that of producing the 

 germ cells, they also secrete substances which play important parts 

 in the development and activity of other parts of the body. Among 

 other things they affect the development of what are termed the 

 secondary sexual characters, that is to say, structures which, while 

 not actually of value in reproduction, serve as distinguishing marks 

 of the two sexes. These are very common in birds where the male 

 is often of quite a different plumage from the female. As an example 

 of the action of the gonads we may consider the case of horns in 

 certain mammals. If the testes be removed from the young animal 

 it does not grow horns even when of full size. Similar instances of 

 other structures behaving in this way could be cited, but this one is 

 sufficient to indicate the mode of action of the gonads as ductless 

 glands. 



