240 The Vertebrate Organ Systems 



amphibians, and fishes. The symptoms are more severe in carnivorous 

 vertebrates than in the herbivorous ones. Perhaps this is related to 

 the fact that carnivorous animals eat their food so rapidly that they 

 are more dependent upon insulin for proper storage than are the her- 

 bivorous forms which usually eat more slowly, 



THE GONADS 



The differences between the two sexes are not always quite so 

 distinct as popular legend would have us believe. Embryologically, the 

 organism has the potentiality of developing into either sex. As develop- 

 ment proceeds, the various hormones influence and control the differen- 

 tiation of the organs and accessory structures typical of one sex or the 

 other. Sexual differences are usually considered to be very evident 

 among the vertebrates, but can be demonstrated even in the protozoans. 

 Sex is best considered to be a matter of one set of characteristics being 

 more evident than a contrasting set. Among human beings, such dif- 

 ferences as hair distribution, voice, general body build, and behavior 

 are considered to be contrasting characters between the two sexes. 



These are considered to be secondary sexual characteristics in 

 contrast to the primary one, which is the production of either sperms 

 or eggs by the individual. The secondary sexual characteristics, how- 

 ever, are dependent upon the elaboration of special hormones by 

 the gonads. 



THE MALE GONAD 



In most mammals, the paired male gonads, the testes, are located 

 in the scrotum which is divided by a median septum into right and 

 left halves. The main portion of each testis is divided into lobules 

 by numerous septa. Within each lobule are numerous convoluted 

 canals, the seininiferous tubules (Fig. 76). Between these tubules, 

 the area is filled in with nerves, connective tissues, blood vessels, and 

 special interstitial cells. The sperm are produced by the cells lining 

 the walls of the seminiferous tubules ; the hormones are elaborated 

 by the interstitial cells. 



The Function of the Interstitial Cells. — The sex hormone pro- 

 duced by these interstitial cells has influence not only over the develop- 

 ment of typical male secondary sexual characters, but also influences 

 behavior. 



