The Endocrine System 241 



Secondary sexual characters vary considerably in the different ani- 

 mal groups. Among human beings, the hairiness of the male, the dis- 

 tribution of body fat, the skeletal outlines, and the deeper voice, which 

 develop after puberty, all are easily recognized as essentially male 

 characters. In birds, feather color, the presence of crests, the pres- 

 ence of spurs, and a distinctive call are all possible male characters. 

 Among fish, the male is often more brightly colored than is the female. 

 Insects very often have conspicuous secondary sexual characteristics 

 such as distinctive wing colors, songs, and differential development of 

 various mouth parts. In all of the various groups, the male is likely 

 to be characterized by a more aggressive behavior. 



Except for a few animals such as man, the rabbit, mouse, rat, and 

 guinea pig, the mating season is not continuous ; rather it occurs at 

 several definite periods during the year. This again appears to be under 

 the control of the sex hormones of the interstitial cells. 



The Composition of the Male Hormone. — In recent years, sev- 

 eral similar materials have been isolated from the testes of various ani- 

 mals. All are of the same general type structure, but vary in detail 

 and in general effectiveness. All of them are steroids having a struc- 

 tural formula similar to that of the steroids of cortin. The most effec- 

 tive of these have been named testosterone. All these various steroids 

 which have certain similar effects are called androgenic hormones. 



The Effect of Hyposecretion. — If an animal is castrated (that is, 

 the testes are removed) before puberty, the secondary sexual characters 

 fail to develop. If testicular function ceases after puberty, there may 

 be diminution of the typical secondary sexual characters. 



Castration has long been practiced in livestock for improvement of 

 temperament and appearance of animals. It also is practiced to a 

 large extent by poultry raisers. 



The Effects of Hyperactivity. — Presumably such activity before 

 puberty would result in premature appearance of the secondary sexual 

 characters. It is difficult to assess the effects of this hyperactivity 

 after puberty. 



THE FEMALE GONAD 



Among mammals, the paired, flattened ovaries are located low 

 in the abdominal cavity along the dorsal peritoneum. Each ovary is 

 covered with a layer of germinal epithelium composed of cuboidal cells. 



