448 



COMPARATIVE ANATOMY 



proper medical treatment. Today, however, as a result of the discovery 

 of insulin and the part it plays in life, thousands of people who a generation 

 ago were destined to prolonged suffering and premature death are able to 

 live a fairly normal life. 



Pancreatic islands occur in all classes of vertebrates, usually in con- 

 nexion with the pancreas, though in some bony fishes the two glands are 



Blood vessel 



Al\ eoli^ 



Fig. 374. — An island of the pancreas with the stirrounding alveoli, from an adult. 

 X400. (From Bremer's "Text Book of Histology.") 



independent. Both arise from the endoderm, but there is no evidence 

 that pancreatic gland cells are ever converted into cells of the blood-islands. 



THE GONADS 



Male Sex Glands. The structure and development of the male gonads 

 have already been described in detail. We are now concerned with their 

 endocrinal function. It has long been known that removal of the gonads 

 in childhood prevents the appearance of secondary sex characters of the 

 male, such as beard, deeper voice, broadened shoulders, etc., the castrated 

 individual becoming a eunuch and lacking the virile traits of the normal 

 male. Evidently, then, the appearance of those traits which distinguish 

 male from female depends upon some physiological action of the testes. 

 That this is an endocrinal effect of the interstitial Leydig cells of the 

 testes and not of the seminiferous tubules which produce the spermatozoa, 

 may be inferred from the fact that male traits develop in the absence of 

 the seminiferous tubules, provided only that the interstitial tissue is 



