38 THE RIDDLE OF MIGRATION 



some of these glands has been successfully isolated 

 and it has been shown that in case of certain diseases 

 due to the cessation of production of one of these 

 vital substances, the deficiency can be made up by 

 administration of extract. Thus the condition 

 known as diabetes, due in the first place to degener- 

 ation of the islet cells of the pancreas which results 

 in the disturbance of the sugar metabolism of the 

 body, can be alleviated by use of the extract known 

 as insulin. It matters nothing if the preparation 

 is made from the pancreas of hog or steer, it is 

 equally efiicacious in cats or men. The active 

 principle appears to be the same. 



The true endocrine organs include thyroid, para- 

 thyroid, suprarenal, pituitary, pineal, pancreas and 

 gonad. It is to the last that we shall confine our 

 attention. Like the pancreas, the reproductive 

 organs have a double function. They not only 

 produce the actual germ cells — ova and spermatozoa 

 — but also internal secretions liberated directly into 

 the bloodstream. In order to understand the situa- 

 tion it is necessary to acquaint ourselves with the 

 structure of the organs. They differ in the two 

 sexes, those of the male being termed the testes, 

 those of the female, the ovaries. Both are usually 

 paired structures, but in birds the right ovary dis- 

 appears during development although it may occa- 



