:59« THE MAINTENANCE OF THE INDIVIDUAL 



after puberty, it undergoes involution and finally disappears, its 

 place being taken by fibrous tissue. While there is some evidence 

 that extirpation of the pineal gland accelerates the development of 

 the sexual organs of the male, as found in experiments on the guinea 

 pig, its functioning is still a moot question. 



While the thymus has long been a subject of controversy, it now 

 appears likely to many students that this gland will not prove to 

 belong to the endocrine group. 



Reproductive Organs as Regulators 



It has been known for many years that the gonads are structures 

 in which are produced the eggs and spermatozoa that are essential 

 to reproduction in most forms of life. However, scientists have 

 learned within comparatively recent years that the reproductive or- 

 gans function also as ductless glands, producing hormones associated 

 with the development of those features known as secondary sexual 

 characters. One group of these hormones, though partially under the 

 control of that "generalissimo" of the endocrines, the pituitary, is 

 really responsible for the normal cyclical functioning of the sex 

 glands. Besides producing eggs and sperm, the ovaries and testes 

 play a vital part in the development of those mental and physical 

 characteristics which constitute maleness or femaleness. The exist- 

 ence of some regulatory mechanism has been clearly demonstrated 

 in various animals by the removal of the sex glands and the subsequent 

 failure of certain secondary sexual characteristics to develop. Nu- 

 merous examples might be cited. Male deer (Cervidae), for instance, 

 are typically adorned with antlers that are annually renewed. A 

 young castrated buck fails to grow antlers, thus suggesting that the 

 key to this phenomenon lies in the production of some secretion of 

 the testes. 



Many other experiments, performed in recent years upon the lower 

 vertebrates, tend to support the idea that such secretions are indis- 

 pensable to the proper development of many male and female char- 

 acteristics. When emasculated male rats or guinea pigs are given 

 ovarian transplants, the skeleton and hair soon begin to resemble 

 those of a female and before long the mammary glands enlarge to 

 functional size. These results suggest that the effect is due to some- 

 thing secreted in, or by, certain cells of the transplanted gonad. 



Other experiments indicate that the hormones of one sex dominate 

 expression of the other sex. Such a case is that of the "free-martin," 



