THE ENDOCRINE SYSTEM 217 



females, such as the comb, wattles, and spurs in the cock, or 

 beard, voice, body size, and strength in a man. Removal of the 

 gonads in young animals prevents or alters the full development 

 of such characters in the adult. There is, however, no general 

 rule for predicting the results of gonadectomy. Removing the 

 testis of a young Brown Leghorn cock interferes with the com- 

 plete development of the male type of plumage and of the head 

 furnishings. Otherwise such birds, known as "capons," are 

 large in size, have spurs, and look like cocks, though they lack the 

 pugnacity and temperament of the normal male bird. The result 

 of removing the ovary of the Brown Leghorn early in life is com- 

 plicated by conditions peculiar to birds. In birds only the left 

 ovary is functional, the right gonad being very rudimentary. 

 If the functional ovary of the Brown Leghorn is completely 

 removed, in a certain percentage of cases the rudimentary right 

 gonad is changed into a testis, with the result that the spayed 

 bird later develops all the pugnacious qualities and sexual 

 instincts of a normal male. Such birds, however, retain their 

 smaller size and develop a male plumage that is succeeded by a 

 permanent typical hen plumage. The female in this case has the 

 potentiality of both maleness and femaleness, the outcome 

 in the normal cases being determined by the development of the 

 left gonad and the suppression of the right. 



In mammals extracts have been made from various parts of the 

 ovary and their properties studied. Theelin has been made from 

 the follicles in which the eggs develop. After an egg is dis- 

 charged from a follicle, the cavity becomes filled with cells, 

 forming what is called a corpus luteum, from which another 

 hormone, progestin, has been extracted. Theelin has also been 

 obtained during pregnancy from the placenta. It is also present 

 in the blood of the female from puberty on. When injected into 

 immature female rats, it causes the uterus and vagina to become 

 mature. Theelin partially stimulates the proliferation of the 

 uterine mucosa in anticipation of the implanting of the fertilized 

 egg; and progestin holds the functioning of the ovary in abey- 

 ance until the birth of the young and also augments the action of 

 theelin on the uterus. The production of both theelin and 

 progestin is controlled by the gonadotropic hormone of the 

 anterior pituitary. The latter is necessary for the normal 

 growth of the follicles and the production of corpora lutea. 



