362 THE VERTEBRATE BODY 



The gonads are glands that have already been studied under the re- 

 productive system in the role of producers of gametes, sperms, and eggs. 

 Like the pancreas, however, they are dual in their function, secreting 

 hormones in addition to reproductive cells. Since these hormones differ 

 according to sex we will study them separately. 



In the male the testes secrete the hormone testosterone that is re- 

 sponsible for the development of the characteristics that we think of as 

 masculine. If a boy is castrated before puberty the voice will remain at 

 a high pitch, the beard will not grow normally, the muscles do not be- 

 come masculine, and he will not develop the aggressive personality, nor 

 the attraction toward women, characteristic of normal men. 



Hypofunction of this gland will produce undersexed individuals that 

 may be benefited by hormone treatment. 



In the female the ovaries secrete two hormones. The first of these, 

 estrogen, is produced in the follicles containing the maturing eggs. This 

 is the female hormone that seems to be responsible for the development 

 of feminine characteristics. In addition, estrogen plays an important 

 part in the reproductive cycle of a woman. It is produced in greatest 

 quantities in the maturing follicle before ovulation and this prepares the 

 walls of the uterus for the possible reception of the fertilized egg. After 

 ovulation the ruptured follicle changes its appearance and becomes a 

 small yellow body on the ovary. This yellow body is called the corpus 

 luteum and secretes the second female hormone, progesterone. This 

 hormone seems to effect the final uterine changes necessary for gestation, 

 corpus luteum will remain and continue to secrete its hormone which 

 maintains the uterus in the normal condition for the development of the 

 embryo. It also stimulates enlargement of the breasts during preg- 

 nancy. In case there is no fertilization, the corpus luteum will disin- 

 tegrate and, in the absence of its hormone, the uterine wall will break 

 down, releasing cells and blood in the process of menstruation. Then, 

 as estrogen production increases from another follicle, the uterus again 

 is prepared for reception of a fertilized egg and the cycle is repeated. 



Progesterone has great medical value. It may be injected to prevent 

 threatened abortions, since one of its functions is to keep the uterus in 

 condition to retain the embryo. It may also help to relieve the nausea 

 of early pregnancy. 



The pituitary gland consists of three lobes which have different 

 embryonic backgrounds and secrete different hormones. They are 

 known as the anterior, intermediate, and posterior lobes. The inter- 

 mediate lobe secretes a hormone, intermedin, which exerts an influence 

 on the pigment cells, chromatophores, of fishes, amphibians, and reptiles. 

 The color changes which many of these animals make in response to 



