ENDOCRINE GLANDS AND THEIR FUNCTIONS 675 



frog results, among other things, in the bleaching of the animal 

 and the inability of the frog to alter the color scheme of its skin 

 to agree with the surroundings. When in a strong light or on a 

 light background the retinas of the frog's eyes are stimulated by 

 light rays, and some of the impulses reach the pituitary 's posterior 

 lobe, resulting in a suppression of its secretion, and consequently 

 a lightening of the frog's skin. When the light is decreased the 

 pituitary increases its secretion and the frog has a darker pigmenta- 

 tion. These reactions probably do not occur so directly in the higher 

 groups of vertebrates. Kemoval of the pituitary gland tends to 

 cause atrophy of the other endocrine glands. 



The Thymus Gland 



The thymus, a small glandular structure located in the chest be- 

 tween the upper part of the sternum and the pericardium, is a tem- 

 porary organ, which normally atrophies in human beings by the time 

 of the onset of puberty. When the gland is too active, a condition 

 is found in children in which an enlargement of the organ results, 

 and breathing is rendered difficult. No distinct hormone has been 

 obtained from this gland. 



It has been claimed recently that accruing acceleration in the rate 

 of growth and development occurs when successive generations of 

 rats are given daily injections of thymus extract. In third and 

 fourth generations, the rats at twelve days of age compared favor- 

 ably with controls of twenty days. Introduction of thymus extract 

 in young tadpoles causes them to grow rapidly to the size of the 

 adult frog but still retain their tadpole form and appearance. The 

 disappearance of the thymus at the time of puberty permits the 

 differentiation of mature animals and particularly the onset of 

 activity of the sex glands. Some workers claim that the thymus 

 contributes to the orderly and proper development of the bones of 

 the skeleton. 



The Gonads and Sex Hormones 



In addition to the usual function of producing germ cells for re- 

 production, the gonads produce hormones which influence the devel- 

 opment of secondary sexual characters and which have a regulatory 

 effect on the reproductive processes and activities. Sex differences 

 are caused in part by various hormones which have a selective 

 action on the male or on the female secondary and accessory sex 



