204 ESSENTIALS OF ZOOLOGY 



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 

 characteristics. The earlier concept that the male sex produces 

 ''male" hormones exclusively and the female produces only ''fe- 

 male" hormones is no longer held. For example, extracts have been 

 prepared from the urine of women as well as from men which on 

 injection into capons caused growth of the comb which ordinarily 

 fails to develop as it would in the cock. The sex hormone sub- 

 stances affecting the male are spoken of as androgenic and those 

 affecting the female as esti^ogenic. The important sex hormones are 

 androsterone, testosterone, theelin, and prog ester 07ie (progestin). The 

 first two are male hormones, and the others are female. 



Androsterone is found in male urine and can be crystallized from 

 it. It has a stimulating effect on development of secondary sex char- 

 acteristics and a definite regenerative effect on accessory organs 

 (seminal vesicles, prostate glands, and penis) of castrated male ani- 

 mals. Testosterone is produced in the interstitial tissue of the testis 

 but is absent from the urine. This hormone is several times as 



