EVOLUTION OF NERVOUS SYSTEM 97 



glands enlarged and even gave milk, but what is perhaps more 

 remarkable was that its whole nervous constitution gradually 

 assumed the female role. It became rather timid and retreat- 

 ing, mothered the young of other females and in practically all 

 respects exhibited the traits of the female of its kind. Or- 

 ganically it displayed practically the full range of secondary 

 female characteristics though it was an animal destined by 

 nature to have been a male. 



Steinach also undertook the conversion of females into 

 males and though this from the operative standpoint proved 

 to be a more difficult problem, in that testes had to be repeat- 

 edly introduced, he nevertheless also succeeded in this en- 

 deavor and individuals were produced that though destined 

 by nature to be female had the physical and psychical traits 

 of males. Such individuals were aggressive and quarrelsome, 

 and attempted, though ineffectually of course, to copulate with 

 females. 



Thus the presence of a particular sex gland during adoles- 

 cence seems to be the necessary factor in determining the type 

 of secondary sexual characters that a given individual will 

 show. When the sex glands that had been transplanted in 

 these experiments and that had sojourned in their new habita- 

 tion for a considerable time were studied histologically, 

 they were found to be considerably modified. The sexual cells 

 proper had often largely disappeared and the cells that filled 

 the spaces between the groups of sex cells, the so-called inter- 

 stitial cells, had often greatly increased. It is these cells rather 

 than the sex cells which produce the hormones that determine 

 the secondary sexual characters, and the sex glands must, there- 

 fore, be looked upon as double in nature. They produce, first, 

 the sexual elements, the sperm cells or the egg cells, and 

 secondly, from another source, the hormones necessary for the 

 development of the secondary sexual characters. They are at 

 once sex glands and puberty glands. 



