SECONDARY SEXUAL CHARACTERS 309 



but aside from these characters her modification is usually 

 less extreme than in the male. 



In certain cases these secondary sexual characters are 

 of evident use. Horns are of service to the male in de- 

 fending the female or in battling for a mate. The brood 

 pouches of many females, such as the opossum and kanga- 

 roo, serve to carry the young. Organs of scent, phosphor- 

 escent organs, and sound producing structures as in the 

 katydid are believed to attract members of the opposite 

 sex. But there are very many other characters of this 

 class, such as the brilliant plumage of male birds and 

 the colors of many insects, as well as spines, knobs, 

 combs, wattles, etc., whose significance at the present 

 time is an unsolved problem. 



Body and Germ Cells. — As a result of the growth 

 and differentiation of a sexually developed individual two 

 classes of cells are produced. The first group comprises 

 the reproductive or germ cells, which are protected and 

 nourished by the second class, the body cells. This last 

 named aggregate, forming by far the greater bulk of the 

 individual, performs many duties, including food gather- 

 ing, avoidance of enemies, and various activities in con- 

 nection with the propagation and nourishment of the 

 young. The germ cells, on the contrary, take no part in 

 these purely individual processes, and are solely concerned 

 with the perpetuation of the race. 



Furthermore, it is important to note, both of these 

 cell classes are equally the descendants of an original 

 fertilized egg. The hen, by which is meant the cells of 

 the body, does not produce eggs; it merely lays egg cells 

 that during the course of development have become dif- 

 ferentiated and set aside much as muscle or bone cells 

 are differentiated and grouped into tissues. Each egg 

 cell, when fertilized, produces in due time other eggs as 

 well as the protecting body, and while the body dies 

 after each period of sexual maturity, some germ cells 

 persist from generation to generation. 



