44 ZOOLOGY 



in cooperation with certain of the body cells, forming the inner 

 sex glands, — testes and ovaries. 



4. Sexual Maturity and the Perfection of the Ova and Sperm. — 

 In this parallel development of the germ cells and the body cells 

 it is certain that they modify each other profoundly. The 

 germ cells depend for their supply of food and oxygen, etc., upon 

 the activities of the body cells. Undoubtedly the wastes of the 

 body cells also influence the germ cells. However, it seems that 

 the presence and products of the germ cells even more modify 

 the growth of the body. This is especially true as the germ cells 

 and glands approach maturity. In a number of species it has 

 been shown that the development of certain parts of the body 

 is very much changed if the maturing sex glands are removed 

 early in life. In general it is believed that the special {i. e., 

 secondary sexual) differences in the bodies and the instincts of 

 the males and females of a species are largely due to the effect 

 of the development of sperm and ova within the bodies. That 

 is to say, the sex qualities in the bodies and instincts are directly 

 produced by secretions (hormones) manufactured by the inner 

 sex organs (testes and ovaries). 



In the last steps whereby the spermatogonia produce sperm 

 and the oogonia produce eggs, there are two happenings very 

 important in development : (a) the union of the simple chromo- 

 somes of a nucleus, originally derived from the two parents, to 

 form the bivalent chromosomes; and (b) the separation of these 

 at the next division in such a way as to reduce the simple 

 chromosomes to one-half the number usually found in the cells 

 of the species (Figs. 12, 13). 



It will be recalled that the chromosomes found in any primor- 

 dial germ or body cell are descendants of chromosomes that came 

 in equal numbers from the egg and sperm nuclei when these first 

 united. We have seen that every cell gets its part of every one of 

 these chromosomes. It is believed, when these chromosomes 

 unite in pairs in the act of maturing, that they do not unite in a 

 chance manner, but that those pairing are corresponding chromo- 

 somes coming originally from the mother and father. By "cor- 

 responding" we mean in general the chromosomes that tend to 

 determine the same traits. In other words this union is the 



