THE CONTINUITY OF LIFE 243 



resulting cells. Indeed, exactness of chromatin distribution ap- 

 pears to be the primary object of mitosis. 



The significance of the nicety of chromatin distribution lies 

 in the fact that not only are the various chromosomes qualitatively 

 different but also each chromosome is qualitatively different from 

 one end to the other, and these different parts of the chromosomes, 

 known as genes, are the determiners of characters which are 

 handed on from cell to cell. And since cell division is reproduction, 

 the chromosomes are the chief agents in the transmission of char- 

 acters from parent to offspring in inheritance. We shall consider 

 this important fact when we discuss heredity, but now we must 

 return to the origin of the gametes. 



2. Chromosomes of the Germ Cells 



It is clear that the spermatogonia and oogonia in the reproduc- 

 tive organs, together with all the cells forming the body proper, 

 are direct descendants by mitotic cell division from the fertilized 

 egg which gave rise to the individual organism. This, we have 

 just seen, is equally true of the chromosomes and, therefore, every 

 cell of the animal body has the same number of chromosomes as 

 the fertilized egg. Furthermore, since fertilization always consists 

 in the fusion of two gametes — a fusion of nucleus with nucleus 

 and cytoplasm with cytoplasm to form a zygote — one of two 

 things must happen. Either the zygote, which is one cell recon- 

 structed from two, must have double the chromosome number, 

 that is, a set contributed by both egg and sperm; or some method 

 must exist by which the chromosomes of the gametes are reduced 

 in number to one-half that characteristic of the somatic cells. As 

 a matter of fact, a reduction of the number of chromosomes always 

 does take place in animals during the final stages in the develop- 

 ment, or maturation, of the gametes. 



The maturation or 'ripening' of the germ cells of animals in- 

 volves two cell divisions by which each spermatogonium gives 

 rise to four sperm, and each oogonium to one functional egg and 

 three tiny, abortive cells known as polocytes; each and all with 

 one-half the number of chromosomes of the somatic cells and of the 

 germ cells up to this point in their development. Consequently 

 these two divisions, termed maturation divisions, must be ex- 

 amined in some detail if we are to appreciate the nicety of the 

 process by which the chromosome number is reduced one-half 



