THE BIOLOGICAL BACKGROUND OF GENETICS 



207 



there are large numbers of chromosomes, very many different assort- 

 ments of maternal and paternal chromosomes are produced. This 

 shuffling and dealing of parental heredity materials constitutes one of 

 the principal means of increasing diversity in organisms, and therefore 

 one of the most important mechanisms of evolution. A relatively 



oosrmatogenesis 

 1 



Spermato- 

 gonia 



{Multiplication Period . 



Growth Period 



Goaonia 



4^j J Pairing of Chromosomes 



Spermatocyte l\?J^ 



/ \ } Reducing div'n 



ision 



Secondary I &/ 

 Spermoco- 

 CyUS 



IKT@ © Q © 



Sperm- 

 atozoa 



Primary dStylt 



Secondary oocyte 

 {/lyunj and first 

 fjo'or body) 



I tf~\ nature ovum 



J and polar bodies 



/yf \ nature ofum 



fo/1 number of 



C/iromosomes 



restored 



Fig. 49. — Diagram showing the parallel between maturation of the sperm- 

 cell and maturation of the ovum. {From Guyer.) 



simple case of the assortment of different chromosomes to gametes is 

 shown in Figure 99. 



THE UNION OF GAMETES — FERTILIZATION 



Once the gametes are mature they are read}' for the fertilization 

 process (Fig. 50). It seems to be a matter of pure chance that any 

 particular sperm finds and enters any particular egg. Since there are 

 usually hundreds of different kinds of eggs and equally large numbers 

 of different kinds of sperms, the number of possible kinds of zygotes 

 produced is extremely great. If, for example, a species has only 



