FERTILIZATION 115 



one or the other of the daughter cells. These are associated with the 

 determination of sex and will not be considered until Chap. LXXIII is 

 reached. 



141, Significance of Synapsis. — In all of the somatic cells of the body 

 the chromosomes of maternal and paternal origin remain separate in 

 cell division, and since the corresponding ones from the two parents are 

 similar, they appear in pairs of like chromosomes. In the process of 

 synapsis in gametogenesis these like chromosomes unite, and this is 

 followed by their separation again in chromosome reduction. In mitosis 

 every chromosome is divided and the two daughter cells have the full, 

 or diploid, number, but in meiosis whole chromosomes pass to the 

 daughter cells, which thus acquire the haploid number. Since it is a 

 matter of chance to which of the two poles of the spindle any particular 

 chromosome goes, there is an assorting and chance distribution of 

 chromosomes to the mature egg cell or sperm cell. Thus every individual 

 not only represents a chance mingling of the chromosomes of its parents 

 but also receives a chance selection from those of previous generations, 

 the probable number received from each generation being progressively 

 smaller in going back from one generation to the previous one. In 

 one sense this union of the chromosomes in synapsis may be thought of 

 as the final step in fertilization. 



