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IN VER TEBRA TE ZOOLOG Y 



sion resulting in a reduction of number of chromosomes is known 

 as meiosis. The chromosome number in cells which have not 

 undergone maturation is said to be diploid, because each cell 

 contains two sets of chromosomes, one of which is derived from 

 its male parent and the other from the female parent. Following 

 maturation, the gametes are said to have the haploid or reduced 

 number of chormosomes. 



Fig. 9. — Maturation of the egg and fertilization in Ascaris. A, spermatozoon 

 about to enter egg; B, spermatozoon inside egg; egg nucleus in anaphase of first 

 maturation division; C, completion of first polar body; D, late anaphase in forma- 

 tion of second polar body; E, masturation of egg completed, male and female 

 pronuclei, each with two chromosomes, meeting; F, formation of first cleavage 

 spindle from centrosome of spermatozoon, with two paternal and two maternal 

 chromosomes in late prophase. {From. Sharpe after Hcrtwig). 



The reduction in chromosome numbers is in preparation for 

 fertilization or the union of two gametes to form a fertilized egg 

 or zygote (Fig. 9). By this reduction phenomenon the fertilized 

 egg, and consequently all of the cells resulting from its mitotic 

 division, retain a constant number of chromosomes from genera- 

 tion to generation. Through fertilization a sperm cell with the 

 haploid number of chromosome unites with an egg cell with the 

 haploid number to form a zygote whose nuclear composition is 

 diploid. 



