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Gametes 



258 Experimental Transformation of Species 



When gametes are being formed in the mature insect, 

 the reduction division, which cuts in half the number of 

 chromosomes in a germ cell, results in producing two simi- 

 lar Q^^ cells but two different sperm cells (see Fig. 70) . Sex 



determination through 

 chromosomes has been 

 found to hold for other 

 species. In man the ac- 

 tual number of chromo- 

 somes has been given as 

 24 pairs or 48. One of 

 these pairs, however, 

 seems to be different in 

 the two sexes. There is, 

 in fact, an apparent re- 

 duction of one of these 

 pairs in the male, almost 

 to the vanishing point, 

 so that cells of the male 

 show only 47 chromo- 

 somes distinctly, as 

 against 48 in the female 

 (Fig. 71). The reduc- 

 tion division that brings 

 about the formation of 

 eggs cells thus produces 

 gametes containing each 

 24 chromosomes, where- 

 as the reduction divi- 

 sion leading to the for- 

 mation of sperm cells 

 is always unequal and 

 yields two kinds of 

 sperm cells, one with 24 

 chromosomes and one with 23 chromosomes (and the very 

 small one which is sometimes overlooked by the observer) . 

 If an Q^g cell is fertilized by one of the former there results 



Fig. 70. Reduction Division in 

 Relation to Sex Determination 



The chromosomes of the male (light) par- 

 ent (P) differ from those of the female parent 

 (dark) in the form of the Y chromosome. As 

 gametes are formed, one of each pair of the 

 X chromosomes (black) goes to each egg. In 

 the formation of sperms, however, half get the 

 X chromosome and half get the Y chromo- 

 some: there is only one kind of egg, but there 

 are two kinds of sperms. In the following 

 generation, Fj, the sperms containing the X 

 chromosomes give rise to females, and those 

 containing the Y chromosomes give rise to 

 males. Both males and females of this genera- 

 tion, however, derive half the chromosomes 

 from the father and half from the mother. 



