MATERIALS OF HEREDITY 



19 



chromosome that is present in the female, but is lacking 

 in the male, is known as the X-chromosome. In many species 

 the X-chromosome differs in size or form from the other 

 chromosomes. 



But in many species, the male, instead of lacking the 



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d 



e 



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Figure 6. Male and female groups of chromosomes in an animal 

 (Lygaeus) in which the male has a small Y-chromosome, after Wil- 

 son, a, The male group of fourteen chromosomes as seen under the mi- 

 croscope, b, The process of division of this group into two groups of 

 seven chromosomes each, in the formation of germ cells, c, The two 

 groups of seven chromosomes each in the two kinds of male germ 

 cells formed, one containing the chromosome X, the other the smaller 

 chromosome Y. d, The female group of fourteen chromosomes, as 

 seen under the microscope, e, The seven pairs constituting the female 

 group, in order of size. 



X-chromosome entirely, has in place of it a small chromo- 

 some, which is almost without function (figure 6) . This small 

 one, present In the male, but not in the female, is com- 

 monly called the Y-chromosome. In man the male has such 

 a small Y-chromosome, while the female has in place of 

 it a larger X-chromosome (figure 19). 



