CONSTITUTION OF THE CHROMOSOMES 



159 



somes side by side, as shown in figure 37. While it is im- 

 possible to prove that these thickenings represent or con- 

 tain the genes, it seems probable that they do. For their 



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Figure 37. Structure of chromosomes as seen under the microscope, 

 showing the minute paired particles or chromomeres, of which each 

 is composed. The chromomeres presumably represent the genes. A, B, 

 C, chromomeres in the chromosomes of the grasshopper, after Wen- 

 rich. The paired condition of the chromomeres (genes?) is clearly 

 shown. In B, the two members of one of the chromosome pairs are 

 approaching each other in conjugation, but are not completely con- 

 jugated. D, E, chromomeres (genes?) in the chromosomes of the lily, 

 after Belling. E shows a portion of D at higher magnification. 



arrangement is exactly that which experimentation shows 

 must be the arrangement of the genes. 



Maps of the Chromosomes: Since the genes are thus 

 known to be in serial order, maps can be made of them, 

 provided there is a suflUciently detailed knowledge of the 

 genetics of the organism in question. Such maps show the 

 order of the genes in the chromosomes. In making such 

 maps the guiding principle is the fact that two genes having 



