X SEX 1 01 



small elongated bodies which owe their name to the 

 fact that they stain more deeply with various dyes 

 than the rest of the cell protoplasm. At certain 

 stages they stand out very clearly when appropriately 

 stained, and it is then apparent that as regards 

 number they are constant throughout the body cells 

 of any given individual. They may vary greatly in 

 size and shape, but these differences are remarkably 

 definite, and a similar collection of chromosomes is 

 to be found in the nucleus of every cell of an in- 

 dividual. Moreover, apart from the sex differences 

 about to be mentioned, this resemblance in number 

 and in shape persists throughout all the individuals 

 of a given species. Distinct species, as a rule, 

 exhibit a distinct and definite collection of chromo- 

 somes, so that an expert investigator can often, from 

 the microscopic examination of a minute fragment 

 of tissue, decide the species of animal from which it 

 was taken. From the standpoint of this discussion 

 there is one other feature to be mentioned. The 

 number of the chromosomes in a cell of the body is 

 generally an even one. This is because they normally 

 exist in pairs. If, for example, the number is twelve, 

 careful examination will show that there are six 

 pairs. Each member of a given pair is visibly 

 indistinguishable, though the members of different 

 pairs may differ very greatly in size and shape. In 

 such a case six different kinds of chromosomes can 

 be distinguished. It is a peculiarity of the chromo- 

 somes that in the formation of the gametes their 

 number is reduced by a peculiar and complex process 

 to one half of the number found in the body cells. 

 In our imaginary case, for instance, where the body 



