DISTRIBUTION OF THE CHROMOSOMES 195 



cide whether a mutant factor A^ has as its mate, i.e. 

 ahvays pairs at matm-ation with, a special one of the 

 remaining A's or may become the mate of any one of 

 the three. On the chromosome hypothesis we should 

 expect, on the whole, the latter to be true. Which- 

 ever of these views becomes established the parallel 

 between the double set of chromosomes and the 

 double set of factors is the important fact. Gregory 

 admits this, but adds the caution: "Yet on the other 

 hand the tetraploid number of chromosomes may be 

 nothing more than an index of the quadruple nature 

 of the cell as a whole." 



In the preceding cases it has been shown that the 

 factors and the chromosomes have the same method 

 of distribution. In the case of sex and sex Hnked fac- 

 tors it can even be shown that they have the same 

 distribution as the sex chromosomes. This identity 

 of distribution holds not only for Fo results and Fg 

 tests, but for all kinds of backcrosses as well. The 

 relation holds, moreover, for all known sex linked 

 factors, of which in Drosophila there are more than 

 forty cases, and for all combinations of sex linked 

 factors. Not to interpret this evidence to mean 

 that the factors are contained in and carried by the 

 chromosomes is to reject a mechanistic basis known 

 to exist in the cell. Nothing is gained if, in order to 

 avoid the obvious connection between the inheritance 

 of the character and the transmission of the chromo- 

 some, we assume that som(^thing else in the cell, a 

 portion of the cytoplasm, pcrha])s, also follows the 

 distribution of the sex chromosomes. Such a postu- 



