DISTRIBUTION OF THE CHROMOSOMES 195 
cide whether a mutant factor A! has as its mate, i.e. 
always pairs at maturation 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 linked 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 F, results and F; 
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 something else in the cell, a 
portion of the cytoplasm, perhaps, also follows the 
distribution of the sex chromosomes. Such a postu- 
