THE CHROMOSOMES 153 
resting period of the cells, but this must be received 
with some caution. In many animals and in some 
plants the chromosomes are of very different sizes 
and shapes, and many, or even all of them, can be 
identified at each division. It is found that: these size 
relations hold throughout all divisions of the cells. 
While this evidence appears at first sight to show 
that the chromosomes are structures that perpetuate 
themseives, preserving their identity, yet it might be 
maintained, in fact it has been maintained, that each 
species has its own peculiar protoplasm from which 
chromosomes of a particular kind and number are, 
as it were, crystallized out anew before each cell 
division. This point of view can not, however, be rec- 
onciled with the evidence that follows. In Meta- 
podius, Wilson has found that individuals may differ 
in the particular chromosome that he calls. the m 
chromosome. While the normal individuals have a 
pair of m chromosomes, one individual had three 
m’s; but all of the cells of any given individual 
have the same number. These chromosomes furnish 
strong support of the continuity of the chromosomes; 
for, in whatever number they enter the individual 
during fertilization, they retain that number through- 
out all the subsequent generations of cells. The same 
is true, of course, for the sex chromosomes. 
Corroborative proof is found in certain hybrids, 
where the evidence is even more significant, because 
in such cases the chromosomes introduced by the 
male are, as it were, in a foreign medium. For 
example, Moenkhaus first pointed out that when 
