AND OF HYBRID PIGEONS. 
47 
of its nature and actions, seems to be intimately bound up in the 
phenomena of inheritance. It is reasonable to suppose, there¬ 
fore, that it constitutes at least a part of the material basis for 
variation in the germ. The question then arises as to whether 
there is any correlation between the, distribution of chromatin 
as seen in the germ-cells of hybrids, and the marked variability 
which characterizes the offspring of fertile hybrids. 
If we accept the view that chromatin is a substance capable 
of varying in qualities in the different regions of the chromo¬ 
some, then in fertile hybrids where irre.gular mitoses occur, the 
different germ-cells will certainly not be qualitatively similar 
after division and one would expect the offspring produced from 
such cells to be variable. That the chromatin of each parent 
species often retains its individuality, is indicated by the fact 
observed in many primary spermatocytes where two separate 
groups of the small or single type of chromosome exist. The 
division of such a cell into three or four, as the case may be, re¬ 
sults in the formation of new cells, some of which will mani¬ 
festly contain chromatin from only one of the original parent 
species and some, only from the other. Some of the sperma¬ 
tozoa then, will bear chromatin from only one of these species. 
In the offspring from such a cell one would expect a much closer 
return to whichever one of the parent forms it represented than 
in the offspring of a “mixed” spermatozoon. 
In discussing irregular divisions, however, it must not be 
forgotten that many apparently normal divisions of the primary 
spermatocytes also occur in all hybrids, and constitute by far 
the predominant kind of division in hybrids from closely related 
forms. Unequal distributions of chromatin cannot therefore play 
the most important part in variation or reversion. There seems to 
be no other interpretation, indeed, than that in the many 
normal mitoses of the bivalent chromosomes which occur, the 
chromatin of the father and of the mother is set apart so that 
the ultimate germ-cells are what might be termed “pure” cells; 
that is, a given egg or sperm-cell contains exclusively or at 
least predominantly qualities from one parent. The offspring 
from fertile hybrids of the same parentage might then be sim¬ 
ilar to the mixed type of the original hybrid, or revert to one of 
the grandparent types, dependent upon the chances of the vari- 
