MENDELIAN RATIO AND CHROMOSOMES 499 
in synapsis, and in the reduction division are separated into 
groups which are neither purely paternal nor purely maternal. 
This last suggestion, I believe, was pure theory advanced to meet 
the known experimental facts which show that either parent may 
transmit the characters of its ancestors of the opposite sex. It 
is strange that so careful an observer should have overlooked the 
very thing that was present (the unequal tetrad was first found 
on Sutton’s slides) offering definite chromosomic proof for his 
theory. For, I think, there can be little doubt that the dyads of 
the tetrad described in the foregoing pages are distinct physio- 
logical individuals, representing respectively the paternal and 
maternal contribution to the formation of some character or char- 
acters; and, as each can be identified, they furnish an excellent 
means of tracing the process of segregation and recombination. 
None of the female germ cells in maturation stages being avail- 
able at present, definite knowledge of what occurs there is lacking. 
- But it seems necessary to assume the presence of an unequal tetrad 
there also, for if its place were filled by an ordinary chromo- 
some of equal parts we would sometimes find two large or two 
small united in the first spermatocyte, but in every one of the 
twenty animals studied the unequal tetrad was present. The 
only alternative would be to conclude that one-half the spermato- 
zoa are not functional, for which there is not a shred of evidence. 
If this assumption that after maturation one-half the ova con- 
tain the large and one-half the small dyad be correct, then selec- 
tive fertilization becomes a necessity, since a spermatozoon con- 
taining the large dyad could only fertilize an ovum containing 
the small. This would in no way interfere with the ratio of Men- 
delian characters, for, owing to the abundance of spermatozoa, all 
the ova would be fertilized; nor does it necessitate the extension 
of the theory to those pairs which are quantitatively equiva- 
lent. But, whether further evidence shall show selective fertili- 
zation to be a fact or not, and regardless of the mode of origin of 
the inequality, its absolutely constant occurrence and the alter- 
nate distribution of the dyads in even one individual is sufficient 
for the essential part of this work—the segregation of at least part 
