72 
Th. II. Montgomery, _jr. 
and two sliorter chromosomes. These phenomena indicate that from 
tlie time of the maturation and fertilization of the egg on into the 
cleavage not ouly do the chromosomes persist as distinct entities, 
hat also they retain their morphological differences. That is to say, 
the sperm and the egg not only contribute chromosomes in equal 
number, but also chromosomes of relatively corresponding individual 
leugths, becanse each pronucleus furnishes a shorter and a longer 
one. 
I would seek to forestall at this place an objectiou that might 
he made to my interpretatiou of the chromosome difference in this 
egg. One might object that the relative lengths of the chromosomes 
are suhject to fluctuating Variation, a matter of mathematical chance, 
and that conseqiiently the mere distinction of two larger from two 
smaller need not prove constant constitutional difference. But if the 
leiigth of a pai’ticular chromosome were not a more or less constant 
character, then hy the probabilities of chance we ought to find in 
not more than half the cases two distinctly larger and two distinctly 
smaller. What we do find, however, in almost all the cases of the 
first cleavage is a pair of larger and a pair of smaller chromosomes, 
the former beiug more like each other in length than either of them 
is to either of the smaller pair. Further, the egg retaius two that 
are always dissimilar, and the two from the sperm are also always 
dissimilar, and we do not find at cleavage three larger and one 
smaller chromosome, or three smaller and one larger. Therefore 
the conditions described are not the resultant of mere chance varia- 
tious, but represeut individual chromosome difference. 
At the same time the two elements of a particular pair rarely 
appear precisely alike in length or volume, usually more or less of 
a difference is apparent. Such appareut difference between the two 
of the same pair is uudouhtedly due in some cases to the chromo- 
somes lying iu different planes, which would result in more or less 
ocular aberration. In by far the greater number of cases the difle- 
rence between the two chromosomes of the same pair is decidedly 
less than the difference between either of them and either of the 
chromosomes of the other pair. In most cells it is the two smaller 
chromosomes that show greater differences in volume or length than 
the two larger do; rather marked cases in instauce are Figs. 2, 5, 
8, 9 of PI. 6. Xow curiously euough it is the smaller dyad of the 
secoud polar spindle that also shows in some cases marked size 
difference of its two elements, while the larger dues not; thus of 
