346 T. H. Morgan 
need not be repeated here in extenso. The most serious one is 
that although the hypothesis is ostensibly based on the presence 
of certain chromosomes which are assumed to be male and female 
determining respectively, yet to these chromosomes, which are to all 
appearances identical are ascribed exactly opposite functions; and 
in order to make the scheme workable it must further be assumed 
that selective fertilization occurs—a process still unproven. 
For these and other reasons, sufficiently discussed in the preced- 
ing pages, it seems more probable that the relation of these chromo- 
somes to sex may be a quantitative one. ‘This interpretation 
I urged especially two years ago in my general treatment of the 
problem of sex in my Experimental Zoology. The same view 
I had previously argued in the special case of the bee, and in the 
case of gynandromorphism in the bee and silk-worm moth. I 
shall repeat here, therefore, in conclusion, the essential part of the 
general argument advanced in the Experimental Zoélogy. It was 
pointed out that we can explain the cases in which an accessory 1s 
present on the assumption that the sperm with the accessory brings 
more chromatin into the egg than does the sperm without that 
chromosome. In the bee the result is due to two nuclei leading 
to the formation of a female, one to the formation of a male. I 
pointed out that on this view sex is not laid down in the egg or 
sperm but is determined later by quantitative relations that 
appear as a result of certain combinations. ‘The objection to this 
view is found in those cases where no accessories exist, but even here 
the recent results of Baltzer show that differences between the 
pair of sex chromosomes may still be present. It remains to be 
discovered whether this relation holds in general or not. It would 
obviously go beyond the evidence to assign the determination of sex 
to differences in the chromosomes in those cases where no differ- 
ence has been observed. “The remarkable cases here described for 
Phylloxera fallax show that the determination of male and female 
lines may take place in the presence of all the chromosomes. We 
have in this case strong evidence in favor of the view that other 
processes than the number of chromosomes may initiate changes 
that ultimately lead to the production of one or the other kind of 
individuals. For obviously it is as important to discover what 
