86 EDMUND B. WILSON 
numerary Y-chromosomes into the female has no visible effect 
upon any of the characters of the animal, sexual or otherwise; 
and this might be urged against the whole conception of qualita- 
tive differences among the chromosomes and of their determina- 
tive action in development. It is especially in view of these 
and certain other general questions that I wish to indicate some 
of the many possibilities that must be taken into account in the 
consideration of this problem. My discussion is throughout 
based upon the asswmption that the chromosomes do in fact 
play some definite role in determination, and that they exhibit 
qualitative differences in this respect. I do not hold that they 
are the exclusive factors of determination; though it is often con- 
venient, for the sake of brevity, to speak of them as if they were 
such. 
(2) Cytologically considered, the morphological dimorphism 
of the spermatozoa seems to have arisen by the transformation 
of what was originally a single pair of chromosomes comparable 
to the other synaptic pairs. We have at present no information 
as to whether the members of this pair were equal or unequal in 
size; but in either case there are grounds for the assumption that 
its two members differed in some definite way in respect to the 
quality of the chromatin of which they were composed. This 
pair, which may be called the primitive X Y-pair, has undergone 
many modifications in different species, but without altering its 
essential relation to sex. In the insects (Hemiptera, Coleoptera, 
Diptera) its most frequent condition is that of an unequal pair, con- 
sisting of a ‘large idiochromosome’ or ‘X-chromosome,’ and a 
“small idiochromosome’”’ or ‘ Y-chromosome,’ the latter being con- 
fined to the male line, while the former appears in both sexes— 
single in the male and paired in the female. That all gradations 
exist between cases where X and Y are very unequal (as in many 
Coleoptera and Diptera and in some Hemiptera) and those in which 
they are nearly or quite equal (Mineus, Nezara, Oncopeltus) gives 
some ground for the conclusion that in the original type the 
XY-pair was but slightly if at all unequal. 
By disappearance of the free Y-member of this pair has arisen 
the unpaired odd or ‘accessory’ chromosome, which accordingly 
