STUDIES ON CHROMOSOMES 85 
are to be considered, namely, (1) that it may be the female which 
(as in sea-urchins) is the digametic sex, and (2) that one sex or 
the other may still be physiologically digametic even though this 
condition is not visibly expressed in the chromosomes. The 
first of these possibilities may readily be tested by cytological 
examination of the female groups. The second can only be 
examined by means of experiment, and especially by experiments 
on sex-limited heredity. It is interesting that the work of Don- 
caster and Raynor, cited above, and the more recent one of Morgan 
on Drosophila (10) have given exactly converse results, the former 
demonstrating a sexual dimorphism of the eggs, the latter of the 
spermatozoa. This agrees with the cytological data, as far as 
they have been worked out. The researches of Stevens (’08, 10), 
on the Diptera establish the cytological dimorphism of the sper- 
matozoa in these animals, while all observers of the Lepidoptera 
have thus far failed to find such dimorphism in this group. It 
thus becomes a very interesting question whether a cytological 
dimorphism of the mature eggs may be demonstrable in the 
Lepidoptera; though a failure to find it would in no wise lessen 
the force of the experimental data. Physiological differences be- 
tween the chromosomes are of course not necessarily accompanied 
by corresponding morphological ones—indeed such a correlation 
is probably exceptional. 
(1) (a) Composition and origin of the XY-pair. The facts 
seen in Nezara again force upon our attention the puzzle of the 
Y-chromosome or ‘small idiochromosome.’ It is remarkable 
that two species so nearly akin as N. hilaris and N. viridula should 
differ so widely in respect to this chromosome; though this is 
hardly so surprising as the fact that in Metapodius this chromo- 
some, as I have shown (’09, 710) may actually either be present or 
absent in different individuals of the same species. These facts 
show, as I have urged, that although the Y-chromosome shows a 
constant relation to sex when it is present, it can not be an essen- 
tial factor in sex-production. As the case now stands this might 
be taken as a direct piece of evidence against the view that the 
idiochromosomes are concerned with sex-heredity. Further,: as 
I have pointed out (710) in Metapodius the introduction of super- 
