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, '10) 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 ('10) in Metapodius the introduction of super- 



