402 Edmund B. Wilson. 



somes we have good reason to conclude that it may be true of all, 

 as Montgomery has urged and as Sutton has so cogently argued, 

 from a study of the size-relations. It is a fair working hypothesis 

 that the idiochromosomes represent a pair of corresponding or 

 allelomorphic qualities, or group of qualities, that are respectively 

 maternal and paternal, as Sutton, building on the basis laid by 

 Montgomery and himself, has argued for the chromosome-pairs 

 in general. The argument of Montgomery and Sutton is based, 

 it is true, on the fact that chromosomes of different sizes in the 

 spermatocytes are represented by symmetrical chromosome-pairs 

 of corresponding sizes in the spermatogonia; and to this the idio- 

 chromosomes in most of the cases described form an exception in 

 being unequal. If this appears to be a difficulty it is removed by 

 the case of Nezara, where the idiochromosomes are of equal size. 

 Even in the more usual case, where they are unequal, symmetrical 

 synapsis takes place between all the other chromosome-pairs. 

 If the theory of the individuality of chromosomes be granted no 

 other conclusion seems possible, accordingly, than that the remain- 

 ing two, despite their size-difference, are respectively the paternal 

 and maternal elements of the remaining pair; and if Sutton's 

 general hypothesis be well founded, these elements may be 

 assumed to be physiological correlates or allelomorphs. Their 

 marked difference in size suggests a corresponding qualitative 

 differentiation, and this inevitably suggests a possible connection 

 between them and the sexual differentiation. The visible dimor- 

 phism of the spermatid-nuclei in such forms as Lygaeus, Ccenus 

 or Podisus shows too obvious a parallel to the sexual dimorphism 

 of the germ-cells, indicated by so much of the recent work on sex- 

 determination, to be ignored; while in Nezara, where no visible 

 dimorphism exists, the spermatozoa nevertheless fall into two 

 equal groups in respect to the previous behavior of one of the 

 chromosomes. But such a suggestion as to the possible signifi- 

 cance of the idiochromosomes immediately encounters the diffi- 

 culty that both idiochromosomes are present in the male cells 

 (spermatogonia, and spermatocytes), just as McClung's similar 

 hypothesis regarding the accessory chromosome is confronted with 

 the fact, determined by Montgomery and Gross, that in the Hem- 

 iptera both sexes show the same number of chromosomes. 

 Whether these difficulties can be met by assumptions of dominance 

 and the like remains to be seen; but the fact should be recognized 



