STUDIES ON CHROMOSOMES 391 



the identity of the sex-chromosomes in Oncopeltus. It may there- 

 fore be taken as an estabhshed fact that a pair of sex-chromosomes 

 may be recognizable as such even in cases where they show no 

 perceptible difference of size, and where no constant differences 

 between the diploid chromosome-groups of the two sexes can be 

 seen. 



Such cases are of course fatal to the view that the nuclear differ- 

 ences between the sexes are reducible to one of general chromatin- 

 mass; and, as I have elsewhere urged, these chromosomes can 

 be regarded as factors in sex-production only by assuming some 

 kind of difference between the substance of X and Y. I will 

 not here enter upon the discussion of a point that has been fully 

 considered in several earlier papers (see especially Wilson,' 11a, 

 '11 b). I will only again express the view that the differential 

 factor between X and Y may plausibly be regarded a specific 

 chemical substance (the 'X-chromatin') that is either confiiled to 

 the X-chromosome or is there present in relative excess, and in 

 respect to which the two sexes differ correspondingly. If this is 

 correct, the sexual differences may be at bottom dependent upon 

 a fundamental quantitative difference of metabolism, as stated 

 in my first paper on this subject ('05 a) . Such nuclear differences 

 between the sexes may of course exist not only in forms where no 

 difference of total chromatin mass is visible, but even where no 

 special 'sex-chromosomes' are differentiated. The surprising 

 thing, indeed, is that they should in some instances be expressed 

 in, or accompanied by, visible sexual differences of the chromo- 

 some-groups. 



III. CRITICAL CONSIDERATIONS ON THE MATURATION-PHENOM- 

 ENA BASED ON A COMPARISON OF THE HEMIPTERA, 

 TOMOPTERIS, BATRACOSEPS AND SOME 

 OTHER FORMS 



As has been indicated, my conclusions concerning synapsis 

 and reduction in Hemiptera are largely tentative in character. If 

 I nevertheless venture to make some critical comment on the 

 general problem it is mainly because of the opportunity I have had 

 to reexamine these phenomena in Tomopteris and Batracoseps. 



