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 assumption 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 XY-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 



