Studies on Chromosomes 189 



chromosomes and the mass of protoplasm, but I have found the 

 errors of measurement of the cell size to be too great to give any 

 trustworthy result regarding the relative chromatin mass. 



Despite the difficulties in the way of an accurate direct deter- 

 mination, I believe the facts on the whole warrant the conclusion 

 that the relative chromatin mass shows no constant correlation 

 with sex. The most probable conclusion is that the male-produc- 

 ing spermatozoa in Metapodius are distinguished by the same 

 characters as in other forms having unequal idiochromosomes, 

 the former class being those that receive the large idiochromosome, 

 the latter those that receive the small one, irrespective of the super- 

 numeraries that may be present in either class. For reasons that 

 I have elsewhere stated, I believe that if the idiochromosomes be 

 the sex-determinants their difference is probably a qualitative 

 one, and since the small idiochromosome may be lacking it would 

 seem that the large one must in every case play the active role — 

 perhaps as the bearer of a specific substance (enzyme .^) that calls 

 forth a definite reaction on the part of the developing individual. 

 If this be so, we can comprehend the fact that the presence of 

 additional small idiochromosomes (supernumeraries) in either 

 sex does not affect the development of the sexual characters in that 

 sex. 



b The possible Origin of the unpaired Idiochromosome {''odd" 

 or "accessory" Chromosome) and of the Supernumeraries 



The explanation of the unpaired idiochromosome offered in the 

 second and third of my "Studies on Chromosomes" ('05, '06) was 

 suggested by the fact that various degrees of inequality exist in the 

 paired idiochromosomes, there being an almost continuous series 

 of forms connecting those in which the idiochromosomes are 

 equal (Nezara, Oncopeltus) with those in which they are so very 

 unequal that the small one appears almost vestigial (Lygaeus, 

 Tenebrio). It is evident that by the further reduction and final 

 disappearance of the small member of this pair the large one would 

 be left without a mate, and its history in the maturation process 

 would become identical with that of an "odd" or "accessory" 



