Studies on Chromosomes. 539 



chromosome has assumed its unpaired character by the reduction 

 and final disappearance of its parental mate or homologue (t. e.^ a 

 small idiochromosome); and it is highly probable that this pro- 

 cess has occurred in one sex only, namely, the male.^ If this be 

 the fact, it is evident that the heterotropic chromosome that 

 remains in the male is the maternal mate or homologue of that 

 which has vanished. I think therefore that we may expect to find 

 that the heterotropic chromosome present in the male is derived 

 in fertilization from the maternal group of chromosomes; and 

 also that the female will be found to possess one more chromosome 

 than the male (exactly the opposite of McClung's assumption), 

 the additional chromosome being the homologue of that which 

 has vanished in the male.' If this be the fact, it follows with 

 great probability that in the egg-synapsis this chromosome pairs 

 with its paternal homologue (originally the heterotropic chro- 

 mosome) to form a symmetrical bivalent, and that all the eggs 

 receive eleven chromosomes; while in the male the heterotropic 

 chromosome fails to pair (having no mate) and hence remains 

 univalent. The expectation may therefore be stated as follows: 



Egg II + spermatozoon lo = 21 (male). 

 Egg II + spermatozoon 11 = 22 (female).' 



Important direct evidence in favor of this expectation is given 

 by the discovery by Stevens, briefly referred to in my preced- 

 ing paper, that in the beetle Tenebrio a small chromosome, 

 evidently analogous to the small idiochromosome of Hemiptera, 

 is present in the somatic cells of the male only, while in the female 



'I will here not go into the somewhat intricate difEculties encountered under the supposition that 

 it has occurred in both sexes, except to point out that if an unpaired heterotropic chromosome be present 

 in the female and is allotted to only half the eggs (as in the male) it is necessary to assume a fertiliza- 

 tion of each form of egg by the opposite form of spermatozoon, since otherwise three forms of offspring 

 would result. Such a mode of fertilization is a priori very improbable. Still greater difEculties stand 

 in the way of assuming that an unpaired heterotropic chromosome, present in the female, is retained in 

 all of the eggs. 



^Montgomery ('04) has in fact found in the oogonia and follicle-cells of the female Anasa twenty- 

 two chromosomes, and Gross ('04) reports the same number in those of the female Syromastes. But 

 since the first-named observer is certainly, and I believe the second-named is probably, in error as to 

 the number in the male, both these cases require reexamination. On the other hand Sutton has found 

 twenty-two in the oogonia and follicle-cells of the Orthoptera (Brachystola) while the spermatogonia! 

 groups show twenty-thiee; but here again I think a result so important should be supported by more 

 adequate evidence than he has brought forward. I now have this subject under investigation. 



'For the confirmation of this, see Appendix. 



