32 Edmund B. Wilson 



Now it is clear that if the relations of the chromosomes to sex- 

 production be the same here as in the second type, the chromo- 

 some m must alternate in successive generations between the male 

 and the female (like the large idiochromosome or the heterotrqpic 

 chromosome to which it corresponds), and hence also shows an 

 alternation of dominance, being dominant in the former sex and 

 recessive in the latter. If, therefore, dominance and recession 

 be inherent in the chromosomes, there must be such a relation 

 between them that m is always dominant to the chromosome (/) 

 of the male, and always recessive to the chromosome / of the 

 female, and that the latter two chromosomes (/ and (/)) are never 

 interchanged between the sexes. This last assumption is not so 

 improbable as it may at first sight appear; for in the second type 

 it is certain, as already pointed out, that the small idiochromo- 

 some ((/) under the general assumption) never enters the female, 

 while the large idiochromosome, m, like the heterotropic, alter- 

 nates between the two sexes in successive generations. 



A strict Mendelian interpretation of sex-production may unques- 

 tionably, I think, be constructed upon the foregoing assumptions. 

 But an interesting suggestion for a somewhat modified Mendelian 

 interpretation is given by the possibility that the dominance of 

 the sex-chromosomes is determined by extrinsic factors, namely, 

 by conditions in the protoplasm of the zygote. If this were the 

 case it is evident that the idiochromosomes could not be considered 

 as sex-determinants in the strict sense of the word. The determi- 

 nation of sex would in this case be due to factors preexisting in 

 one or both of the gametes, irrespective of the sex-chromosomes, 

 and the latter could only be considered as a means by which the 

 sex-characters are transmitted or inherited. The possibility is 

 here clearly offered that either or both forms of gametes may be 

 predetermined as males or females (or at least male-producing 

 and female-producing) prior to fertilization and irrespective of the 

 chromosomes; and thus an interpretation of the ordinary forms 

 of gametes would be reached in harmony with such cases as 

 Dinophilus and other forms in which male-producing and female- 

 producing eggs are distinguishable in size prior to fertilization. 

 Such an interpretation would further be perfectly consistent with 



