28 Papers from the Marine Biological Laboratory at Tortugas. 
But as Stevens (1905) points out, it remains a question whether the 
accessory chromosome is really a sex chromosome in the sense that it deter- 
mines sex or merely represents sex-characters. Bateson (1907) suggests 
that the accessory body may be merely associated with the cause of sex. 
Wilson (1906) suggests that the heterochromosomes (therefore accessory 
chromosome) may merely transmit sex-characters, sex being determined by 
cytoplasmic conditions external to the chromosomes ; or again, that the acces- 
sory may be a sex-determinant.only by virtue of a difference in activity or 
amount of chromatin. In view of its apparent function as a sex-determinant 
(whether of sex-condition or sex-characters), it hardly lends itself to the 
interpretation suggested by Paulmier and Montgomery to the effect that it 
is a degenerating chromosome—* such [heterochromosomes] as are in the 
process of disappearance in the evolution of a higher to a lower chromosome 
number” (Montgomery). Nevertheless, Wilson’s further suggestion that 
the accessory of Orthoptera is the homologue of the large member of the 
idiochromosome group in certain Hemiptera, and that its missing mate is 
the homologue of the small idiochromosome—the accessory thus perhaps 
representing the residue of a pair of idiochromosomes after the loss of a 
pair of microchromosomes—is very helpful in formulating a working 
hypothesis in regard to the accessory chromosome considered as a sex- 
determinant. 
Expressed in Wilson’s (1906) formula for sex-determination, the facts 
in Aplopus mayeri are as follows: 
A. Egg (18 chrom.) -++- Spermatozoon (18 chrom.) = female (36 chrom.) 
B. Egg (18 chrom.) ++ Spermatozoon (17 chrom.) = male (35 chrom.) 
Castle (1903) developed a theory of sex in which he applied a modifica- 
tion of Mendel’s principle of segregation to sex-phenomena. This has 
recently been more fully elaborated and applied to the case of the accessory 
chromosome by Wilson (1906). Castle’s theory involves several assump- 
tions: (a) the fact of two kinds of eggs (male and female), as also of two 
kinds of spermatozoa, which have been actually many times observed; and 
(b) selective fertilization or infertility of gametic unions of like sex-chromo- 
somes, 7. e., an egg with a female determinant must be fertilized by a sperma- 
tozoon with a male determinant, and vice versa. Castle further believes 
that there are no individuals pure in regard to sex, but that only hybrids are 
produced. Observation also seems to show the dominance of the female 
over the male determinant. 
If the accessory is actually a sex-determinant, and as such represents the 
homologue of the large idiochromosome as suggested by Wilson, then, since 
an egg fertilized by a spermatozoon lacking the accessory chromosome pro- 
duces a male, the egg itself must contain the factor that determines male- 
ness, and the missing chromosome must be the female determinant. Con- 
sequently, since an egg fertilized by a spermatozoon containing the acces- 
