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.) 4" 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 

 (6) selective fertilization or infertility of gametic unions of like sex-chromo- 

 somes, /. 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- 



