Studies on Chromosomes 29 



this chromosome produce only females, it must be assumed that 

 the maternal mate or fellow, with which it becomes associated on 

 entering the egg, is a dominant female-determinant. Further, 

 since males result from fertilization by spermatozoa that do not 

 contain the heterotropic chromosome, the latter must in male- 

 producing eggs be derived from the egg-nucleus (cf. the diagram, 

 Fig. 6). The general interpretation, therefore, must include the 

 assumption that there are two kinds of eggs (presumably in 

 approximately equal numbers) that contain respectively the male- 

 and the female-determinant, 1 and that the former are fertilized only 

 by spermatozoa that lack the heterotropic chromosome (/. e., the 

 male determinant) and vice versa, 2 giving the combinations (m)f 

 (female) and m (male). Such a selective fertilization is there- 

 fore a sine qua non of the assumption that the heterotropic chro- 

 mosome is a specific sex-determinant. 



A nearly similar, though somewhat more complex, result follows 

 in the case of the idiochromosomes. In respect to sex-production 

 the large idiochromosome is identical with the heterotropic chro- 

 mosome, and the morphological evidence is nearly or quite 

 decisive that the heterotropic chromosome is actually a large 

 idiochromosome, the smaller mate of which has disappeared. 

 The small idiochromosome may therefore be regarded as a 

 disappearing, or even vestigial, female-determinant that is recessive 

 to its larger fellow (the male-determinant); and its reduction in 

 size may plausibly be regarded as an atrophy resulting from its 

 invariably recessive nature (this chromosome being strictly con- 

 fined to the male). Precisely as in case of the heterotropic 

 chromosome, the large idiochromosome of the male (male- 

 determinant) must be derived in fertilization from the egg-nucleus 

 (Fig. 6); and, as before, it must be assumed that eggs that contain 

 this chromosome are fertilized only by spermatozoa that contain 

 the small idiochromosome, those that contain the female-determi- 



'This would follow from the coupling of the two sex-chromosomes in synapsis to form the bivalent 

 (m)j, and its division in such a way as to leave in the egg either the male- or the female-determinant 

 indifferently. 



Otherwise the combinations mm or / might result, which is contrary to observation, since the sex- 

 chromosomes are in this type never paired in the male or unpaired in the female. 



