26 J. E. WODSEDALEK. 



the determining factor. This view has been recently substan- 

 tiated by the careful investigations of McClung, Stevens, Wilson, 

 Morgan, Payne and others, who found that in many invertebrates 

 such as certain insects, myriopods, arachnids and nematodes, two 

 different kinds of spermatozoa are produced. The spermatozoa 

 differ from each other by one chromosome, or by a group of 

 chromosomes, called the accessory chromosomes, or X-chromo- 

 somes. 



In some forms it was found that the spermatozoa each contain 

 an accessory chromosome, but that in half the sperms this chromo- 

 some was larger in its chromatin content than in the other. 

 These are called the X and Y elements respectively. All the 

 eggs produced (except in a few cases), on the other hand, contain 

 the X-element. 



It has been shown in some of the Tracheata that whenever an 

 egg is fertilized by a sperm containing the X-chromosome it 

 develops into a female, while an egg fertilized by a sperm without 

 the X-element, or by one containing the Y-element, gives rise 

 to a male individual. 



In the vertebrates, Guyer was able to identify an X-element 

 in such diverse forms as the guinea, rooster, pigeon, rat, and man, 

 and other investigators have recently brought forth similar 

 evidence. Patterson and Newman ('10) record its presence in 

 the armadillo, Jordan ('n) in the opossum, Stevens ('n) in the 

 guinea-pig, and King ('12) in Necturus. Up to the present in- 

 vestigation, however, a dimorphism in the number of the chromo- 

 somes in the male and female vertebrates has not been recorded. 

 Guyer ('10) after speaking of the difference in the number of 

 chromosomes in the somatic cells of the male and female Tracheata 

 which possess the X-elements says: 



11 In the light of these facts we should expect the somatic cells 

 of man to contain twenty-two and of woman, twenty-four chro- 

 mosomes. The tissues of the female have not yet been studied 

 with this in mind. Flemming ('97) records the somatic number 

 of chromosomes, determined from corneal cells, as twenty-four 

 but unfortunately he does not record the sex of the subjects from 

 which the material was obtained. If it were a female his count 

 would bear out the interpretation given above." 



