Jauttaet 8, 1909] 



SCIENCE 



59 



Spermatocyte 



Division Spermatozoa 



Actual 

 Somatic 

 Number of 

 Zygotes Chromosomes 



f/o^o W ^ 



Lygaeus 



21 



22 



Ordinary chromosomes — ^unshaded. 

 X element — ^black. 

 y element — ^with cross-bars. 



ly, but eonclusively, that the two classes of 

 spermatozoa thus formed are, respectively, 

 female-producing and male-producing (I 

 do not say female- or male-determining), 

 as already stated. In hoth sexes the so- 

 matic groups are identical save in respect 

 to the X- and T-elements; and the differ- 



ence can only be a result of fertilization by 

 the two respective classes of spermatozoa. 

 This is at once proved in species having a 

 T-element by the fact that this chromosome 

 is found only in the male. The evidence 

 given by the X-element alone is equally 

 decisive. This is present in both sexes, but, 



