THE DETERMINATION OF SEX 



575 



shown, those in which it is present are the female-producing or 

 " X-class." The chromosome in question is variously known 

 as the "accessory chromosome " (McClung), "odd chromosome," 

 " heterochromosome " (Montgomery), " heterotropic chromo- 

 some," "idiochromosome," " X-chromosome" (Wilson), etc. 1 

 Fig. i, a shows the differential division of Protenor (second 



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jT 00 



M 



tik 



d 



x 



x- 



f 



** 



a 



c 



e g 



Fig. i. — Exact drawings of the chromosomes in Protenor (a-e), Euschistus (/), and 

 Gelastocoris {g) ; those of Protenor and Gelastocoris from photographs. 



a, side-view of the differential division (second spermatocyte) in Protenor, showing the large X-chromosome 

 approaching one pole ; b and c the two daughter chromosome-groups, both from the same spindle, of a 

 stage like «, seen from one pole, showing all the chromosomes, the large X-chromosome (somewhat 

 foreshortened) at one pole ; d, diploid (spermatogonial) group of the male ; e, diploid group of the 

 female. 



f, differential division of Euschistus in side-view, X and Y separating. 



g, corresponding view of differential division of Gelastocoris (from Payne), at a slightly later stage. 



spermatocyte-division) in side view, with the large X-chromo- 

 some nearly at one pole ; b and c show the two poles (from 



1 A fact of importance for the practical study of the X-chromosome is that in 

 the differential division it often lags behind the others. This is, however, by no 

 means invariable, even in the same group. Thus, in Anasa or Prote?tor it always 

 lags, but in the nearly related Syro7)tastes it always passes to the pole in advance 

 of the others and never even enters the equatorial plate. The latter case is the 

 rule in the Orthoptera. In the dragon-fly, A mix, it sometimes lags, sometimes 

 precedes the others. Another interesting and perhaps important detail is that the 

 X-chromosome differs from the others in behaviour during the growth-period of 

 the spermatocytes, always having throughout this period a compact consistency 

 and rounded form like a nucleolus, while the ordinary chromosomes are in a diffuse 

 and lightly staining condition. In the spermatogonia, too, it often differs from the 

 others in certain definite ways. 



