Studies on Chromosomes. 531 



immediately preceding the reduction-division /. <?., in case of the 

 m-chromosomes to the prophases of the first division, at the very 

 end of the growth-period, and in case of the idiochromosomes to a 

 still later period following the first division (though a temporary 

 or preliminary union frequently occurs at a much earlier period). 

 The "accessory" or heterotropic chromosome, finally, does not 

 undergo synapsis at all, since it is without a mate with which to 

 pair. 



As regards their behavior in the growth-period, the idiochro- 

 mosomes and the heterotropic chromosome agree in being "hetero- 

 chromosomes" in Montgomery's sense /. e. y are distinguished 

 from the other chromosomes by their compact form -and deep- 

 staining capacity. The m-chromosomes, on the other hand, 

 may remain in a diffused condition throughout the early and 

 middle growth-periods, only condensing to the compact form at 

 the same time as the ordinary chromosomes (Anasa, "Charies- 

 terus"); their condensation may, however, take place in the 

 middle growth-period (Alydus), or even earlier (Protenor, ac- 

 cording to Montgomery). An analogous difference in the time 

 of condensation exists in case of the idiochromosomes, which in 

 case of Lygaeus do not condense as early as in Ccenus or 

 Euschistus. 



My observations prove definitely in some cases (Alydus, 

 Anasa, Archimerus, "Chariesterus"), and I think render it prob- 

 able for all cases, that in those Hemiptera that possess an "acces- 

 sory" or heterotropic chromosome and two equal spermatogonial 

 microchromosomes (m-chromosomes), the large chromosome- 

 nucleolus of the synaptic and growth-periods is not, as other 

 observers have supposed, the microchromosome-bivalent 

 ("chromatin nucleolus" of Montgomery) but the heterotropic 

 chromosome, precisely as in the Orthoptera. This error of 

 identification has led Montgomery to designate three quite 

 distinct kinds of chromosomes by the same name of "chromatin- 

 nucleoli." These are (i) the equal paired spermatogonial micro- 

 chromosomes and the corresponding bivalent of the first sper- 

 matocyte division; (2) the idiochromosomes, which are typically 

 unequal and do not form a bivalent in the first division; and (3) 

 the heterotropic chromosome as it appears in the growth-period. 

 It is therefore desirable, despite some repetition, to bring together 

 in brief form the principal distinctions between these three. 



