1 6 Edmund B. Wilson 



may correspond to the microchromosomes, or ra-chromosomes, 

 that are so characteristic of the first type (m, in Figs. 2, 3). 



e. Euschistus 



In E. variolarius the inequality of the idiochromosomes (Fig. 4, 

 a) is greater than in any other of the observed forms excepting 

 Lygaeus turcicus. The sister spermatid-groups (Fig. 4, &) consist 

 in each case of a ring of six ordinary chromosomes with the idio- 

 chromosome near its center. In the outer ring may be distin- 

 guished as a rule four or five different sizes of chromosomes, the 

 largest and smallest (/) being always recognizable, and usually 

 also a second largest and second smallest. The large idiochro- 

 mosome is always distinctly larger than the smallest chromosome 

 (/) of the outer ring, while the small idiochromosome is very much 

 smaller than either, and in long extracted preparations looks 

 exactly like a centrosome. The spermatogonial groups corre- 

 spondingly show seven pairs of chromosomes (Fig. 4, /), of which 

 the small idiochromosome, the smallest pair of ordinary chro- 

 mosomes, and two large pairs are recognizable. The remaining 

 seven include three equal pairs, while the seventh is the large 

 idiochromosome, but it is impossible to identify this chromosome 

 more nearly. The oogonial groups show fourteen equally paired 

 chromosomes, as shown in Fig. 4, g; but my preparations do not 

 show this so well in this species as in the others. 



E. ictericus shows a similar spermatogonial group (Fig. 4, ;) 

 except that the small idiochromosome is relatively a little larger 

 and the small pair of ordinary chromosomes but slightly smaller 

 than the others. The oogonial groups (Fig. 4, k, an unusually 

 open specimen) very clearly show the absence of the small idio- 

 chromosome, but the equal pairing of the chromosomes is less 

 obvious than in the following species. 



In E. tristigmus (Fig. 4, e, /, m) the small idiochromosome is 

 relatively much larger than in the foregoing species, while in 

 E. servus, it is usually a little larger still (Fig. 4, c, d, h). In both 

 these forms the smallest pair of ordinary chromosomes are at once 

 recognizable in the spermatogonia (j, Fig. 4, h, I) and the equal 

 pairing of the others is evident. In E. servus the oogonial groups 



