376 Edmund B. Wilson. 



these bodies shows a more primitive condition in the former genus. 

 I have followed their behavior in the early stages less completely 

 in Euschistus and Podisus, but their behavior during the matura- 

 tion-divisions in these forms is closely similar to that of the others 

 and leads to an exactly similar result. Lygaeus is in some respects 

 the most favorable of all these species owing to the remarkable 

 disparity in size between the idiochromosomes, and to the fact 

 that both are so much smaller than the other chromosomes as to 

 admit of their immediate identification at every period. 



In all the species, the chromosomes show distinct and constant 

 size-differences. A largest chromosome or macrochromosome 



O 



may be distinguished in all, and in most cases a second largest; 

 and in all, the small idiochromosome is the smallest of the group 

 and typically lies near the center of the equatorial plate. (Figs. 

 ib, 2<r, 2d, 3#, d, /, etc.) It is difficult to be sure of the size-differ- 

 ences in case of the other chromosomes, owing to variations in 

 form and position,which produce various degrees of foreshortening. 

 In all the forms, with the exception of Nezara, the larger chromo- 

 somes of the first division are typically arranged in an irregular 

 ring within which lie the two idiochromosomes, side by side, but 

 always quite separate (Figs. l&, 2a, 30, d, /, etc.). This grouping 

 is apparently invariable in Lygaeus, but in Coenus and Euschistus 

 the larger idiochromosome frequently lies in the outer ring (Figs. 

 2b, 3*). In Lygaeus the two idiochromosomes, within the ring, 

 are always much smaller than any of the outer ones, and the 

 smaller is so minute that at first sight I mistook it for a centro- 

 some. 1 In Coenus both the idiochromosomes are relatively 

 larger than in Lygaeus, and their inequality of size is less striking 

 (Fig. 2, <7, I, g, ). The larger one is about equal in size- to the 

 smallest of the peripheral chromosomes and hence cannot be 

 certainly distinguished when it lies in the outer ring (Fig. 26). 

 In both species an equatorial plate occasionally occurs in which 

 nine chromosomes clearly appear (Figs, id, 2<r), but this is excep- 

 tional, and I have never found a spindle showing this body in 

 division. The presence of this additional chromosome is probably 

 due to a failure of synapsis between two of the spermatogonial 

 chromosomes which normally conjugate to form a bivalent body, 

 and it is evidently to be regarded as an abnormal condition. 



l Cf. Montgomery's Fig. 105, of Corizus, '01, i. 



