Studies on Chromosomes 15 1 



A GENERAL DESCRIPTION 



Since the phenomena as a whole are somewhat complicated, I 

 have thought it desirable to bring the most essential facts together 

 for ready comparison in a prehminary general account illustrated 

 by a limited number of selected figures (Figs, i, 2). The funda- 

 mental type of the genus is, I believe, represented by individuals 

 that possess 22 chromosomes in the somatic groups of both sexes, 

 and in which no supernumeraries are present (Fig. i,d-f). Two 

 of the chromosomes are a pair of very small m-chromosomes, like 

 those of other coreids; two are a pair of idiochromosomes consist- 

 ing in the male of a large and a small member, in the female of 

 two large ones; while the remaining 18 are ordinary chromosomes 

 or "autosomes." These chromosomes have in the spermato- 

 genesis the same general history as in other Hemiptera heteroptera. 

 In the first division the idiochromosomes are separate univalents, 

 their position being typically (but not invariably) outside a ring 

 formed by the nine larger bivalents within which lies the small 

 m-chromosome bivalent (Fig. i, d, Photo 2). This division 

 accordingly shows 12 separate chromosomes (one more than the 

 reduced or haploid number.) In the second division, as des- 

 cribed beyond, they are always united to form a dyad or bivalent, 

 composed of two unequal halves, and the number of separate 

 chromosomes is 11. The spermatogonial groups possess 22 chro- 

 mosomes (Fig. I, e) of which the small idiochromosome may often 

 be recognized as the smallest of the chromosomes next to the 

 m-chromosomes; but it does not differ sufficiently in size from the 

 other chromosomes to be always certainly distinguishable.® In 

 the growth period the idiochromosomes, as usual, have the form 

 of condensed deeply-staining chromosome-nucleoli, while the other 

 chromosomes are in a vague, faintly staining condition. They 

 are usually in contact but not fused (Fig. i, /, Photo 25), thus form- 



* In considering the relative size-relations it is important to bear in mind that the apparent size, as 

 seen in polar view, varies considerably with the degree of polar elongation. Still more important is 

 the fact (which I have emphasized in a preceding paper) that in the first division univalent chromosomes 

 always appear relatively much smaller than they do in the spermatogonia. This is the case with the 

 idiochromosomes and the supernumeraries, which are always readily recognizable in the spermatocj^e- 

 divisions, but are often difficult to distinguish in the spermatogonia. 



