352 EDMUND B. WILSON 



fully established by study of the growth-period and the pro- 

 phases) ; for, were one or both bivalent, the spermatogonial groups 

 should show one or two corresponding pairs, which is not the case, 

 In Lygaeus (fig. 7) the grouping is the same, but only six bivalents 

 are present, and the sex-chromosomes are conspicuously unequal 

 in size. 



The composition of these chromosomes is better seen in smears 

 than in sections, and better in Protenor (described beyond) than 

 in either of these forms. In sections, side views of the full meta- 

 phases (figs. 14 to 17, photo. 4) usually show all of the chromosomes 

 as simple dumb-bell figures, though indications of a quadripartite 

 form sometimes appear. In smears the bivalents are often seen 

 to be quadripartite, owing to the presence of a longitudinal split 

 in addition to the transverse constriction; but this never appears 

 in the univalents. All the chromosomes alike divide 'trans- 

 versely' that is, across the constriction of the dumb-bell. In 

 case of the bivalents, therefore, the early anaphase-chromosomes 

 are double bodies, while the sex-chromosomes are single, but this 

 contrast only appears clearly in smears, owing to the close union 

 of the two halves. In this respect the relations are less clearly 

 seen in these forms than in some others, such as Anax, where the 

 anaphase-chromosomes are clearly double (cf. Lefevre and Mc- 

 Gill, '08), or Aprophora, where the same condition is conspicuously 

 shown (Stevens, '06). 



The anaphases are of particular interest because, as has been 

 mentioned, a conjugation between the X- and F-chromosomes 

 takes place in the later stages. As. the division begins and the 

 daughter-chromosomes are separating, a marked contrast in form 

 often appears between the sex-chromosomes and the autosomes 

 (figs. 19 to 21). The latter are more or less extended transversely 

 and often show a slight constriction, thus giving evidence of their 

 double nature, which is accentuated by the very conspicuous 

 double fibres by which they are connected. The latter are so 

 thick, and stain so deeply, as to appear as if spun out from the 

 chromosomes themselves (as has been noted by other observers). 

 On the other hand, the sex-chromosomes do not show such a con- 

 striction, remaining nearly circular in outline, while the connect- 



