STUDIES ON CHROMOSOMES 77 



b. The double chromosome. A second interesting feature of the 

 second division that I formerly overlooked is the presence of a 

 remarkable double chromosome which in the metaphase has ex- 

 actly the appearance of a butterfly with widespread wings. This 

 chromosome (which may be called the d-chromosome) is shown in 

 profile view in 2 b-e and 1 a-d, 16, 17, 20, 24, 25. This is the only 

 chromosome in the second division that shows any approach to a 

 quadripartite form, and its characters are so marked as to constitute 

 the most striking single feature of the division. As the figures 

 show, it is one of the largest of all the chromosomes. It always 

 has an asymmetrical tetrad shape, giving exactly the appearance 

 of a smaller and a larger dyad in close union; and it always lies 

 in the outer ring, so placed as to undergo an equal division, and 

 with the larger wings of the butterfly turned towards the axis of 

 the spindle. In polar view (3 j-rri) the duality is far less apparent 

 and sometimes invisible, even upon careful focussing. In N. 

 viridula the duality is always apparent in side view, but the but- 

 terfly shape is usually less evident than in N. hilaris. 



In the initial anaphases the d-chromosome divides symmetri- 

 cally, drawing apart into two bipartite chromosomes (2 j, k, 1 g) ; 

 but this is seldom evident save in profile view. Viewed from the 

 pole the duality does not now ordinarily appear, though it may 

 still sometimes be seen upon careful focussing. In the later ana- 

 phases the two components tend to fuse, and often can no longer 

 be distinguished. Not seldom, however, the duality is visible 

 even in the final anaphases; and sometimes this is so conspicuous 

 that the spermatid-group seems at first sight to comprise eight 

 instead of seven separate chromosomes (n, r, s, t). 



Since the duality of this chromosome does not certainly appear 

 in the spermatogonial groups or in the first spermatocyte-division, 

 its peculiar form in the second division might be supposed to 

 result from some special mechanical relation to the spindle-fibers 

 in that division. This is, however, excluded by examination of 

 the interkinesis, in which the chromosomes are irregularly scat- 

 tered. In these stages, even when the spindle is still very small 

 and the chromosomes lie in a quite irregular group, the butterfly 

 shape is already perfectly evident; and it shows no constancy of 



