STUDIES ON CHROMOSOMES Ch 
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, andits characters are so marked asto 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—m) 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 7, 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, 7, 8, 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 
