380 Edmimd B. Wilson. 



and it may now be seen, even before the equatorial plate is formed, 

 that the two idiochromosomes, retaining their characteristic size- 

 relations, have conjugated to form an asymmetrical dyad, which 

 shows the most striking contrast to the six other dyads, all of 

 which have a symmetrical dumb-bell shape (Fig. i^). The seven 

 dyads are now drawn into the equatorial plate, the asymmetrical 

 one invariably lying at the center of a ring formed by the six 

 symmetrical ones (Figs, i/, m, 2k, ^b). These dyads place them- 

 selves with their long axes parallel to that of the spindle, so that 

 when seen in polar view they present a circular or more or less 

 ovoidal outline; if they lie in a slightly oblique position, as is fre- 

 quently the case, they may give a bipartite appearance. Since in 

 polar view the small idiochromosome lies above or below the large 

 one it is usually invisible, and hence only the larger one appears at 

 the center of the equatorial plate (Figs, ii, j, 2/, y). In the early 

 metaphase the chromosomes, especially in Ccenus, are often 

 rather widely separated, so that the equatorial plate may be 

 nearly or quite as wide as in the first division (cf. Figs. 2a, b, 21, /). 

 Such figures might at first sight readily be mistaken for those of 

 the first division, but without exception, in my material, both the 

 chromosomes and the cell-bodies of the 7-chromosome cells are 

 much smaller than those of the 8-chromosome ones; and the com- 

 pleteness of the series and the great number of division-figures 

 that I have had under observation precludes, I think, the possi- 

 bility of error on this point. 



In the ensuing division each of the dyads draws apart into two 

 spheroidal single chromosomes, the peripheral ones dividing 

 equally, while the idiochromosome-dyad separates into its two 

 unequal constituents — invariably, I believe, leading the way in 

 the division (Figs, im, 2/, 3^, g, k). Owing to this fact the unequal 

 division of the central dyad may be seen with unmistakable clear- 

 ness. In polar or slightly oblique view of the late anaphases, 

 when both daughter-groups are visible, the asymmetrical result 

 may plainly be seen. In such figures the two daughter-groups 

 show a most striking contrast to those of the first division, being 

 no longer duplicates of each other, and both showing seven instead 

 of eight chromosomes. Each daughter-group shows, as in the 

 metaphase-group, a ring of six larger chromosomes (now single 

 spheroidal bodies) within which lies at one pole the smaller, at 

 the other pole the larger, of the idiochromosomes (Figs, in, o, 2m, 



