64 
EDMUND B. WILSON 
seen in side views of the late metaphases or earliest anaphases 
(fig. 5^) this chromosome always appears darker and more con- 
spicuous than the others (probably because it is not drawn out 
along the spindle fibers) and owing to this circumstance its history 
during these stages may be followed with an ease and certainty 
of which the figures give but an imperfect idea. As the bipartite 
chromosomes separate in the anaphases the chromsome in ques- 
tion is usually left lagging near the equator of the spindle, though 
not infrequently it lies in one of the daughter groups (figs. 5, h-n). 
In the late anaphases, as the cell-body is dividing, it may be seen 
passing, without constriction, diminution in size, or other sign 
of division, into one of the daughter-nuclei (fig. 50). I desire 
especially to emphasize the fact that these processes are seen 
with such clearness and in so great a number of cells, as to leave 
not the remotest doubt that this chromosome neither divides 
nor separates from an accompanying mate. It is therefore a 
typical odd or accessory chromosome, or unpaired idiochromo- 
some, identical in its general history with that seen in Anasa, 
Protenor, and so many other forms. In the second maturation- 
division, accordingly, one of the daughter-cells in each case re- 
ceives one chromosome less than the other; and since there are 
two classes of secondary spermatocytes, there are four classes of 
spermatids and of spermatozoa. All receive nine ordinary 
chromosomes and one m-chromosome. Two-fourths contain and 
two-fourths lack a second small chromosome ; and each of these two- 
fourths falls into two classes, one containing and one lacking the 
accessory chromosome. These four classes are readily distinguish- 
able in polar views of rather late anaphases, particularly in cases 
where the accessory chromosome lies at the same level as the chro- 
mosomes of one daughter group. Fig. 5p,q show two such daugh- 
ter groups, from the same spindle and in the same section. In 
each case two small chromosomes are present, and one group 
contains 11 chromosomes, the other 12. I could not find a single 
case of the other type (with 10 and 11 chromosomes) in which 
both daughter-groups appear in the same spindle: but two ana- 
phase groups from different spindles are shown in fig. 5r, s, the 
former containing 11 chromosomes, the latter 10. 
