64 EDMUND B. WILSON 



seen in side views of the late metaphases or earliest anaphases 

 (fig. 50) 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-ri) . 

 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. 



