l62 Edmund B. Wilson 



Photo ll), 12 chromosomes passing to each pole. The second 

 division immediately follows without the intervention of a "resting 

 stage," and the chromosomes undergo the same regrouping as 

 that described for the 21-chromosome forms. As this takes place, 

 the two idiochromosomes conjugate to form an unequal bivalent 

 (precisely as in Lygaeus or Euschistus); so that when the equato- 

 rial plate reforms but 11 (instead of 12) chromosomes appear in 

 polar view (Fig. 5, a-c, Photo 12). The idiochromosome-biva- 

 lent now usually lies near the center of the group (contrasting 

 with the first division), and the m-chromosome is usually not far 

 from it. Such views are almost indistinguishable from those of 

 the 21-chromosome individuals, since the small idiochromosome 

 is covered by the large one and only appears in side view. In the 

 course of the division the idiochromosome bivalent separates into 

 its two components, which pass to opposite poles, while all the 

 other chromosomes divide equally. The idiochromosomes at first 

 separate more rapidly than the other daughter-chromosomes (Fig. 

 5, /, h), as in other genera, but as the division proceeds the reverse 

 condition prevails, so that the two idiochromosomes are seen lag- 

 ging on the spindle between the diverging daughter groups (Fig. 5 

 /-/). In the later stages one passes to each pole. There is 

 much variation in this process. Often the two move at the same 

 rate so that in the late anaphases one may be seen entering each 

 pole (Fig. k, /, Photo 17). Not uncommonly, however, one or 

 the other lags behind upon the spindle (usually the large one, 

 though Fig. 5, y, shows the reverse case) giving a condition that 

 exactly resembles that seen in the 21-chromosome forms (Fig 5, 

 m, n), but earlier anaphases in the same cysts at once show the 

 difference. It is no less conclusively shown by polar views of the 

 late anaphases, in which each daughter-group is seen to consist 

 of 1 1 chromosomes, ten of which are duplicated in the tw^o while the 

 the eleventh is in one case the large, in the other the small idio- 

 chromosome (Fig. 5, q, r, s, t). 



The difference between the two types is shown with almost equal 

 clearness by the chromosome-nucleoli of the growth-period. In 

 the 21-chromosome type, as already stated, this body is single. A 

 similar appearance is sometimes given in the 22-chromosome indi- 



