37$ Edmund B. Wilson. 



In side views of the spindles both species usually show the chro- 

 mosomes of a symmetrical dumb-bell shape (Figs, la, e, 2cT), 

 though one or more of them may appear quadripartite, as is espe- 

 cially common in case of the largest one or macrochromosome 

 (Fig. 2b, i). In both forms all of the eight chromosomes are 

 symmetrically divided in the first mitosis (Figs, if, 2e, /), giving 

 rise to two exactly similar daughter-groups of eight chromosomes 

 each (Figs, i^, h, 2g, /?). The rate at which the daughter- 

 chromosomes separate varies widely in different cases. Fre- 

 quently the idiochromosomes lead the way in the march toward 

 the poles and may be widely separated at a time when one or more 

 of the larger chromosomes are only just separating (Fig. I/), 

 while the macrochromosome often lags behind the others (Fig. 2*); 

 but now and then a spindle shows the reverse condition, the small 

 idiochromosome being the slowest of the group. In the end the 

 daughter-chromosomes come to lie at the same level, and in the 

 final anaphase are drawn more closely together. At this period 

 the grouping of the chromosomes is exactly the same in the two 

 species, the two idiochromosomes lying close together and closely 

 surrounded by a ring formed by the six larger chromosomes. In 

 spindles that lie vertically or slightly obliquely the two daughter- 

 groups may in both species be seen, with the greatest clearness, to 

 be exact duplicates of each other, proving beyond doubt the equal 

 division of all of the eight chromosomes of the first mitosis (Figs. 

 i, h, 2g, /?), and the ske-relations of the chromosomes persist 

 without noticeable change. In Lygaeus at this period the two 

 idiochromosomes are still as a rule clearly separated; in Ccenus 

 this may be the case, but they sometimes lie in close contact, 

 already forming a dyad almost identical in appearance with the 

 central unequal dyad of the second mitosis (Fig. 2/?). 



as accurately as possible, and none of the figures are schematized in this respect, except that in a few 

 cases one or more of the chromosomes have been drawn out of their natural positions in order to avoid 

 confusion in the figures. It should be noted that in the division-stages there is some variation in the 

 actual size of the chromosomes, and this is more or less exaggerated in the figures owing partly to slight 

 differences in position, which cause foreshortening in various degrees, and partly to differences in 

 form (different degrees of elongation of the chromosomes cause corresponding variations in thickness 

 as seen in polar view). No attempt has been made to represent the minuter details of the spindle- 

 fibers or asters, though the figures are but slightly schematized in this respect. Some of the stages of 

 the growth-period (especially stages b, d and /) are difficult to represent adequately in pen drawings; 

 though I have attempted to show them as accurately as possible. 



In all the figures the idiochromosomes are marked ;', the plasmosome p. 



