STUDY OF CHROMOSOMES IN NOTONECTA 125 



general it may be described as consisting of four open U's, with 

 the arms often pulled out transversely, sometimes giving it the 

 appearance of being combined with a smaller chromosome. The 

 darker portions in the drawings represent dark areas seen on the 

 chromosome, probably due to the overlapping of two parts, 

 causing thickenings in these regions. Frequently, when the 

 chromosome is seen in face view, that is, on the top of the spindle 

 instead of at the side, a characteristic double cross is apparent 

 (fig. 27). In polar view, the usual appearance is of two diverg- 

 ing, slightly curved arms or open U's (figs. 20 to 22) ; this proba- 

 bly represents only half of the entire chromosome, for often a 

 more complex structure is evident, consisting of a system of 

 superimposed open U's (fig. 28). In early anaphase, the chro- 

 mosome appears somewhat irregular (figs. 29 and 30), but the 

 structure of each sister half is clear in later anaphase (figs. 31 

 and 32). Each part consists of two slightly curved arms or open 

 U's in contact at the base, and bending away from each other. 

 The peculiarity and irregularity of the chromosome in metaphase 

 is probably due to a difference in the amount of fusion and the 

 arrangement of its component parts. The structure of the 

 next largest chromosome, which I described as sometimes simi- 

 lar in appearance to the largest chromosome in the metaphase, is 

 in the anaphase similar in structure, only the parts are smaller 

 and not quite so curved (figs. 31 and 32)-. These chromosomes 

 in polar views of the anaphase give approximately the same ap- 

 pearance as in side view (fig. 33) ; this figure shows the entire 

 group of chromosomes in polar view, similar in grouping and 

 size relations to the polar metaphase. Occasionally the chro- 

 matic substance from the large chromosome seems to run down 

 over the spindle fibers in the anaphase, giving the figure a very 

 curious appearance (fig. 34). Other observers have noted a 

 thickening of the spindle fibers where attached to the chromo- 

 somes, as though some of the chromatic substance contributed to 

 them. This appearance is usual in Notonecta, as I have repre- 

 sented in the drawings, and it is probably an extreme exaggera- 

 tion of this tendency that is represented in figure 34. 



