STUDY OF CHROMOSOMES IN NOTONECTA 133 



irregular at first sight, but closer examination shows a more or 

 less regular ring of ten chromosomes, with the two smallest ones 

 in the center and two intermediate ones between these and the 

 periphery. This arrangement seems quite constant, but owing 

 to the fact that it is somewhat complex, I can not be sure that 

 it is invariable. A slight shifting of one of the chromosomes 

 from the periphery would serve to confuse the arrangement. 

 In side views, the position of these two ex-centric chromosomes 

 is a little diffcult to determine, except when they happen to lie 

 in the section which includes the two small chromosomes and in 

 a plane determined by the central and outermost chromosomes, 

 as indicated by focusing. The ex-centric chromosomes deter- 

 mined in this way are indicated in light gray in figures 81, 82 

 and 85 B, It appears that these are not the smallest chromo- 

 somes after the central one, but are among the intermediate 

 ones. In the polar views, the large double chromosomes appear 

 almost like crosses; in many cases it is impossible to tell whether 

 the division line between the two curved bars or U's is in line 

 with the diameter of the circle or at right angles to it. In fact 

 the chromosome is sometimes turned so that the line of separa- 

 tion lies midway between these two positions (fig. 87). In the 

 anaphase, a peculiar shape is often assumed by X (fig. 89), but 

 after the two sister elements are separated, each shows again the 

 appearance of being composed of a larger and smaller parts (figs. 

 90 and 91). In these figures are also shown the sister elements 

 of the large compound chromosome (in figure 91, both large 

 ones) , each consisting of two curved bars or open U's, bent away 

 from each other. In figure 92, A, B, C, are given serial sections of 

 a spindle in anaphase, showing all the 14 chromosomes in each 

 sister group. 



The appearance of X- in the early prophase of the first division 

 is even more remarkable than in later stages. Its main central 

 mass is split longitudinallj'^, and at each end of each bar are 

 small knobs attached to the main mass by thin threads. The 



- Professor Wilson has called my attention to his description of the X 

 chromosome in Lygaeus bicrucis ('12), where in the growth period it consists 

 of several, usually three, paired segments, somewhat resembling the X chromo- 

 some of X. indica. 



