140 ETHEL NICHOLSON BROWNE 



chromosome is separating into two parts. If so, then the large 

 chromosome of N. insulata is of particular interest since it is 

 composed of a larger and a smaller part and the notch indicates 

 another line of separation. If the unequal parts are in the proc- 

 ess of fusion, an equal division of the chromosome means a re- 

 distribution of particles in the resulting chromosomes; if the un- 

 equal parts are in the process of splitting, then the chromosome 

 is splitting at two points, one split being realized in the first 

 division, the other being only indicated. Various stages in the 

 formation of these large compound chromosomes are to be found 

 in the different species. In the case of the largest chromosome, 

 of N. glauca and of the two largest ones in N. indica, the com- 

 pound nature is evident in the first division. In the case of the 

 largest chromosome of N. insulata and the second large chro- 

 mosome of N. glauca, the compound nature is not noticeable in 

 the metaphase of the first division except in polar view, but is 

 evident in the second division. In the second largest chromosome 

 of N. insulata and the largest chromosome of N. irrorata, a split 

 is noticeable in some cases in polar view of the first division, but 

 the chromosome does not appear double in the second. In the 

 largest chromosomes of N. shooterii and N. undulata there is no 

 evidence of* a compound structure. In general, it may be said 

 that the larger the chromosome is, the greater the tendency is 

 for its component parts to separate. 



b. Sex chromosomes 



In comparing the chromosome groups of the six species of 

 Notonecta, one of the most striking features is in regard to the 

 XY pair. This pair is almost equal, occasionally quite so in 

 N. shooterii, more unequal in N. glauca, still more so in N. in- 

 sulata and N. irrorata, very unequal in N. undulata and ex- 

 tremely so in N. indica. This difference in the size relations of 

 X and Y in the different species is caused apparently both by X 

 and Y. In comparison with the other chromosomes, X is the 

 fourth largest chromosome in N. irrorata, the third in N. glauca, 

 N. insulata (probably — the lack of diploid groups prevents an 



