STUDY OF CHROMOSOMES IN NOTONECTA 139 



termediate step and N. shooterii another. Such a process of 

 fusion and separation of chromosomes is analogous with that de- 

 scribed in the case of the sex chromosomes referred to under N. 

 indica, and in the case of the sex chromosomes with other chro- 

 mosomes in the phasmids (Sinety '01), a few other Orthoptera 

 (McClung '05), one of the mosquitoes (Stevens '11) and Ascaris 

 megalocephala (Boring '09, Boveri '09, Edwards '10). In the 

 different species of Drosophila, Metz ('14) has attributed the 

 difference in number to a separation of certain V-shaped chro- 

 mosomes into single bars. 



In all the species of Notonecta described, there are two hro- 

 mosomes larger than any of the others, particularly striking as 

 four large ones in the diploid groups. Together with X, which 

 is usually" the fifth largest, we find five large chromosomes in the 

 male and six large ones in the female diploid groups. N. irro- 

 rata forms an exception in having seven large chromosomes in 

 the spermatogonial groups. When these chromosomes attain 

 an especially large size, they appear in the spermatocyte divi- 

 sions as large compound chromosomes. These are made up of 

 four curved bars or U's in the first division, so arranged as to 

 form a cross, though this structure is not always evident owing 

 to an overlapping of parts. Two of these bars go to each sister 

 cell, and in the second division, their line of separation lies along 

 the equatorial plane so that one goes to each spermatid. These 

 chromosomes are really typical tetrads whose four parts remain 

 separated until finally distributed in the second division. I have 

 called them double or compound chromosomes because their 

 component parts are distinguishable, giving them a composite 

 appearance, and also because the final element often looks as 

 though it were composed of two equal parts, owing to a notch 

 in the center. Whether this notch is merely a remnant of the 

 original bend in the curved bar, or whether it indicates that the 

 chromosome is in the process of splitting into two equal parts 

 may possibly be determined by work on other species. Since 

 there must be a limit to the size of a chromosome, and since 

 these large ones, especially the largest one in N. glauca, have 

 become quite massive, it seems plausible to suppose that this 



