STUDY OF CHROMOSOMES IN NOTONECTA 137 



Reference should be made to the XY chromosome as described 

 by Stevens ('07) for Drosophila, where there is a third element 

 which looks as though it were separating from the X Y pair in 

 the maturation divisions, although no trace of this was to be 

 found in the diploid groups. Metz ('14), however, who has stud- 

 ied the chromosomes of Drosophila carefully, says that he finds 

 no indication of such a separate element in the X Y pair. 



VII. COMPARISON OF THE CHROMOSOMES IN THE SIX SPECIES 



The chromosome groups of the different species of Notonecta 

 are in general very similar in number, arrangement and size rela- 

 tions. An effort has been made to account for the slight differ- 

 ences which do occur in the different species. 



a. Autosomes 



The typical arrangement of the chromosomes in the first mat- 

 uration division of the six species is a ring of chromosomes of 

 varying sizes, surrounding one or two small chromosomes in the 

 center, and in the second division a ring surrounding the XY 

 pair. The only departures from this grouping are in the case 

 of N. shooterii where one small one is usually a little within the 

 peripheral ring in the first division; and in the case of N. indica, 

 in the first division, where two intermediate ones are somewhat 

 within the ring. The arrangement in N. shooterii may be ex- 

 plained on the assumption that the second small chromosome, 

 which is in the center in N.undulata and N. indica, is in the proc- 

 ess of shifting its position to the outside ring, possibly a stage 

 preparatory to fusion with the largest or some other chromosome, 

 or the reverse, a stage in separation. If this be true, we have 

 represented in N. shooterii another step in the change in number 

 of chromosomes in the different species, comparable with that 

 represented in N. insulata, as pointed out in my former paper. 

 The divergence from the typical arrangement in N. indica may 

 be explained on the ground that this species is an offshoot from 

 X. undulata, and its chromosome grouping correspondingly a 



