STUDY OF CHROMOSOMES IN NOTONECTA 129 



sion, they do not state the number of chromosomes, but a figure 

 of a polar view shows 12, including one inside the ring, and the 

 'chromosome exceptionelle' having a cross shape, the next larg- 

 est of a similar shape, and two small ones, a group similar to 

 those that I have figured. These authors have apparently over- 

 looked entirely the XY chromosomes, and this may explain 

 their hesitancy in stating the number of chromosomes in the 

 two divisions, which should differ by one with these chromo- 

 somes present, and should be the same if they are absent. 



V. THE CHROMOSOMES OF N. SHOOTERII 



A study of the chromosomes of N. shooterii has shown that 

 the number is the same as in N. undulata, but that the grouping 

 is more like that of N. irrorata and N. glauca. 



a. Observations 



The complete number of chromosomes in N. shooterii in the 

 first spermatocyte division is 14, consisting of one small central 

 one surrounded by a ring of 13 (fig. 51). It is characteristic of 

 this species that a second small chromosome lies usually just 

 within the peripheral ring, but sometimes a little more toward 

 the center (fig. 52). It does not lie typically close to the central 

 one, as is characteristic of N. undulata. The position of these 

 two small chromosomes is more strikingly seen in side view, one 

 of them appearing on the central fibers, and the other one some 

 little distance away on the peripheral fibers (figs. 53 and 54 B). 

 In the actual sections, the position of the second small chromo- 

 some somewhat within the peripheral ring is perfectly evident 

 from the fact that it lies at a different focus from the others in 

 the periphery; this cannot be so well shown in the drawings, al- 

 though I have attempted to do so by different shading. A some- 

 what oblique section perhaps brings this out more clearly (fig. 

 55). It often happens in N. shooterii, as described also for N. 

 irrorata and N. glauca, that the components of the central dyad 

 lie side by side, instead of being linearly arranged (fig. 56). 

 When such a spindle is followed through in consecutive sections, 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 1 



