MALE GERM CELLS IN NOTONECTA 67 



(fig. 18). Their behavior resembles that of the components of 

 the A^F-pair in the second division, which may or may not con- 

 jugate before going to the poles; and it is also analogous to that 

 of the ?^ -chromosomes of the Coreidae which conjugate very late 

 and do not fuse. As in N. undulata, one chromosome in the 

 peripheral ring is larger than the others, and it is here in some 

 cases longitudinally split (fig. 16). There are in this species two 

 small chromosomes in the peripheral ring. 



In the second division, the presence of an unequal XF-pair 

 in the center of the spindle is e\ddent from side views (fig. 19 B). 

 Serial sections of a spindle in side view (fig. 19 A, B, C), and 

 polar views (fig. 20), show that there are 12 chromosomes, in 

 contrast to the 13 of N. undulata. Here too, the components 

 of the XF-pair may fail to conjugate before the second division, 

 and lie on separate spindle fillers in the center of the spindle 

 (figs. 21 B, 22). It is evident that A^ and F are less unequal in 

 size than in N. undulata, X being comparati\'ely smaller and F 

 larger. The largest chromosome is distinguishable among the 

 others, and also the three small ones of the first division (the one 

 in the center and the two peripheral ones). The fact that the 

 result of this division will be two kinds of cells (ultimately sper- 

 matozoa) differing in chromatin content in respect to one chro- 

 mosome is apparent from sister anaphase groups (fig. 23 A, B). 



Only one clear spermatogonial group has been found (fig. 24) ; 

 the number here is 24, including three pairs of small chromosomes, 

 corresponding to the three small ones of the spermatocyte 

 divisions; the largest pair corresponding to the large one of the 

 haploid groups; two other large pairs, and one odd large one. 

 This is doubtless the X-chromosome; the F-chromosome is indis- 

 tinguishable, but must be one of the smaller intermediate ones. 



3. Notonecta insulata. N. insulata has proved an extremely 

 interesting species from the fact that two distinct types of chro- 

 mosome groups occur in the first division, in approximately equal 

 numbers and side by side in the same cyst. One type has 14 

 chromosomes including two small ones in the center, like N. undu- 

 lata (figs. 25, 26) ; the other type has 13 chromosomes, including 

 only one small one in the center, like N. irrorata (figs. 27, 28). 



