66 ETHEL NICHOLSON BROWNE 



with regard to the XF-pair. The two components frequently 

 fail to conjugate, and lie in the second metaphase side by side, on 

 separate spindle fibers (figs. 8, 9). A little later stage is shown in 

 figure 10 A, where the small component is evidently going to one 

 pole, the large one to the other; these are frequently connected 

 at this time by an oblique fiber (fig. 11). The size relations of the 

 chromosomes are evident from an inspection of figures 5, 6, 7, 9, 

 10. The two smallest chromosomes which were in the center in 

 the first division are now in the peripheral ring. The third small 

 one of the first division was apparently the F-chromosome. The 

 largest chromosome is again evident in the peripheral ring. The 

 X-chromosome is one of the larger chromosomes, probably the 

 third largest. The size relations come out very clearly in figure 

 12 A, B, which are sister anaphase groups from the same spindle. 

 It is apparent from these groups that X is present in one of the 

 resulting cells, Y in the other, and since these cells develop directly 

 into the spermatids and thence into spermatozoa, the latter must 

 be of two types in respect to the chromosome content. 



In the spermatogonia! groups, there are 26 chromosomes 

 (figs. 13, 14), among which a largest and a smallest pair can easily 

 be recognized. There are two pairs of very small ones, evidently 

 corresponding to the two small bivalents in the center of the spin- 

 dle in the first spermatocyte division. There are two very large 

 chromosomes corresponding to the one large one in the haploid 

 groups. Then there is a pair slightly smaller than these and an- 

 other odd large one. This is evidently the X-chromosome, and 

 F is distinguishable as the fifth small chromosome which clearly 

 has no mate of its own size. 



2. Notonecta irrorata. In N. irrorata, the typical arrangement 

 is a ring of 12 chromosomes surrounding one small one (figs. 

 15, 16). The second small chromosome which occurred inside the 

 ring in N. undulata is here lacking. Serial sections of a spindle 

 in side view showing the total number and the typical arrange- 

 ment, are represented in figure 17 A, B, C. A few cases have been 

 observed where the components of the central pair apparently 

 fail to conjugate and lie on separate fibers in the metaphase; 

 these are distinctly univalent in contrast to the other bivalents 



