2O ETHEL NICHOLSON BROWNE. 



give the appearance of being on the same spindle fiber (Fig. 4). 

 There are 13 chromosomes in the second division, a ring of 12 

 chromosomes surrounding the idiochromosome-pair in the center 

 (Figs. 5-10). The spermatogonial number is 26, including two 

 pairs of small chromosomes, the same ones, evidently, which lie 

 in the center in the first division (Fig. n). 



In N. irrorata there are but 13 chromosomes in the first division, 

 a ring of 12 surrounding one small chromosome in the center 

 (Figs. 12-14). In the second division there are 12 chromosomes 

 including the idiochromosome-pair in the center (Figs. 15-19). 

 The spermatogonial number is 24, the smallest pair of chromo- 

 somes corresponding to the small chromosome within the ring 

 in the first division (Fig. 20). 



In N. insulata there are two types of first division groups, 

 occurring in the same testis, in approximately equal numbers. 

 One type has 14 chromosomes, a ring of 12 surrounding two small 

 ones (Figs. 21-23); the other type has but 13 chromosomes, a 

 ring of 12 surrounding only one small one (Figs. 24-26). The 

 apparent discrepancy is accounted for by the fact that the second 

 small chromosome which lies in the center in the 14-group is 

 frequently found in the ij-grotip attached to the largest chromosome. 

 Serial sections of four spindles as seen in side view are given 

 in Figs. 27-30. In the first three series (Figs. 27, 28, 29), there 

 is only one small chromosome in the center of a ring of 12 chromo- 

 somes; the second small one is attached to the largest chromo- 

 some (Ma). In the fourth series (Fig. 30), the two small chro- 

 mosomes lie free in the center of a ring of 12. A polar view 

 showing the compound character of the large chromosome in the 

 13-group is given in Fig. 31 (Ma). No case of such a compound 

 chromosome has been found associated with the 14-group, and 

 some fifty clear cases have been observed in the 13-group. In 

 many cases, however, the compound nature of the large chromo- 

 some could not be determined in the 13-group, the two com- 

 ponents having evidently fused beyond recognition. The fusion 

 must take place in all cases before the second division, for there 

 are always, so far as I have observed, 12 chromosomes in this 

 division, including the idiochromosome-pair in the center of the 

 group (Figs. 32-36). All trace of the original composition of 



