138 ETHEL NICHOLSON BROWNE 



modification of that of N. undulata, indicated by the displace- 

 ment of two of the chromosomes from the outer ring. 



With regard to number, the six species may be grouped into 

 two classes, one class having a diploid number of 26, first sper- 

 matocyte number of 14, second spermatocyte number 13, and 

 the other class with a diploid number of 24, first spermatocyte 

 number 13, second spermatocyte number 12. To the first class 

 belong N. undulata, N. shooterii and N. indica; to the second 

 class belong N. irrorata and N. glauca. N. insulata belongs 

 rather to the latter group, though it is somewhat intermediate, 

 since many of the first division cells show 14 chromosomes. 

 The difference in number in the two classes can probably be at- 

 tributed, as previously suggested, to the presence or absence of 

 a particular small chromosome which, when present, lies in the 

 center of the group in the first division, as in the case of N. undu- 

 lata and N. indica, or just within the peripheral ring as in N. 

 shooterii. In N. insulata its presence or absence can be directly 

 attributed to its separation from or its fusion with the largest 

 chromosome, and I have attributed its absence in N. irrorata to 

 its permanent fusion with the largest chromosome. This expla- 

 nation will likewise apply to N, glauca, and I believe that this 

 is the correct explanation. There is, however, another possi- 

 bility which may apply to both of these cases or to one of them. 

 This is that the second small chromosome in present in the periph- 

 eral ring, and one of the other peripheral chromosomes repre- 

 sents two chromosomes of the 14-group. In this event, N. shoot- 

 erii represents the step in transition, the small chromosome be- 

 ing intermediate in position between the central and peripheral 

 positions. There is a small chromosome in the peripheral ring 

 in both N. glauca and N. irrorata, but there is nothing in its po- 

 sition or behavior to show any relationship with the one in the 

 center in the 14-groups, as is the case in N. shooterii. Moreover, 

 there is a small chromosome in the periphery in N. insulata even 

 when the two small ones are present in the center. It seems 

 therefore more plausible to asssume that in N. irrorata and in 

 N. glauca, the fusion of the second small chromosome and the 

 largest one is permanent, and that N. insulata presents one in- 



