68 ETHEL NICHOLSON BROWNE 



The discrepancy in number was very perplexing until consecu- 

 tive sections of complete spindles were examined as they appeared 

 in side view. It was then discovered that in many cases the dis- 

 crepancy is accounted for by the fact that the second small chromo- 

 some which appears in the center in the 14-type is frequently found 

 attached to the largest chromosome in the 13-type. In figures 29- 

 31 A, B, C are shown serial sections of three spindles which have 

 only one small chromosome in the center, the other small one being 

 attached to the large chromosome forming the compound chro- 

 mosome Ma (macrochromosome -1- small autosome) . Polar views 

 of the compound chromosome are rather difficult to obtain owing 

 to the small size of the smaller component. Such a view, from a 

 spindle cut somewhat obliquely, is given in figure 32 where both 

 components show very clearly. In figures 33, MA, B, C, are 

 shown spindles of the other type, where both small chromosome 

 pairs are in the center and the large chromosome is not compound. 

 In over thirty cases where the chromosomes have been counted 

 in consecutive sections in side view, the apparent 13-type has 

 been found to be due to the attachment of the second small chro- 

 mosome to the large chromosome. It is always this particular 

 chromosome, the largest one, with which the little one is associated. 

 In many cases, however, when only one small chromosome appears 

 in the center, the compound character of the large one cannot be 

 detected, the two components having probably fused beyond rec- 

 ognition. When there are two small ones in the center, there are 

 14 chromosomes, and the large chromosome is never compound. 



Besides the two small chromosomes at the center of the spindle 

 (or one in the center and the other attached to M) it is clear from 

 an inspection of the figures that there is always another small 

 chromosome in the peripheral ring. Attention may also be called 

 to the fact that the largest chromosome is usually longitudinally 

 split, as it is occasionally in N. irrorata (figs. 25-28, 32). 



In the second division, the number of chromosomes is always 

 12, so far as I have observed (figs. 35-37). As in the other two 

 species, an unequal XF-pair is here present in the center of the 

 spindle (fig. 37 -^4), and the components are frequently found 

 side by side, having apparently failed to conjugate (figs. 38, 39). 



