124 ETHEL NICHOLSON BROWNE 



presented by the next largest chromosome (fig. 20), though in 

 the majority of cells this is not the case (figs. 21 and 22). A 

 longitudinal section through the center of the spindle shows the 

 small dyad on the central fibers and the large compound chromo- 

 some and the next largest one on the peripheral fibers (fig. 23). 

 Consecutive sections of a spindle show the appearance of the 

 entire group of 13 in side view, including the small central dyad, 

 the large compound chromosome, the next largest, somewhat 

 similar in appearance, and another small one, a trifle larger than 

 the central one (fig. 24 A,'B, C). Very frequently, the compo- 

 nents of the small central dyad lie on different fibers and are not 

 linearly arranged (fig. 25 B). This has been noted previously 

 in N. irrorata, and likewise occurs in N. shooterii. This pecu- 

 liarity is probably due to the fact that the components of this 

 dyad do not become associated in the late prophase as the com- 

 ponents of the other chromosomes do; even when linearly ar- 

 ranged, the components of this small chromosome are usually 

 considerably separated in the full metaphase. Such a preco- 

 cious separation of the parts of the small central chromosome 

 (the m-chromosome) seems to be characteristic also of the co- 

 reids (Wilson et al.). That the small chromosome? in the cen- 

 ter in figure 25 B are the components of 'one central dyad, and 

 do not represent two small dyads is shown by the fact that these 

 chromosomes are distinctly univalent in contradistinction to the 

 other bivalents, and they are passing to opposite poles; moreover, 

 the complete number of 13 is given, only if these are regarded 

 as one chromosome (fig. 25 A, B). Particular attention has 

 been paid to this question, for if this interpretation be no', correct, 

 the explanation that the difference between the 14 chromosome 

 groups of some species and the 13 groups of other species is due 

 to the presence or absence of the second small chromosome in 

 the center, would not stand. However, I am convinced from a 

 very careful study of many spindles in side and polar view, 

 that there can be no question that these are the components of 

 one small central dyad and are not two separate dyads. 



The large compound chromosome presen:s a variety of ap- 

 pearances as seen in side view (figs, 23, 24 C, 25 A and 26). In 



