180 Edmund B. Wilson 



are shown in Fig. 9, /?, q. Fig. 9, m, shows a spermatogonial group 

 from fern., No. 42, that is abnormal in showing with perfect clear- 

 ness 27 instead of 26 chromosomes (cf. Fig. 2, /z). 



5 Individuals with twenty-four Chromosomes; two 

 Supernumeraries 



The material for these individuals and those of the 25-chromo- 

 some class, is less satisfactory than in the preceding case, but the 

 relations are undoubtedly quite analogous to those just described. 

 The 24-chromosome class is represented by 9 males and 4 females, 

 and occurs in all three species. In one of the males one of the 

 supernumeraries is large (of the same size as a small idiochromo- 

 some) and one small; in all the others both are large. Additional 

 figures of the first division, showing variations in the grouping, are 

 given in Fig. II, a-g; of spermatogonial groups in Figs, n, i-r. 

 Of particular interest is the male, term., No. 22, shown in Photo 30 

 and in Fig. 1 1, p-r. This individual was, unfortunately, immature 

 showing only spermatogonia and cells in the growth-period; but 

 many perfectly clear spermatogonial groups are shown. These 

 groups uniformly show 24 chromosomes, of which three are very 

 small, while in many cases two others are slightly but distinctly 

 smaller than the others. The latter are evidently the small idio- 

 chromosome and the larger supernumerary, while the three small 

 ones represent the w-chromosomes and the small supernumerary. 



In the second division the two idiochromosomes and the super- 

 numeraries are frequently united to form a tetrad element, various 

 forms of which are shown in Fig. n, s-w. The distribution of 

 these four components is not so well shown in this material as in 

 that of the 26-chromosome class, described above. It is, however, 

 clear that this distribution is inconstant. In cases like those shown 

 in Fig. n, j, t, it is probable that the tetrad divides in the middle, 

 so that each idiochromosome is accompanied by a supernumerary, 

 and each pole receives 12 chromosomes. The cases shown in 

 Fig. n, <y, w, prove however that this is not always the case; for 

 in w the large idiochromosome is seen passing to one pole while 

 both supernumeraries, attached to the small idiochromosome, 



