172 Edmund B. Wilson 



4 Individuals with twenty-six Chromosomes; four 

 Supernumeraries 



It will be convenient to consider this type before the 24- and 25- 

 chromosome forms, since the material is more favorable for an 

 account of the remarkable phenomena occurring in the second 

 division. Of these individuals there are seven males and three 

 females, all three species being represented. Unfortunately very 

 few perfectly clear spermatogonial groups are shown; but the 

 spermatocyte-divisions and cells of the growth-period are particu- 

 larly well shown and in large numbers of cells. In all but one of 

 these individuals the four supernumeraries are large and of nearly 

 equal size. In one (femoratus No. 40) two are large and two 

 small. The latter case, already shown in Fig. 2, /-/, is further 

 illustrated by Fig. 9, h, /, y, n, o. Two of these {h and /') show 

 but three supernumeraries in the first division, a common appear- 

 ance in this individual (see p. 186). Fig. 9, a-l, show varying 

 arrangements of the 16 chromosomes that appear in the first 

 division, the most typical ones being k and /. In 9, a-c, k, /, both 

 idiochromosomes and the four supernumeraries lie outside the 

 ring. In 9, gy all but the large idiochromosome are inside the 

 ring. 



In some of these slides the compound chromosome-nucleoli are 

 shown with great distinctness in many cells of the growth-period. 

 This body usually has the form of a flat plate that Hes next the 

 nuclear wall (Fig. 10, q, r) so that a clear view of all the compo- 

 nents can only be had in tangential sections. Thus viewed (Fig. 

 10, s-Uy Photo 28) it may often be seen to consist of six components 

 one of which (the large idiochromosome) is about twice the size of 

 the others and is usually at one side or end of the group. The 

 other five evidently represent the small idiochromosome and 

 the four supernumeraries. In side view (Fig. 10, q, r) not more 

 than three or four of the components, can as a rule be recognized. 

 In a considerable number of cases these six chromosomes are not 

 aggregated to form a single body but form two or more simpler 

 bodies. 



The second division in these forms presents an extraordinary 



