528 Edmund B. Wilson. 



is sometimes hardly distinguishable from the latter though, as in 

 Fig. 4, /', it can often be identified on careful scrutiny. Second, 

 the small idiochromosome, now only half as large as in the first 

 division, has conjugated in typical fashion with the larger one, 

 so as to be visible, as a rule, only in side view (Fig. 4, /), though 

 careful focusing will often reveal it also in polar view, especially 

 when the idiochromosome-dyad lies in a slightly oblique position. 

 In this way the idiochromosome-dyad has been positively identi- 

 fied in Fig. 4, h, /. In side-view the second division shows with 

 entire clearness the separation of the idiochromosome-dyad into 

 its two unequal constituents, precisely as in Lygaeus, Euschistus, 

 etc., while all the other dyads, including the small heterotropic, 

 divide equally (Fig. 4, /-/). From this it follows that four visibly 

 different classes of spermatid chromosome-groups are formed in 

 equal numbers. Two primary classes are formed that possess 

 respectively fourteen and thirteen chromosomes, according to the 

 presence or absence of the heterotropic chromosome; and each 

 of these falls into two secondary classes, one of which contains 

 the large idiochromosome, the other the small. Although this 

 result necessarily follows from the mode of division, it is not a 

 matter merely of inference, but of observed fact; for with a little 

 pains spindles of both classes in the anaphases may readily be 

 found in a vertical position that show both the sister-groups. 

 Such a pair, from the early anaphase of a fourteen-chromosome 

 spermatocyte, are shown in Fig. 4, m, the two groups exactly 

 corresponding, chromosome by chromosome, except in case of 

 the idiochromosomes (which are shown by focusing to be more 

 widely separated than the others). A similar pair from a some- 

 what later anaphase of the thirteen-chromosome class is shown in 

 Fig. 4, o, the relations being as before save that the heterotropic 

 chromosome is lacking. A pair from a later anaphase of the 

 fourteen-chromosome type is shown in Fig. 4, , showing a prin- 

 cipal ring of ten ordinary chromosomes within which lie four 

 others. Two of these (below) are ordinary chromosomes; the 

 other two are, at one pole the heterotropic and the small idio- 

 chromosome, at the other pole the heterotropic and the large 

 idiochromosome. 



