80 EDMUND B. WILSON 



In this division the d-chromosome can not be identified in either 

 species. Figs. 3 e, /, h, i, show all the chromosomes of the two 

 species, in each case from a single spindle in side view. Most 

 of them have a simple bipartite form, but in each species two or 

 three of them often appear more or less distinctly quadripartite 

 as is, of course, often the case with the bivalents in this division. 

 In N. hilaris one of the largest chromosomes is usually more 

 elongated than the others, and each half shows a slight trans- 

 verse constriction. I suspect that this may be the d-chromosome, 

 but cannot establish the identification. 



3. The growth-period and spermatocyte-prophases 



These stages fully bear out the conclusions based upon the 

 divisions and establish the identity of the idiochromosome-pair 

 with the chromatic nucleolus of the growth-period. Throughout 

 the growth-period each nucleus contains a single intensely stain- 

 ing spheroidal chromatic nucleolus and in addition a very large, 

 clearly defined pale plasmosome, which is sometimes double. 

 Series of drawings of these two bodies (in each case from the same 

 nucleus, and in their relative position) are given in figs. 4 i-l and 

 m-p, from cells of the middle growth-period. They are also 

 shown in figs. 26-29. In these stages no sign of duality is to be 

 seen in the chromatic nucleolus, even after long extraction or in 

 saffranin preparations. In later stages, as the chromosomes begin 

 to condense, this nucleolus becomes less regular in outline, and 

 gradually assumes a tetrad form, which becomes very clear as 

 the chromosomes assume their final shape. This transformation 

 may be traced without a break, successive stages being often seen 

 within the same cyst. Just before the nuclear wall breaks down 

 this tetrad is still clearly distinguishable from the others by its 

 asymmetrical quadripartite form, as seen in 4 y, z, which show all 

 the chromosomes (in each case from two successive sections). 

 Figs. 4 q-t show four views of this tetrad at this period in N. 

 hilaris, while u-x are corresponding views of N. viridula. These 

 figures (which might be indefinitely multiplied) show the marked 

 differences between the two species in respect to this tetrad, 

 obviously corresponding to that seen between the idiochromosome 



