80 EDMUND B. WILSON 
In this division the d-chromosome can not be identified in either 
species. Figs. 3 e, f, h, 7, 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 7-1 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 g-t show four views of this tetrad at this period in N. 
hilaris, while w-v 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 
