SOMATIC CHROMOSOMES IN TRADESCANTIA 
345 
fine reticulum is the direct outgrowth of the progressive transformation 
shown in figures 5-12, it seems more probable that the "chromatin granules" 
are merely the heavier portions of the alveolated and reticulate chromo- 
somes, and that the lighter "supporting network" consists simply of the 
thinner portions of the same together with the delicate anastomoses. As 
pointed out in the case of Vicia, the finer strands stain much less deeply 
than the coarser portions, so that one easily gains the impression of separate 
chromatin granules connected by delicate threads of another material. 
But in a tapering strand the color grows gradually deeper in passing from 
the thinner to the thicker portion, a fact which indicates that the reticulum 
consists of but a single substance, or, more probably, that the chromatin 
substance is very fluid in consistency and free to diffuse about within 
the other material which composes the framework, as suggested by Gregoire 
(1906). Although much evidence which has been brought forward by 
various workers indicates the presence of two principal and morphologically 
distinct elements in the nuclear reticulum, it is nevertheless probably 
true that the above interpretation will apply to many accounts describing 
autonomous chromatin units on a supporting achromatic network. 
Prophase. As the prophasic changes begin, the reticulum becomes 
somewhat coarser and commences to break up into irregular band-like 
portions (fig. 14). As pointed out in the preceding section, the telophasic 
changes in rapidly multiplying cells may go no further than the stage 
represented in figure 11, where the limits of the chromosomes can easily 
be made out. If, now, such a nucleus should enter upon the prophase, 
there can be little doubt that it would be along the lines of chromosome 
union that the reticulum would break down, since along these lines are the 
delicate anastomoses, which would be the first to give way as the band-like 
portions begin to condense. A reticulum in which the telophasic trans- 
formation has been carried further, as in figure 12, also breaks down along 
its lighter zones. From what has been seen in the case of figure 11, it 
seems evident that these zones for the most part represent the interchromo- 
somal spaces, so that here also argument may be made for the structural 
continuity of the individual chromosomes through the interphase or resting 
stage. In the case of a nucleus with such a fine and uniform reticulum as 
that of figure 13, it is manifestly impossible to determine by direct observa- 
tion whether or not the lines of prophasic separation coincide with those 
of the preceding telophasic union. The evidence for the structural indi- 
viduality of the chromosomes must here be indirect, and such indirect 
evidence is afforded by the many known instances in which the chromosomes 
in successive mitoses, although lost tb view during the resting stages, not 
only remain constant in number but maintain constant differences in size 
and shape. Additional evidence is found in the fact that in cells dividing 
repeatedly in one plane, as in the root meristem, the separate bands or 
chromosomes appear in the prophase with the same orientation as that 
