k, which separate from one another and condense so as to produce
chromosomes for a new division, correspond to the segments produced from
the chromosomes of the preceding division. The behaviour of such nuclei
as possess chromosomes of unequal size affords confirmatory evidence of
the permanence of individual chromosomes in corresponding sections of an
apparently uniform nuclear network. Moreover at each stage in division
chromosomes with the same differences in size reappear. Other cases are
known in which thicker portions occur in the substance of the resting
nucleus, and these agree in number with the chromosomes. In this
network, therefore, the individual chromosomes must have retained
their original position. But the chromosomes cannot be regarded as the
ultimate hereditary units in the nuclei, as their number is too small.
Moreover, related species not infrequently show a difference in the
number of their chromosomes, whereas the number of hereditary units
must approximately agree. We thus picture to ourselves the carriers of
hereditary characters as enclosed in the chromosomes; the transmitted
fixed number of chromosomes is for us only the visible expression of
the conception that the number of hereditary units which the chromosomes
carry must be also constant. The ultimate hereditary units may, like
the chromosomes themselves, retain a definite position in the resting
nucleus. Further, it may be assumed that during the separation of the
chromosomes from one another and during their assumption of the rod-like
form, the hereditary units become aggregated in the chromomeres and
that these are characterised by a constant order of succession.
The hereditary units then grow, divide into two and are uniformly
distributed by the fission of the chromosomes between their longitudinal
halves.

As the contraction and rod-like separation of the chromosomes serve
to isnure the transmission of all hereditary units in the products of
division of a nucleus, so, on the other hand, the reticular distension
of each chromosome in the so-called resting nucleus may effect a
separation of the carriers of hereditary units from each other and
facilitate the specific activity of each of them.

In the stages preliminary to their division, the chromosomes become
denser and take up a substance which increases their staining capacity;
this is called chromatin. This substance collects in the chromomeres
and may form the nutritive material for