the carriers of hereditary units
which we now believe to be enclosed in them. The chromatin cannot itself
be the hereditary substance, as it afterwards leaves the chromosomes,
and the amount of it is subject to considerable variation in the
nucleus, according to its stage of development. Conjointly with the
materials which take part in the formation of the nuclear spindle and
other processes in the cell, the chromatin accumulates in the resting
nucleus to form the nucleoli.

Naturally connected with the conclusion that the nuclei are the carriers
of hereditary characters in the organism, is the question whether
enucleate organisms can also exist. Phylogenetic considerations give an
affirmative answer to this question. The differentiation into nucleus
and cytoplasm represents a division of labour in the protoplast. A
study of organisms which belong to the lowest class of the organic world
teaches us how this was accomplished. Instead of well-defined nuclei,
scattered granules have been described in the protoplasm of several of
these organisms (Bacteria, Cyanophyceae, Protozoa.), characterised by
the same reactions as nuclear material, provided also with a nuclear
network, but without a limiting membrane. (This is the result of the
work of R. Hertwig and of the most recently published investigations.)
Thus the carriers of hereditary characters may originally have been
distributed in the common protoplasm, afterwards coming together and
eventually assuming a definite form as special organs of the cell. It
may be also assumed that in the protoplasm and in the primitive types
of nucleus, the carriers of the same hereditary unit were represented in
considerable quantity; they became gradually differentiated to an extent
commensurate with newly acquired characters. It was also necessary that,
in proportion as this happened, the mechanism of nuclear division must
be refined. At first processes resembling a simple constriction would
suffice to provide for the distribution of all hereditary units to each
of the products of division, but eventually in both organic kingdoms
nuclear division, which alone insured the qualitative identity of the
products of division, became a more marked feature in the course of
cell-multiplication.

Where direct nuclear division occurs by constriction in the higher
organisms, it does not result in the halving of hereditary units. So far
as my observations go, direct nuclear division occurs in the m