of unicellular animals. As already noticed, those pieces
which possess a nucleus are able to continue their life and reproduce
themselves, while those without a nucleus are incapable of reproduction.
With greater force still is the fact shown by the process of
fertilization of the egg. The egg is very large and the male
reproductive cell is very small, and the amount of material which the
offspring derives from its mother is very great compared with that which
it derives from its father. But the child inherits equally from father
and mother, and hence we must find the hereditary traits handed down in
some element which the offspring obtains equally from father and mother.
As we have seen (Figs. 34-44), the only element which answers this demand
is the nucleus, and more particularly the chromosomes of the nucleus.
Clearly enough, then, we must look upon the nucleus as the special agent
in reproduction of cells.

Again, we have apparently conclusive evidence that the _nucleus_
controls that part of the assimilative process which we have spoken of
as the constructive processes. The metabolic processes of life are both
constructive and destructive. By the former, the material taken into the
cell in the form of food is built up into cell tissue, such as linin,
microsomes, etc., and, by the latter, these products are to a greater or
less extent broken to pieces again to liberate their energy, and thus
give rise to the activities of the cell. If the destructive processes
were to go on alone the organism might continue to manifest its life
activities for a time until it had exhausted the products stored up in
its body for such purposes, but it would die from the lack of more
material for destruction. Life is not complete without both processes.
Now, in the life of the cell we may apparently attribute the destructive
processes to the cell substance and the constructive processes to the
nucleus. In a cell which has been cut into fragments those pieces
without a nucleus continue to show the ordinary activities of life for a
time, but they do not live very long (Fig. 25). The fragment is unable to
assimilate its food sufficiently to build up more material. So long as
it still retains within itself a sufficiency of already formed tissue
for its destructive metabolism, it can continue to move around actively
and behave like a complete cell, but eventually it dies from starvation.
On the other hand, those fragments which retain a piece