tic was that they were made of
cells without nucleii, giving the name _Monera_ to the group. As the
method of studying cells improved microscopists learned better methods
of discerning the presence of the nucleus, and as it was done little by
little they began to find the presence of nucleii in cells in which they
had hitherto not been seen. As microscopists now studied one after
another of these animals and plants whose cells had been said to contain
no nucleus, they began to find nucleii in them, until the conclusion was
finally reached that a nucleus is a fundamental part of all active
cells. Old cells which have lost their activity may not show nucleii,
but, so far as we know, all active cells possess these structures, and
apparently no cell can carry on its activity without them. Some cells
have several nucleii, and others have the nuclear matter scattered
through the whole cell instead of being aggregated into a mass; but
nuclear matter the cell must have to carry on its life.

[Illustration: FIG. 24.--A cell cut into three pieces, each containing a
bit of the nucleus. Each continues its life indefinitely, soon acquiring
the form of the original as at _C_.]

Later the experiment was made of depriving cells of their nucleii, and
it still further emphasized the importance of the nucleus. Among
unicellular animals are some which are large enough for direct
manipulation, and it is found that if these cells are cut into pieces
the different pieces will behave very differently in accordance with
whether or not they have within them a piece of the nucleus. All the
pieces are capable of carrying on their life activities for a while. The
pieces of the cell which contain the nucleus of the original cell, or
even a part of it, are capable of carrying on all its life activities
perfectly well. In Fig. 24 is shown such a cell cut into three pieces,
each of which contains a piece of the nucleus. Each carries on its life
activities, feeds, grows and multiplies perfectly well, the life
processes seeming to continue as if nothing had happened. Quite
different is it with fragments which contain none of the nucleus (Fig.
25). These fragments (1 and 3), even though they may be comparatively
large masses of protoplasm, are incapable of carrying on the functions
of their life continuously. For a while they continue to move around and
apparently act like the other fragments, but after a little their life
ceases. They are incapable of a