As a result of this excessive belief in the efficiency of protoplasm the question of the presence of a nucleus in the cell was for a while looked upon as one of comparatively little importance.  Many cells were found to have nucleii while others did not show their presence, and microscopists therefore believed that the presence of a nucleus was not necessary to constitute a cell.  A German naturalist recognized among lower animals one group whose distinctive characteristic 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 assimilating food and incapable of reproduction, and hence their life cannot continue very long.  Facts like these demonstrate conclusively the vital importance of the nucleus in cell activity, and show us that the cell, with its power of continued life, must be regarded as a combination of protoplasm with its nucleus, and cannot exist without it.  It is not protoplasm, but cell substance, plus cell nucleus, which forms the simplest basis of life.