organic connexion, each of these cells is in itself an
"individual," capable of self-nourishment, reproduction, and, generally,
of independent existence. Consequently, when the growth of a Protozooen
ends in a division of its substance, the two parts wander away from each
other as separate organisms. (Fig. 27.)

[Illustration: FIG. 27.--Fission of a Protozooen. In the left-hand
drawing the process is represented as having advanced sufficiently
far to have caused a division and segregation both of the nucleus
and the vesicle. In the right-hand drawing the process is
represented as complete. _n_, N, severed nucleus; _vc_, severed
vesicle; _ps_, pseudopodia; _f_, ingested food.]

The next point we have to observe is, that in all cases where a cell or
a Protozooen multiplies by way of fissiparous division, the process
begins in the nucleus. If the nucleus divides into two parts, the whole
cell will eventually divide into two parts, each of which retains a
portion of the original nucleus, as represented in the above figure. If
the nucleus divides into three, four, or even, as happens in the
development of some embryonic tissues, into as many as six parts, the
cell will subdivide into a corresponding number, each retaining a
portion of the nucleus. Therefore, in all cases of fissiparous division,
the seat or origin of the process is the nucleus.

Thus far, then, the phenomena of multiplication are identical in all the
lowest or unicellular organisms, and in the constituent cells of all the
higher or multicellular. And this is the first point which I desire to
make apparent. For where the object is to prove a continuity between the
phenomena of growth and reproduction, it is of primary importance to
show--1st, that there is such a continuity in the case of all the
unicellular organisms, and, 2nd, that there are all the above points of
resemblance between the multiplication of cells in the unicellular and
in the multicellular organisms.

It remains to consider the points of difference, and, if possible, to
show that these do not go to disprove the doctrine of continuity which
the points of resemblance so forcibly indicate.

The first point of difference obviously is, that in the case of all the
multicellular organisms the two or more "daughter-cells," which are
produced by division of the "mother-cell," do not wander away from one
another; but, as a rule, they continue to be held in more or less clo