*How the Cells enable the Body to Grow.*—Every cell is able to take new
material into itself and to add this to the protoplasm. This tends to
increase the amount of the protoplasm, thereby causing the cells to
increase in size. A general increase in the size of the cells has the
effect of increasing the size of the entire body, and this is one way by
which they cause it to grow. There is, however, a fixed limit, varying
with different cells, to the size which they attain, and this is quite
low. (The largest cells are scarcely visible to the naked eye.) Any marked
increase in the size of the body must, therefore, be brought about by
other means. Such a means is found in the formation of new cells, or _cell
reproduction_. The new cells are always formed _by_ and _from_ the old
cells, the essential process being known as _cell-division_.

[Fig. 5]


Fig. 5—Steps in cell-division (after Wilson). Note that the process begins
with the division of the attraction sphere, then involves the nucleus, and
finally separates the main body.


*Cell-Division.*—By dividing, a single cell will, on attaining its growth,
separate into two or more new cells. The process is quite complex and is
imperfectly understood. It is known, however, that the act of separation
is preceded by a series of changes in which the attraction sphere and the
nucleus actively participate, and that, as a result of these changes, the
contents of the old cell are rearranged to form the new cells. Some of the
different stages in the process, as they have been studied under the
microscope, are indicated in Fig. 5.