discovery of the cells. For a number of years, however, the matter
was in dispute, evidence being collected which about equally attested
each view. It was a Scotchman, Dr. Barry, who finally produced evidence
which settled the question from the study of the developing egg.

The essence of his discovery was as follows: The ovum of an animal is a
single cell, and when it begins to develop into an embryo it first
simply divides into two halves, producing two cells (Fig, 8, _a_ and
_b_). Each of these in turn divides, giving four, and by repeated
divisions of this kind there arises a solid mass of smaller cells (Fig.
8, _b_ to _f_,) called the mulberry stage, from its resemblance to a
berry. This is, of course, simply a mass of cells, each derived by
division from the original. As the cells increase in number, the mass
also increases in size by the absorption of nutriment, and the cells
continue dividing until the mass contains thousands of cells. Meantime
the body of the animal is formed out of these cells, and when it is
adult it consists of millions of cells, all of which have been derived
by division from the original cell. In such a history each cell comes
from pre-existing cells and a cytoblastema plays no part.

[Illustration: FIG. 8.--Successive stages in the division of the
developing egg.]

It was impossible, however, for Barry or any other person to follow the
successive divisions of the egg cell through all the stages to the
adult. The divisions can be followed for a short time under the
microscope, but the rest must be a matter of simple inference. It was
argued that since cell origin begins in this way by simple division, and
since the same process can be observed in the adult, it is reasonable to
assume that the same process has continued uninterruptedly, and that
this is the only method of cell origin. But a final demonstration of
this conclusion was not forthcoming for a long time. For many years some
biologists continued to believe that cells can have other origin than
from pre-existing cells. Year by year has the evidence for such "free
cell" origin become less, until the view has been entirely abandoned,
and to-day it is everywhere admitted that new cells always arise from
old ones by direct descent, and thus every cell in the body of an
animal or plant is a direct descendant by division from the original
egg cell.

==The Cell==.--But what is this cell which forms the unit of life, and to
which al