it existed between the cells or sometimes within them.  For example, the fluid part of the blood is the cytoblastema, the blood corpuscles being the cells.  From this structureless fluid the cells were supposed to arise by a process akin to crystallization.  To be sure, the cells grow in a manner very different from that of a crystal.  A crystal always grows by layers being added upon its outside, while the cells grow by additions within its body.  But this was a minor detail, the essential point being that from a structureless liquid containing proper materials the organized cell separated itself.

This idea of the cytoblastema was early thrown into suspicion, and almost at the time of the announcement of the cell doctrine certain microscopists made the claim that these cells did not come from any structureless medium, but by division from other cells like themselves.  This claim, and its demonstration, was of even greater importance than the 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.