Schwann and his immediate followers, while recognizing that the bodies of animals and plants were composed of cells, were at a loss to explain how these cells arose. The belief held at first was that there existed in the bodies of animals and plants a structureless substance which formed the basis out of which the cells develop, in somewhat the same way that crystals arise from a mother liquid. This supposed substance Schwann called the _cytoblastema_, and he thought 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.]