Physiology: The Science of the Body

by Ernest G. Martin

We now know that the nucleus, or rather the chromatin that it contains, plays a remarkable and interesting rôle in the life of the cell. To this we shall return presently. The remainder of the protoplasm, outside of the nucleus, shows the greatest possible variety of form, according to the kind of cell at which we happen to be looking. In some of the simpler types this part of the protoplasm seems to be merely a nearly uniform mass, perhaps with tiny particles scattered through it. In other types the protoplasm is drawn out into long slender threads, and these threads may have many branches; or the protoplasm may be distorted into a thin shell inclosing a mass of fat; or it may be subdivided into dense and thin portions with sharp lines of division between them. These various forms are related to the special functions which the cells have, and we shall learn more about them as we take up the different functions in order. On the whole, study of cell structure shows clearly that the protoplasm outside the nucleus carries on the greater part of the metabolism or power development, and is correspondingly important as the seat of the special functions shown by the cell. If it is a muscle cell, this is the part that does the moving; if a gland cell, this is the part that secretes. Nevertheless, the nucleus is a vital part of the cell. It has been definitely proven that a cell from which the nucleus is lost cannot survive more than a brief time. To gain some idea of the actual part played by the nucleus, we shall have to return to it in some detail.

Before undertaking a further description of the nucleus itself, we shall be helped to an understanding of its function if we trace briefly the history of the cells which make up our body. At the beginning, as we probably all know, we start life as a single cell. This cell, after a series of events which will be described in a later chapter, begins the process known as development. Development consists of a series of subdivisions of cell material. At first the single cell divides into two; each of these then divides, giving four. At the next stage eight are formed, then sixteen and so on, until finally the millions of cells that make up the body are produced, all derived from the original single cell. We know that in the adult body there are very many different kinds of cells. Since they are all derived from a single cell, these differences must have put in their appearance during the course of the various cell divisions. In fact, this happens all along; at definite points in the process the two cells that come from the subdivision of some particular one will not be alike. The special kinds of cells that are thus produced become the starting points for whole masses of similar cells in the fully developed body. In human beings, and probably in most other kinds of animals, the very first subdivision does not result in any difference between the cells. The proof of this is that sometimes, in fact fairly often, the two cells become separated. When this happens twinning results, and the twins are exactly alike, being known as “identical twins.” Not only are they alike in all other respects, but they are always of the same sex, a fact that has escaped the attention of some writers of fiction, who have made twins, identical in all other features, brother and sister, instead of both boys or both girls. Twins that are not identical come from different original cells that happened to start developing together. Such twins need have no more resemblance than any members of the same family, and may or may not be of the same sex.

In every cell division the first step consists in a division of the chromatin of the nucleus, which is followed by a division of the rest of the protoplasm. The process by which the chromatin is subdivided is so curious as to be worth a brief description. The