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know, that the most important part of the cell is not the lifeless wall of cellulose, but the living substance which is found inside the cell wall, making up a large part of the cell body and cell nucleus. To this substance is given the name protoplasm. We know now that the living substance or protoplasm is the essential part, while the wall may be missing, so that in such a case there is no resemblance to a cell or box. Biologists now understand a cell to be a bit of protoplasm (cell body) containing a nucleus (which is a denser portion of the protoplasm).

Protoplasm, when examined with the highest powers of the microscope, appears as a colorless, semifluid substance, in which are often seen solid particles or granules, which are probably little masses of food. The nucleus, as already stated, is commonly found near the center of the cell, and is composed of protoplasm denser than the protoplasm of the rest of the body of the cell. The appearance and composition of the protoplasm surrounding the nucleus, that is, the cell body, may be well represented by raw white of egg; but in making this comparison one should bear in mind that the white of an egg is not living substance.

Within the pro

6. Assimilation, growth, and cell division. toplasm are foods in solution (such as sugar, protein, and mineral matters). These are used by cells in their growth and repair, and in the various kinds of work that they carry on. In the human body, as in plants, the food materials are gradually changed by protoplasm into living substance like itself. To this process is given the name assimilation (Latin, ad to + similis = like). As a result of the process of assimilation the amount of protoplasm of course increases and the cell grows. Were this process to continue indefinitely, cells would come to be large in size. This, however,

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does not occur; for when a cell reaches its normal size, the nucleus divides (Fig. 4), and the halves separate from each other to form two nuclei. The cell body now divides into two parts, and cell walls are formed between the two cells. Thus are produced two cells, each having its own nucleus, and these in

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4, cell before divi- B, cell with divided C, single cell divided
sion.
into two cells.

nucleus.

FIG. 4.-Cell division.

scope a drop of fresh blood,1 we should find that it is not the simple red liquid it seems to be; it consists of solid particles, called blood corpuscles, floating in a watery liquid known as blood plasma. These corpuscles are single cells. Two kinds can be distinguished, which from their color are known as red corpuscles and white corpuscles (Fig. 5).

There are three hundred to seven hundred times as many red corpuscles as white. We shall first consider the white corpuscles. Each consists of a minute bit of protoplasm in which is imbedded a nucleus. These cells of the blood

1 The blood may be easily obtained by tying a cord tightly about the finger and then pricking it with a needle cleaned by an antiseptic like peroxid of hydrogen or by heating it in a flame. A drop of blood is squeezed out upon a glass slide and covered with a thin cover glass.

have a characteristic method of locomotion, in the process of which they change their shape; they can creep along in a direction opposite to that of the blood current, and they

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have even been seen forcing their way through the walls of small blood vessels by pushing out slender processes called false feet. They then wander about in the tissues of the body, and, as we shall soon see, do us great service. The white corpuscles closely resemble in structure and functions a kind of single-celled animal called the Amoeba (A. B.,1 Fig. 120).

10,000,000 red corpuscles could be enclosed in a single layer within this square inch

FIG. 6.-Number of red corpuscles in a

square inch.

The red corpuscles have no power of independent motion. They are circular disks, concave on both surfaces. Some idea of the minute size of these cells may

be gained from the fact that ten millions of them would just about cover a space one inch square. There is no nucleus in the red corpuscles; they are, however, formed from cells having a nucleus.

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8. Cells in other tissues. It has been demonstrated that nerve tissue, muscle tissue, and other building materials of the body are all composed of cells (Fig. 3). A tissue may now be defined as a building material of the body, composed of cells of the same kind.

CHAPTER II

MICROORGANISMS AND THEIR RELATION TO HUMAN

WELFARE

I. STRUCTURE AND FUNCTIONS OF BACTERIA

9. Bacteria: their microscopical appearance and size. In the preceding chapter we considered to some extent the organs, tissues, and cells of the human body. However, before we discuss further the structure and functions of these various parts of our bodies, we shall study in some detail certain microscopic plants which have a most intimate relation to human welfare. Chief among these are the tiny organisms known as bacteria.

Every one is familiar with the fact that if a bouquet of flowers is left for some time in a vase of water, the stems decay and disagreeable odors are given off. This is a common example of the action of bacteria, for all decay is due to the work of these organisms. When we come to examine the flower stems or the putrid water, we find a slimy scum. If we put a drop of this scum on a slide, cover with a cover glass, and examine with the highest powers of the microscope, we usually see many different forms of living things. Some of them appear relatively large, and these, as we have already seen (A. B., Chapter VI), are single-celled animals. A closer examination will disclose countless numbers of very minute,

1 The substance of this section, and several of those that follow, appear in Part I, "Plant Biology." Many teachers, however, find it impracticable to discuss bacteria until the work in human biology is taken up; hence the repetition of this material in this volume.

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