Let us see some of the facts upon which this conclusion rests: If a very thin slice of a plant-stem be examined under the microscope, it will be found to be constructed of small units fitted together. If the outer epidermis of a leaf be stripped off and properly magnified, it will likewise show cellular structure. (See illustration.) By extending our observations, any part of the plant may be proved to be constructed of cells or their derivatives. In like manner, if the epidermis of an animal be magnified, it will be seen to be constructed of cell elements. (See illustration.) If animal tissue, for example, the liver, be hardened in alcohol and cut into thin sections, it may also be shown to be made of united cells. Now, cartilage presents us with a modification; in it the cells are separated by considerable lifeless substance in which the cells lie. (See illustration.) This lifeless substance has been secreted by the cells and thrown out around them. It may later have a deposit of earthy matter in it—as in bone—or may undergo other changes. These afford a few illustrations of cells in animals and plants. We must understand, however, that there is considerable variation, both as regards size and shape of the different cells, but they are quite uniform for the same kind of tissue.
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Cells from Epidermis of frog (upper left); Epidermis of leaf (upper right); Cartilage showing matrix (lower left); and fiver (lower right). | |
This theory first took definite form in 1839-40 through publication of the observations of Schleiden and Schwann, and is generally known as the cell theory of Schleiden and Schwann. But cells, especially those of plants, had been observed long before this time. Robert Hooke, the English botanist, in 1665 described the plant-tissue in cork as made up of “little boxes or cells distinct from one another.” Malpighi the Italian, Leeuwenhoek of the Netherlands and Grew of England all made sketches within a few years of this date, to show the cellular structure of plants. These, however, were individual observations, and Schleiden's great step consisted in applying the idea to all plants without exception; and Schwann made a general application to animal tissues. Both these men had a wrong idea regarding the cells. They thought of them as “box-like compartments,” something like the cells in honeycomb or a wasp's nest, and, therefore, looked upon the cell-walls as important; but the idea of the cell changed and gradually came to mean the living particles instead of little boxes. Within the cavity of the cell is a jelly-like, viscid substance that is actually alive while the cell-walls are lifeless. The living substance is protoplasm, and in due time a cell came to be defined as a small mass of protoplasm containing a nucleus, for a nucleus or denser portion of protoplasm was found in all living cells. Finally, about 1860, the original idea of Schleiden and Schwann was completely replaced by the true one. Max Schultze did more than any other one man to establish this idea, which is called the protoplasm theory; but the work was greatly aided by De Bary the botanist and Virchow the pathologist.
In the meantime, a new discovery had been made and partly perfected, viz.: that the egg is a cell. It is a modified cell of the body of the parent, and, with this new fact in mind, we can account for the origin of cells in the body. The egg divides into 2, then into 4, 8, 16, 32 cells, and so on, so that what was a single cell becomes many, and each new cell is derived by division from a formerly living cell. The cells produced in this way become grouped into layers and, later, into tissues. As the tissues grow, the cells become changed, both in form and as regards the work they do, and so we have different kinds of tissue. Now, since all animals arise from eggs, the substance of the egg must contain everything that an animal inherits from its parents, and it follows that heredity, in the long run, is a question to work out on cells.
All activities have been shown, likewise, to take place in the protoplasm of cells; for example, the liver does not act as a whole, but each cell of which it is composed is doing a part of the work of the liver, and the combined action of all represents the work of that organ. This is the case with any other organ in the body, and, therefore, the cell is important in physiology as well as in anatomy and development. Many problems are to be solved by work on cells. The recent
