CHAPTER III

THE CELL, CELL DIVISION, AND THE CELL THEORY

The protoplasm of all higher plants and animals is organized into minute units called cells. Whether we study the microscopic structure of the wood, bark, and leaves of a tree, or that of the muscles, glands, bones, or brains of an animal, what we encounter is a multitude of these cellular units. Higher organisms are composed of cells much as a brick house is composed of bricks. This is a somewhat bald statement of the celebrated cell theory, although, as thus formulated, it cannot be said that there is anything theoretical about it at the present time. It is simply a generalized statement of observed facts. From the standpoint of this doctrine, the cell is a unit of composition modified in different ways in relation to the discharge of different functions. To a certain degree the cell has an individuality of its own, while its form and activities are subordinated to the needs of the whole body. The conception of an organism as composed of cells has played a very important part in the development of biology, although this fruitful generalization is a product of relatively recent times.

The discovery of cells had to wait upon the perfection of the microscope, which made possible the revelation of structures hitherto hidden from the eyes of man. The first clear description of cells was given by that versatile genius, Robert Hooke, who observed in 1665 that cork is composed of "little cells or boxes." Hooke saw only the walls of the cells after their living contents had disappeared, and quite naturally he regarded the cell wall as the essential feature of the cell. The name which he employed, however inappropriate it may be in the light of more recent knowledge, still persists. Cells were soon afterward

described in various kinds of plant tissues by the English botanist, Nehemiah Grew (1672), and in both plants and animals by the Italian anatomist, M. Malpighi, the latter regarding the cell as a kind of unit with its own wall, or utriculus. In 1833 Robert Brown observed in many kinds of plant cells a rounded body, which he called the nucleus, and which contains a very much smaller body, the nucleolus. Although many contributions to our knowledge of the cell, and its rôle in physiology and development, were made by Martin Barry, C. F. Wolff, Leeuwenhoek, Meyen, and Treviranus, the credit for the formulation of the cell theory is usually given to two German investigators, Schleiden and Schwann, whose first writings on the subject were published in 1838 and 1839. These investigators recognized the cell as consisting typically of a cell wall and its contained living substance, inclosing a nucleus. In 1846 this living substance was designated as protoplasm by von Mohl who employed a term previously used by Purkinje in a somewhat different sense. Schleiden and Schwann laid much stress on the wall as an essential constituent of the cell, but the discovery of cells in which a definite cell wall is absent, led to a modification of this conception. As Max Schulze defined it in 1861, a cell is "a bit of living protoplasm containing a nucleus."

Inasmuch as the growth and development of an organism involve an increase in the number of its cells, the attention of the earlier investigators was naturally directed to the problem of how new cells arise. Schleiden and Schwann both held that new cells arise by a sort of condensation of living material from a fluid matrix, the nucleolus forming first, then the nucleus, and finally the surrounding protoplasm with its cell wall. New cells were supposed to form within old cells, or at times, according to Schwann, in the nutrient fluid outside the cells entirely; but the labors of subsequent investigators corrected these errors and showed that new cells are furnished by a division of other cells, a conclusion that was formulated by Virchow in his famous aphorism, "Omnis cellula e cellula." Later it was established that the nucleus of each cell arises by a division of the nucleus of the

parent cell (Omnis nucleus e nucleo). The later developments of the cell theory thus lead us to regard a plant or animal as a sort of aggregation of cells, each of which has arisen by the division of preexisting cells. It was inevitable, therefore, that the cell theory should exert a profound influence upon our conceptions of embryonic development and the whole subject of the perpetuation of life. Schwann showed that the ovum, or egg, which forms the starting point of the development of the individual organism, consists of a single cell. Later the spermatozoön by which the egg is fertilized and stimulated to develop, was shown to be also a single cell, although one of very much smaller size. It soon came to be recognized that the development of the individual from the fertilized egg is brought about by the division of cells, their growth and differentiation, and their orderly arrangement. The physiologist may look upon the functioning of the body as a series of coördinated activities of the numerous little cellular units of which it is composed. Whether we are walking, thinking, or quietly digesting our food, we may regard ourselves as a great congeries of multitudinous little individualities, all performing their several tasks in our muscular, nervous, digestive, and other systems of organs. An organism is analogous to a society whose individual members coöperate in the various activities of social life, and the phrase "cell state" is often employed to point out a fundamental likeness between organic and social activities.

It should be clear, I think, that a proper understanding of fundamental life processes involves an acquaintance with the organization and activities of cells. Let us look, then, at the make-up of a typical cell, which we find to be much as follows: Surrounding the whole is a cell wall, which, in plant cells, usually consists of a carbohydrate called cellulose, which is to be regarded as a product formed by the living protoplasm; in the animal cell the wall is a specialized protoplasmic layer. There are many cells of animals and some cells of plants which are devoid of a permanent surrounding membrane. The protoplasm outside the nucleus is now designated as cytoplasm, and it

exhibits the structure described in the last chapter. In the alveoli of the protoplasmic network, or foam work, there is contained the more fluid cell sap, or other inclusions, such as fat or yolk. In addition to the granules previously described, there may be seen in many cells a minute rounded body called the centrosome, often surrounded by a rounded mass of cyto

FIG. 4-A typical cell. The cytoplasm, or protoplasm surrounding the nucleus, appears in the form of a network, or foam-work, containing various other bodies. (From Wilson's The Cell. Courtesy Macmillan Co.)

plasm, known as the attraction sphere. Both of these bodies play an important part in cell division, and the centrosome, since it commonly arises by division from a preceding centrosome, has often been regarded as a distinct and permanent cell organ. In exceptional cases, however, it has been proven that the centrosome may arise de novo. In plant cells especially there are small refractive bodies, the plastids, which often exhibit the power of multiplication by fission. These bodies are associated

with the formation of starch, chlorophyll, and other cytoplasmic products of the plant cell.

Recently much attention has been devoted to a class of bodies called chondriosomes. These frequently appear in the form of threads, which may be demonstrated in the cytoplasm by the use of appropriate stains. In some cases these threads divide with the division of the cell. There may be still other bodies which divide and thus perpetuate themselves, and it has been conjectured that the minute microsomes which occur FIG. 5-Cells from the root of Fritillaria: in the protoplasmic network,

B, older cells showing vacuoles, or spaces also have this property, but this filled with sap, s; C, still older cells with

larger vacuoles; h, cell wall; k, nucleus; kk, has not been established.

nucleolus; p, protoplasm. (After Sachs.) The nucleus of the cell is generally spherical, and inclosed in a distinct nuclear membrane. The framework of the nucleus consists of a substance called

FIG. 6-Chondriosomes in cells from the pancreas of the rabbit: A, cell in resting state; B, in active state; gr, granules of secretion; m, chondriosomes (mitochondria). (After Hoven.)

linin, which frequently appears as a network, or framework resembling that of the cytoplasm. The interspaces of this network