Page:A Text-book of Animal Physiology.djvu/35

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All living things, great and small, are composed of cells. Animals may be divided into those consisting of a single cell (Protozoa), and those made up of a multitude of cells (Metazoa); but in every case the animal begins as a single cell or ovum from which all the other cells, however different finally from one another either in form or function, are derived by processes of growth and division; and, as will be seen later, the whole organism is at one period made up of cells practically alike in structure and behavior. The history of each individual animal or plant is the resultant of the conjoint histories of each of its cells, as that of a nation is, when complete, the story of the total outcome of the lives of the individuals composing it.

It becomes, therefore, highly important that a clear notion of the characters of the cell be obtained at the outset; and this chapter will be devoted to presenting a general account of the cell.

The cell, whether animal or vegetable, in its most complete form consists of a mass of viscid, semifluid, transparent substance (protoplasm), a cell wall, and a more or less circular body (nucleus) situated generally centrally within; in which, again, is found a similar structure (nucleolus).

This description applies to both the vegetable and the animal cell; but the student will find that the greater proportion of animal cells have no cell wall, and that very few vegetable cells are without it. But there is this great difference between the animal and vegetable cell: the former never has a cellulose wall, while the latter rarely lacks such a covering. In every case the cell wall, whether in animal or vegetable cells, is of greater consistence than the rest of the cell. This is especially true of the vegetable cell.

It is doubtful whether there are any cells without a nucleus, while not a few, especially when young and most active, possess several. The circular form may be regarded as the typical form of both cells and nuclei, and their infinite variety in size and form may be considered as in great part the result of the action of mechanical forces, such as mutual pressure; this is, of course, more especially true of shape. Reduced to its greatest simplicity, then, the cell may be simply a mass of protoplasm with a nucleus.