So much for plant cells. Is this principle different in animals? For a long time it was thought that plants and animals were different. But upon investigation it was discovered that animals were comprised of cells just as plants. And not only was this discovered to be true, but also that animal cells corresponded in all respects to plant cells. Hence in animals are to be found cells possessing the cell walls formed of a rather thick membrane, the granular protoplasm or yoke, and the nucleus established in the yoke. The ovum, known as the female egg, is composed of the parts just described. If it is not fertilized when ripe it passes away and dies. If fertilized in a natural way, it enlarges in size and subsequently divides into two cells; and these, passing through similar changes, finally give rise to the various groups of cells from which the body is developed. =The Animal Body a Group Collection.=--The body is, therefore, a mass of cells; not all alike, of course, but grouped together for the purpose of doing certain special kinds of work. In this way we have various groups, with each group a community performing its own function. The brain forms one community; and these cells are concerned with mind acts. The muscle cells are busy in exerting force and action. Another group looks after the secretions and digestive functions, while another group is concerned solely with the function of generation and reproduction. And so it is throughout the body. Both individual cells and group cells are concerned with disease. One cell may be diseased or destroyed, but the surrounding ones may go on just the same. It is when the group is disturbed that the greatest trouble results. =A Word About the Cells.=--The cell always possesses its three parts--membrane, protoplasm, and nucleus. But there is no rule as to the size or shape. Cells may be round or oblong, any shape. Substances pass in and out of the cell walls; and they are in motion, many of them, especially those that line the intestines and the air passages, and the white corpuscles of the blood. More than this, some cells, Dr. Jekyl-like, change their appearance and shape, send out finger-like bodies to catch enemies or food, and even travel all around in the body, often leaving it altogether. BODY TISSUES The animal body contains five forms of tissues: Epithelial, in which the cells are very compact, forming either thin or thick plates; the connective tissue, by which many organs are supported or embedded; muscle tissue, either smooth or striated, and in which the cells are in fibers that contract and shorten; nerve-tissue, that has to do with nerve and ganglion cells by which mental impulses are sent; and blood and lymph tissue or fluid tissues.