The first formation of every animal is accomplished by the agency of cells, and may be illustrated by the egg of any bird or fowl. The exterior consists of a hard shell for protection, and this is lined with a tough skin, to which is fastened the yelk, (which means the _yellow_,) by fibrous strings, as seen at _a_, _a_, in the diagram. In the yelk floats the germ-cell, _b_, which is the point where the formation of the future animal commences. The yelk, being lighter than the white, rises upward, and the germ being still lighter, rises in the yelk. This is to bring both nearer to the vitalizing warmth of the brooding mother. New cells are gradually formed from the nourishing yelk around the germ, each being at first roundish in shape, and having a spot near the centre, called the nucleus. The reason why cells increase must remain a mystery, until we can penetrate the secrets of vital force--probably forever. But the mode in which they multiply is as follows: The first change noticed in a cell, when warmed into vital activity, is the appearance of a second nucleus within it, while the cell gradually becomes oval in form, and then is drawn inward at the middle, like an hour-glass, till the two sides meet. The two portions then divide, and two cells appear, each containing its own germinal nucleus. These both divide again in the same manner, proceeding in the ratio of 2, 4, 8, 16, and so on, until most of the yelk becomes a mass of cells. The central point of this mass, where the animal itself commences to appear, shows, first, a round-shaped figure, which soon assumes form like a pear, and then like a violin. Gradually the busy little cells arrange themselves to build up heart, lungs, brain, stomach, and limbs, for which the yelk and white furnish nutriment. There is a small bag of air fastened to one end inside of the shell; and when the animal is complete, this air is taken into its lungs, life begins, and out walks little chick, all its powers prepared, and ready to run, eat, and enjoy existence. Then, as soon as the animal uses its brain to think and feel, and its muscles to move, the cells which have been made up into these parts begin to decay, while new cells are formed from the blood to take their place. Time with life commences the constant process of decay and renewal all over the body. [Illustration: Fig. 49.] The liquid portion of the blood consists of material formed from food, air, and water. From this material the cells of the blood are formed: first, the white cells, which are incomplete in formation; and then the red cells, which are completed by the addition of the oxygen received from air in the lungs. Fig. 49 represents part of a magnified blood-vessel, _a_, _a_, in which the round cells are the white, and the oblong the red cells, floating in the blood. Surrounding the blood- vessels are the cells forming the adjacent membrane, _bb_, each having a nucleus in its centre.