Development of a Simple Animal.—Many-celled animals are formed in much the same way as are many-celled seed plants. A common bath sponge, an earthworm, a fish, or a dog,—each and all of them begin life in the same manner. In a many-celled animal the life history begins with a single cell, the fertilized egg. As in the flowering plant, this cell has been formed by the union of two other cells, a tiny (usually motile) cell; the sperm, and a large cell, the egg. After the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the blastula is formed; later this ball sinks in on one side, and a double-walled cup of cells, now called a gastrula, results. Practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg.
In animals the body consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called ectoderm, which later gives rise to the skin, nervous system, etc.; an inner layer, developed from the inner layer of the gastrula, the endoderm, which forms the lining of the digestive organs, etc.; a middle layer, called the mesoderm, lying between the ectoderm and the endoderm, is also found. In higher animals this layer gives rise to muscles, the skeleton, and parts of other internal structures.
Stages in the development of a fertilized egg into the gastrula stage. Read your text, then draw these stages and name each stage.
Photograph of a living vorticella, showing the contractile stalk and the cilia around the mouth. Compare this figure with that of the paramœcium. Which cell shows greater division of labor?
Physiological Division of Labor.—If we compare the amœba and the paramœcium, we find the latter a more complex organism than the former. An amœba may take in food through any part of the body; the paramœcium has a definite gullet; the amœba may use any part of the body for locomotion; the paramœcium has definite parts of the cell, the cilia, fitted for this work. Since the structure of the paramœcium is more complex, we say that it is a "higher" animal. In the vorticella, a still more complex cell, part of the cell has grown out like a stalk, has become contractile, and acts like muscle.
As we look higher in the scale of life, we invariably find that certain parts of a plant or animal are set apart to do certain work, and only that work. Just as in a community of people, there are some men who do rough manual work, others who are skilled workmen, some who are shopkeepers, and still others who are professional men, so among plants and animals, wherever collections of cells live together to form an organism, there is division of labor, some cells being fitted to do one kind of work, while others are fitted to do work of another sort. This is called physiological division of labor.
Enlarged lengthwise section of the hydra, a very simple animal which shows slight division of labor. ba, base; b, bud; m, mouth; ov, ovary; sp, spermary.