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 and a cell may be defined as a bit of protoplasm containing a nucleus. This definition includes naked cells as well as cells with walls.

One-celled Animals.—There are countless millions of animals and plants the existence of which was not suspected until the invention of the microscope several centuries ago. They are one-celled, and hence microscopic in size. It is believed that the large animals and plants are descended from one-celled animals and plants. In fact, each individual plant or animal begins life as a single cell, called an egg cell, and forms its organs by the subdivision of the egg cell into many cells. An egg cell is shown in Fig. 6, and the first stages in the development of an egg cell are shown in Fig. 7.

—Egg cell of mammal with yolk.

—Egg cell subdivides into many cells forming a sphere (morula) containing a liquid. A dimple forms and deepens to form the next stage (gastrula).

The animals to be studied in the first chapter are one-celled animals. To understand them we must learn how they eat, breathe, feel, and move. They are called Protozoans (Greek protos, first; zoon, life). All other animals are composed of many cells and are called Metazoans (Greek meta, beyond or after). The cells composing the mucous membrane in man are shown in Fig. 8. The cellular structure of the leaf of a many-celled plant is illustrated in Fig. 1. (See also Chap. I, Human Biology.)