=Cell and Protoplasm.=--Both plants and animals are composed of small parts called _cells_. Cells are usually microscopic in size. They have various shapes, as spherical, flat, cylindrical, fiber-like, star-shaped. The living substance of cells is called _protoplasm_. It is a stiff, gluey fluid, _albuminous_ in its nature. Every cell has a denser spot or kernel called a _nucleus_, and in the nucleus is a still smaller speck called a _nucleolus_. Most cells are denser and tougher on the outside, and are said to have a _cell wall_, but many cells are naked, or without a wall. Hence the indispensable part of a cell is not the wall but the nucleus, 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. [Illustration: FIG. 6.--Egg cell of mammal with yolk.] [Illustration: FIG. 7.--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.) [Illustration: FIG. 8.--MUCOUS MEMBRANE formed of one layer of cells. A few cells secrete mucus.] =Method of Classifying Animals.=--The various animals display differences more or less marked. The question arises, are not some of them more closely related than others? We conclude that they are, since the difference between some animals is very slight, while the difference between others is quite marked.