The science of life : fully illustrated in tone and line and including many diagrams

THE GOMPLEX

the viscera ; it consists, we see, of a number of separate minute flat plates of living substance, lying side by side with their edges touching each other, and separated by thin, wavy outlines, and each has in its middle a conspicuous round body called the nucleus. That is one sort of cell. The “tissue” of which it is the living part is called “‘ pavement epithelium.” In other “ tissues ”’muscle, bone, liver stuff or nervous substance, for example—the cells assume other forms. They may be flat or round or cubical or star-shaped or drawn out into thin threads and fibres ; but it is always possible to make out the essential fact, that the living substance of the tissue we examine, whatever part of the body it comes from, is an aggregate of an enormous number of cells. In the following chapters we shall meet many different kinds of cells. In the developing young every one of these extreme forms began as a simple one, like that to be immediately described. We may compare the body to a community, and the cells to the individuals of which this vast organized population is composed.

It is very important to realize that this is not a merely allegorical comparison. It is a statement of proven fact, for—we resort here to the stress of italics—single cells can be isolated from the rest of the body, and kept alive.

This is a branch of biological investigation only recently developed. Its fascination is evident. The procedure is to take a minute

fragment of living tissue from an animal g and to put it in a drop of blood-serum (the 2

liquid left over after blood has clotted) from the same species. In order to make microscopical examination of the fragment possible, the drop is suspended from a thin square of glass, a “ coverslip,’ over a little depression excavated in a thicker glass slip, the microscope slide. (It is necessary to make the most elaborate precautions, we may note, against bacterial infection when making such a ‘‘ culture,” and there are various other matters that must be attended to if the cells are to be kept alive and active —for example, if the tissue is taken from a warm-blooded animal it must be kept in an incubator at a carefully-regulated temperature—so that the work demands. considerable experience and skill, and elaborate experimental equipment.) After a few hours, if the culture is successful, some of the cells will wander out of the fragment of tissue into the surrounding liquid. They move about in that liquid by themselves, and while they are creeping over-the cover-glass it is possible to examine them with the microscope

BODY-MACHINE AND HOW IT WORKS

and to take note of the most intimate details of their structure. The clearest observations are made by letting an intense beam of light fall obliquely on the cell; the surfaces of the various structures inside it reflect the light and are seen as brilliantly luminous objects on a dead-black background.

A cell is a tiny, flat, irregularly-shaped lump of protoplasm. It is not bounded by any visible wall ; there is just a simple interface (the meaning of this useful new word is too plain to need definition) separating the cell-substance from the surrounding fluid. The outline is constantly changing. Sometimes it changes so rapidly that one can watch bulges being thrust out of the cell, and withdrawn again; sometimes so

A piece of the membrane that lines the abdomen highly magnified and revealed as a smooth pavement of thin flat cells.

Fig. 9.

slowly that ohe must make accurate drawings—or, better, photographs—every few minutes, and compare them with each other before any change can be detected. By means of such movements the cell creeps

slowly about. a The ground-substance, which is called the cytoplasm, is a fluid as clear as glass. Somewhere about the middle of the cell is the large, rounded nucleus ; floating in the transparent cytoplasm round the nucleus are numbers of smaller bodies of various kinds. These bodies are seldom at rest ; in the living cell incessant movement is the rule. Some of the bodies are carried passively hither and thither, by streaming movements of the cytoplasm, while others 29