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

THE GOMPLEX

sub-cultures growing from a scrap of him, fragments of that eminent personage might, for all we know to the contrary, be living now.

This release of living cells to a free life is not restricted to the primitive connectivetissue cell. It has been possible to get similar strains of cells from muscle, or nerve, or kidney. In such cases the cells lose much of their specialization before they begin to wander about and reproduce : they cease to be typical muscle-cells, nervecells, or kidney-cells, and become more like the primitive connective-tissue cell that we have described above. They cease to be parts, they become wholes.

We may say then that every living tissue consists of cells and that every living cell is a potential individual (or sub-individual), containing in its own body all that is necessary for life. But, be it noted, the cell is a unit: it is not possible to isolate parts of cells and keep them alive. If a cell be cut into halyes—one with the nucleus and centrosphere and the other without—the first, which contains all the elements of the original cell, will live on and reproduce itself ; the defective half, on the other hand, may creep about for a time, but it can neither grow nor divide, and presently and surely it will perish. (The experiments on which this statement is based were made, not on cells from higher animals, but on Amoebae and other Protozoa, which are independent creatures resembling the released tissue-cells of a higher animal very closely.) When cells multiply they divide into two by an elaborate process which ensures that each of the two daughter-cells shall have half the cytoplasm with its various inclusions, half the centrosphere and half the nucleus. Thus each of the two resultant cells has all the essential cell-elements : if this were not the case they would not live any more than a man who lacked lungs or heart could live.

Naturally, when they are parts of a living body the cells are disciplined, they do not wander about where they like, growing actively and reproducing themselves, as the cells in a culture do. An organ such as the brain or liver is like the City during working hours, a tissue-culture is like Regent’s Park on a Bank Holiday, a spectacle of rather futile freedom.

The ordinary activities of organs are summations of the activities of their constituent cells. The pull of the muscle, for example, is in reality the synchronized effort of thousands of muscle-cells, and bile is the pooled outpourings of countless liver-cells. Evidently this co-operation involves control.

BODY-MACHINE

AND HOW IT WORKS

No muscle would be of any use if its cells were allowed to contract independently whenever they liked; they wait, tense and expectant, until the muscle is required to move, when they all join together in a hearty but disciplined pull. Even growth is a summation of cell-activities, for the increase in size of any part is due to the orderly growth and multiplication of its constituent cells “ according to plan.”

We shall deal in later chapters with the processes of reproduction and development.

_ We shall see then that the fertilized egg is a

single cell with cytoplasm, nucleus, centrosphere—all the essential elements, that this cell divides into two, that each daughter-cell divides in its turn, and that in this way, by a process of continued subdivision, all the millions of cells in the body are produced. Naturally, this growth process must be regulated like any other cell-activity. The thumb must not be allowed to grow to the size of a leg. Occasionally a group of cells shakes off the normal controlling influence and grows disproportionately, producing a tumour or a cancer, but normally all the cells are kept well in hand.

We have reached an idea whose importance in biological theory can hardly be overestimated, and we may perhaps be pardoned if we emphasize it. Briefly, it is this. The reader has a feeling of single individuality ; he or she feels and acts as one; the various parts of his or her body work smoothly and harmoniously together. But he or she is also a community, a vast assemblage of invisibly small cells. These cells are living together and they are controlled and specialized in divers ways for the common good ; nevertheless they are themselves individuals, for under suitable conditions they can be detached from the body and kept alive indefinitely.

At this point the reader may protest. “T am myself an indivisible individual,” he or she may say, “for if one of my arms or legs be amputated it cannot survive. How then can my body be a community?” It is of course true that an isolated limb will shortly die; but if while a busy factory was working it was suddenly surrounded by a high brick wall, so that its personnel could not get any food, it would soon be starved out and become still. An isolated limb dies for precisely the same reason. Normally its cells are nourished and supplied with oxygen by the blood; when it is amputated its blood-supply is cut off and so the cells are killed (although as a matter of fact they suffocate before they have time

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