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

THE LOWLY AND MINUTE

that may already have become apparent to the reader between the anatomy and general behaviour of amoeba and that of a white blood corpuscle, or of an isolated cell creeping in a tissue-culture. Both are of a size and both are masses of living protoplasm without enduring form. It is possible, because of this great similarity of organization, to look upon an ameeba as a single cell which is living by itself instead of being part of a bodycommunity, and for this reason amceba is generally called a singie-celled or ‘ unicellular? organism.

This view is not unanimously held among biologists. Some point out that ameba is more like a whole human body than one of our constituent cells, for it can do a number of things that our own cells cannot do. Itcan, for example, live in a pond, finding and securing its own nourishment. No single human cell could do that, nor could a cell from any of the higher animals or plants. ‘Tissue-cultures have to be carefully nursed and grown in elaborately standardized and sterilized fluids containing appropriate foods. Moreover, the life-history of amoeba is more complicated than that of a cell in a tissue-culture, for, besides simply tearing itself into two halves, it has other methods of reproduction, which we shall describe later. Amoeba, in short, is a whole and independent organism, while the tissue-cell is only a subordinated organism.

Yet it is an attractive comparison. Our white blood cells creep about our bodies and consume things very much as amoebe creep about and feed. And if our own cells lack the selfcompleteness of amceba, that is closely approached by the cells of many lower invertebrate forms. We have already noted what happens when Obelia or a sponge is passed through gauze. We can see the cells that originally belonged to a manycelled creature wandering about by themselves, lost and disorganized like a routed army. ‘They creep about in the manner of amoebe, and they can be seen to clump together gregariously in enormous flocks ; they arrange and specialize and organize themselves so that the clumps, at first formless, gradually shape into individuals of a higher order, into tiny polyps or sponges. Moreover, we can trace a series of animals leading up by gradual stages from these feebly organized cell-communities to our own strictly disciplined bodies. These are facts, and basing ourselves upon them we make the

statement that amceba is an unicellular organism—unicellular because it is like a single cell, organism because it is a complete living thing in itself—a statement which implies that a man is a great number of amceba-like creatures, their independence sacrificed to the common good.

We may most clearly understand this statement by comparing amceba to a naked and solitary savage on a desert island, and a human tissue-cell to a trim, bowler-hatted clerk, trotting with umbrella and attaché-case . to his appointed desk in the city. Both are built in much the same way, and their corresponding parts work in much the same way. They are homologues. But one is without the versatility of the other. The clerk is a specialist in his own particular job, but in specializing himself he has lost touch

Fig. 114. Ameba multiplies by simply tearing itself into

two Amebe (highly magnified).

with many of the basal activities of men. Remove him from the civilization to which he belongs, isolate him on a desert island, and although he can write and calculate more efficiently than the savage, you will have to take the most elaborate care of him, providing him with shelter and clothing and cooked food, or he will surely perish. The savage, on the other hand, can build a hut and scrape together a meal, and his tough skin can survive exposure to the elements. He is a jack-of-all-trades, albeit doing nothing very efficiently, and it is precisely because he is unspecialized that he is independent. One can trace just the same distinction between a free, unspecialized amoeba and an efficient but limited muscle-fibre or nervecell or gland-cell ; the latter three, like the clerk, are accustomed to a division of labour, and have come to depend on other units for the satisfaction of many of their

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