The Outline of Science, Volume 1

The great naturalist Louis Agassiz once said that the biggest gulf in Organic Nature was that between the unicellular and the multicellular animals (Protozoa and Metazoa). But the gulf was bridged very long ago when sponges, stinging animals, and simple worms were evolved, and showed, for the first time, a "body." What would one not give to be able to account for the making of a body, one of the great steps in evolution! No one knows, but the problem is not altogether obscure. When an ordinary Protozoon or one-celled animal divides into two or more, which is its way of multiplying, the daughter-units thus formed float apart and live independent lives. But there are a few Protozoa in which the daughter-units are not quite separated off from one another, but remain coherent. Thus Volvox, a beautiful green ball, found in some canals and the like, is a colony of a thousand or even ten thousand cells. It has almost formed a body! But in this "colony-making" Protozoon, and in others like it, the component cells are all of one kind, whereas in true multicellular animals there are different kinds of[Pg 68] cells, showing division of labour. There are some other Protozoa in which the nucleus or kernel divides into many nuclei within the cell. This is seen in the Giant Amœba (Pelomyxa), sometimes found in duck-ponds, or the beautiful Opalina, which always lives in the hind part of the frog's food-canal. If a portion of the living matter of these Protozoa should gather round each of the nuclei, then that would be the beginning of a body. It would be still nearer the beginning of a body if division of labour set in, and if there was a setting apart of egg-cells and sperm-cells distinct from body-cells. It was possibly in some such way that animals and plants with a body were first evolved. Two points should be noticed, that body-making is not essentially a matter of size, though it made large size possible. For the body of a many-celled Wheel Animalcule or Rotifer is no bigger than many a Protozoon. Yet the Rotifer—we are thinking of Hydatina—has nine hundred odd cells, whereas the Protozoon has only one, except in forms like Volvox. Secondly, it is a luminous fact that every many-celled animal from sponge to man that multiplies in the ordinary way begins at the beginning again as a "single cell," the fertilised egg-cell. It is, of course, not an ordinary single cell that develops into an earthworm or a butterfly, an eagle, or a man; it is a cell in which a rich inheritance, the fruition of ages, is somehow condensed; but it is interesting to bear in mind the elementary fact that every many-celled creature, reproduced in the ordinary way and not by budding or the like, starts as a fertilised egg-cell. The coherence of the daughter-cells into which the fertilised egg-cell divides is a reminiscence, as it were, of the primeval coherence of daughter-units that made the first body possible.