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 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 Amoeba (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.