Such a colony is represented by magosphæra. This is a microscopic globular form, discovered by Professor Haeckel on the coast of Norway. It consists of a large number of conical or pear-shaped individual cells, whose apices are turned toward the centre of the sphere. The cells are cemented together by a mucilaginous substance. Around their exposed larger ends, which form the surface of the sphere, are rows of flagella, by whose united action the colony rolls through the water. After a time each individual absorbs its flagella, the colony is broken up, the different individuals settle to the bottom, and each gives rise by division to a new colony. This group of cells may be considered as a colony or as an individual. Each term is defensible.
Volvox is also a spheroidal organism, composed often of a very large number of flagellated cells. But it differs from magosphæra in certain important respects. In the first place its cells have chlorophyl, the green coloring matter of plants. It lives therefore on unorganized fluid nourishment, carbon dioxide, nitrates, etc. It is a plant. But certain characteristics render it probable that it once lived on solid food and was therefore an animal. For where almost the sole difference between plants and animals is in the fluid or solid character of their food, a change from the one form into the other is not as difficult or improbable as one might naturally think. And plants and animals are here so near together, and travelling by roads so nearly parallel, that, even if volvox never was an animal, it might still serve very well to illustrate a stage through which animals must have passed.
The cells of volvox do not form a solid mass, but have arranged themselves in a single layer on the outer surface of the sphere. For a time, under favorable circumstances, volvox reproduces very much like magosphæra, and each cell can give rise to a new, many-celled individual. But after a time, especially under unfavorable circumstances, a new mode of reproduction appears. Certain cells withdraw from the outer layer into the interior of the colony. Here they are nourished by the other cells and develop into true reproductive elements, eggs and spermatozoa. Fertilization, that is, the union of egg and spermatozoon, or mainly of their nuclei, takes place; and the fertilized egg develops into a new organism. But the other cells, which have been all the time nourishing these, seem now to lack nutriment, strength, or vitality to give rise to a new colony. They die.
We find thus in volvox division of labor and corresponding difference of structure or differentiation; certain cells retain the power of fusing with other corresponding cells, and thus of rejuvenescence and of giving rise to a new organism. And these cells, forming a series through all generations, are evidently immortal like the protozoa. Natural death cannot touch them. These are the reproductive cells. The other cells nourish and transport them and carry on the work of excretion and respiration. These latter correspond practically to our whole body. We call them somatic cells. In volvox they are entirely subservient to, and exist for, the reproductive cells, and die when they have completed their service of these. The body is here only a vehicle for ova. Furthermore, in volvox there has arisen such an interdependence of cells that we can no longer speak of it as a colony. The colony has become an individual by division of labor and the resulting differentiation in structure.
But hydra gives us but a poor idea of the cœlenterata, to which kingdom it belongs. The higher cœlenterata have nearly or quite all the tissues of higher animals—muscular, connective, glandular, etc. And by tissues we mean groups of cells modified in form and structure for the performance of a special work or function. The protozoa developed the cell for all time to come, the cœlenterata developed the tissues which still compose our bodies. But they had them mainly in a diffuse form. A sort of digestive and reproductive system they did possess. But the work of arranging these tissues and condensing them into compact organs was to be done by the next higher group, the worms.
Let us now take a glance at certain stages of embryonic development which correspond to these earliest ancestral forms. We should expect some such correspondence from the fact already stated that the embryonic development of the individual is a brief recapitulation of the ancestral development of the species or larger group. The egg of the lowest vertebrate, amphioxus, shows these changes in a simple and apparently primitive form.
3. IMMATURE EGG-SHELL FROM OVARY OF ECHINODERM. HATSCHEK, FROM HERTWIG.
The fertilized egg of any animal consists of a single cell, a little mass of protoplasm containing a nucleus and surrounded by a structureless membrane. The egg is globular. The nucleus undergoes certain very peculiar, still but little understood, changes and divides into two. The protoplasm also soon divides into two masses clustering each around its own nucleus. The plane of division will be marked around the outside by a circular furrow, but the cells will still remain united by a large part of the membrane which bounds their adjacent, newly formed, internal faces.
Let us suppose that the egg lay so that the first plane of division was vertical and extending north and south. Each cell or half of the egg will divide into two precisely as before. The new plane of division will be vertical, but extending east and west. Each plane passes through the centre of the egg, and the four cells are of the same form and size, like much-rounded quarters of an orange. The third plane will lie horizontal or equatorial, and will divide each of these quarters into an upper and lower octant. The cells keep on dividing rapidly, the eight form sixteen, then thirty-two, etc. The sharp angle by which the cells met at the centre has become rounded off, and has left a little space, the segmentation cavity, filled with fluid in the middle of the embryo. The cells continue to press or be crowded away from the centre and form a layer one cell deep on the surface of the sphere.