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Updated: September 23, 2025
The lowest cell-bodies develop from one cell; the enormous majority of organic beings are many-celled and among the lower forms these take on similar, and among the higher forms greater variations of, groupings and activities. In the human body for example are bones, muscles, nerves, sinews, ligaments, cartilage, skin, all either made up of cells or originating in them.
But this principle is best illustrated in the other branch of the early many-celled animals, which continued to move about in search of food. Here, as will be expected, we have the main stem of the animal world, and, although the successive stages of development are obscure, certain broad lines that it followed are clear and interesting.
We now reach the realm of the true many-celled animals, or Metazoa, where the biological units are combined to form an organic association displaying many more resemblances to a human society.
Inasmuch as this process still further illustrates the nature of the cell, and has further a most intimate bearing upon the fundamental problem of heredity, it will be necessary for us to consider it here briefly. The sexual reproduction of the many-celled animals is always essentially alike.
Thus a hollow, or fluid-filled, little sphere, like the Volvox, is the natural connecting-link between the microbe and the many-celled body, and may be taken to represent the first important stage in its development. The next important stage is also very clearly exhibited in nature, and is more or less clearly reproduced in the embryonic development of all animals.
A few slides from the aquarium at my side reveal an evolutionary sequence to the heavenly host the simplest of earthly organisms playing fast and loose with the borderland, not only of plants and animals, but of the one and of the many-celled.
The lower one-celled organisms in nature increase by division, just as do the individual cells of a more highly organized, many-celled order of living beings. And in all cases, though death or destruction of the cells is synonymous with the death or destruction of the living organism, the latter in most cases already has recreated itself by reproduction.
It begins its existence as a simple cell, analogous in form to the amoeba, one of the lowest living creatures, and later assumes the gastrula form supposed to have been that of the earliest many-celled animals. From this state it progresses by successive stages, each of which has some relation in form to a lower class.
The graceful Diatom is a morsel of plasm enclosed in a flinty box, often with a very pretty arrangement of the pores and markings. The Desmid has a coat of cellulose, and a less graceful coat of cellulose encloses the Peridinean. Then the social principle enters. First we have loose associations of one-celled plants in a common bed, then closer clusters or many-celled bodies.
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