This original development from the ovum, first into an embryo with the form of a small globe or, more correctly, an oval body consisting of two layers of cells and having a hole at one pole—in other words, a creature with nothing but skin, stomach, and mouth—was found, curiously enough, in other animals besides the medusæ, corals, and sponges. We have the same course of development in representatives of the most varied groups of animals. There are worms, star-fishes, crabs, and snails that develop in the same way. In fact, it was proved in this very year (1867) that the lowest of the vertebrates, the amphioxus (or lancelet), develops in the same way. And this was not all. In the ontogeny of all the higher animals right up to man (inclusive) we find a state of things that most closely resembles the same development. At all events, the fertilised ovum gives rise in all cases to a cluster of cells; this cluster forms something like a flattened or elongated vesicle with a single-layered wall; the single layer of cells is doubled, and in the building up of the body one half makes the external coat or skin and the other half the internal lining or membrane. Haeckel reflected on the whole of the facts, and drew his conclusions. This very curious agreement in the earlier embryonic forms must be interpreted in terms of the biogenetic law. In the case of the higher animals the forms have been profoundly modified by cenogenesis. In the lower animals they are almost or altogether a pure recapitulation of the real primitive course of the development of the animal kingdom. In the earliest times animals were evolved in something like the following way. First, the primitive unicellular protozoa came together and formed crude social bodies, clusters of cells that kept together, but had no special division of labour. As all the members in the cluster pressed to the surface, in order to obtain their food, they came to form, not a solid mass of cells, but a hollow vesicle with a wall of cells. Then the first division of labour set in. Certain cells, those that were situated at the anterior pole, and so were better placed to receive the floating food as the animal moved along, became the eating-cells of the group; they provided nourishment for the others, as the nutritious sap circulated through all the cells in the cluster, as we find in the case of the siphonophores. As these feeding-cells multiplied rapidly at the fore part of the animal, a depression was formed at that pole of the body. In the end the ball or vesicle was doubled in upon itself, until it came to have the form of a cup with a double-layered wall. Externally were the cells in the skin that effected movement and feeling, and afforded protection; inside, forming the internal wall, were the eating- or stomach-cells. An opening remained at the top—the opening of the cup or vase-like body. The food entered by it: it was virtually the “mouth.”