Thus was formed a primitive multicellular animal with division of labour. If we imagine it attaching itself to the bottom by its lower pole, we can see that it would easily become a sponge of the simplest kind, a polyp, a coral, or, detaching itself once more, a medusa. If we imagine it swimming ahead in the water or creeping along the ground in such a way as to assume a bilateral symmetrical structure, like a tube, with right and left, back and belly, and an anus behind, we have a worm. This worm developed, under the action of the Darwinian laws, into a star-fish in one case, a crab or insect in another, a snail or mussel in another, and lastly into the amphioxus, which led on through the vertebrates to the human frame. But the mysterious series of forms always remained in the development of the individual from the egg, pointing more or less clearly to the earlier stages: ovum, cluster of cells, ball, two cell-layers in a cup-shaped form, skin, stomach, and mouth. All animals that exhibit this primitive scheme belong to one great stem. It was not until this skin-stomach-mouth animal was formed that the tree branched out—evolving into sessile, creeping, swimming, and other forms. Let us give a name to this phylogenetic (ancestral) form, which stands at the great parting of the ways in the animal world, as embryology proves. Leaving aside its innumerable relatives in the primitive days, it must have differed essentially from all other living things at the time—all the protists and the plants—by its possession of a skin, stomach, and mouth. _Gaster_ is the Greek for stomach. Let us, therefore, call this primitive parent of all the sponges polyps, medusaæ, worms, crustacea, insects, snails, mussels, cephalopods, fishes, salamanders, lizards, birds, mammals, and man, the _gastræa_, the primitive-stomach or primitive-gut animal. The corresponding embryonic form may be distinguished from it as the _gastrula_. There are still many living species of animals that are very little higher in organisation than the gastræa-form. The _Pemmatodiscus gastrulaceus_, discovered by Monticelli in 1895, corresponds entirely to it. And the gastrula is found, as I said, with astonishing regularity in its precise gastræa-form in representatives of all the higher groups of animals.