The question as to what are the ancestral forms of multicellular animals cannot be solved through direct observation, for there is a lacuna between them and unicellular beings, a lacuna which is due to the disappearance of intermediary forms. It can only be filled by hypotheses, based upon the embryology of those animals which, in their embryonic development, repeat the inferior forms from which they are derived, thus reflecting the general evolution of living beings. It was therefore to the embryology of lower multicellular beings that Metchnikoff turned, in order to endeavour to reconstitute their origin and to show the link between them and unicellular beings. We know that the _ovule_ or primitive genital cell of every animal may be compared to a unicellular organism. After fertilisation the egg undergoes consecutive divisions or segmentation; each segment constitutes a new cell, and their aggregation forms a hollow sphere called a _blastula_, which is similar to a colony of unicellular beings. The blastula differentiates itself into embryonic layers, the _ectoderm_, _endoderm_, and _mesoderm_ already mentioned. In the majority of animals the origin of the first two layers, ectoderm and endoderm, is due to the invagination of one of the poles of the blastula; the invaginated part of the walls forms the internal layer, the endoderm, and lines the cavity produced by invagination; this cavity thus becomes a _digestive_ cavity. This stage of development, called _gastrula_, is similar to a cup with a double wall, of which the outer is the ectoderm and the inner the endoderm. This stage, discovered by Kovalevsky, is to be found in the evolution of most animals and corresponds to the adult stage of some of them. It was consequently considered as the _primitive type_ of multicellular beings. Haeckel founded thereupon his theory of the _gastræa_, according to which the common ancestor of animals was a lower animal, now disappeared, and similar to that stage of development. He therefore gave to this hypothetical animal the name of _gastræa_.