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_.
Metchnikoff, however, discovered among primitive multicellular animals, such as sponges, hydroids, and lower medusæ, a stage of development still more simple than the gastrula; this stage is without a digestive cavity and only assumes the gastrula form in its ulterior evolution. He also made the remarkable discovery that, in the most primitive multicellular animals, the endoderm is formed, not by means of invagination, but by the _migration_ of a number of flagellated cells from one pole of the wall of the blastula into the central cavity. These cells draw in their flagellum, become amoeboid and mobile, multiply by division, fill the cavity of the blastula, and become capable of digesting. They originate the digestive cells of the complete organism and give birth to the mesoderm, which explains how the latter comes to contain a number of devouring cells even though these do not constitute digestive organs properly so called. Metchnikoff gave to that stage the name of _parenchymella_, for the migrating cells constitute the endoderm in the condition of a parenchyma.
The invariable presence of this stage in the simplest multicellular animals, the primitive amoeboid state of the endodermic cells, cases of ulterior transformation of the parenchymella into the gastrula form in certain animals, the absence of a differentiated digestive cavity,--all that proved, according to Metchnikoff, that the parenchymella is more primitive than the gastrula, and is therefore entitled to be considered the prototype of multicellular beings.
He saw a confirmation of this in the fact that primitive adult animals also have no digestive cavity but merely an intracellular digestion (sponges, turbellaria).
He concluded that the common ancestor of multicellular beings was a being constituted by an agglomeration of cells without a digestive cavity, but endowed with intracellular digestion, like that of the "parenchymula" stage of development. He therefore gave to that hypothetical ancestor the name of _parenchymella_.