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.
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 amœboid 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 amœboid 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.
Later, in 1886, he definitely formulated his theory of the genesis of multicellular beings, and having already stated the phagocyte theory, he substituted for the name parenchymella that of phagocytella, which indicated at the same time the primitive mode of digestion of that hypothetical ancestor.
Reduced to its simplest form, it presented, according to Metchnikoff, a certain analogy with a colony composed of unicellular beings of two kinds: the first, flagellated, forming the external layer, and the others, amœboid, occupying the centre of the colony and capable of digesting.