Metchnikoff's studies, carried out on the various groups of animals, contributed towards the foundation of comparative embryology. Owing to the comparative method, he had made himself familiar not only with the morphological and functional continuity of divers organisms, but also with that of their constituting cells; a comparison between the latter and unicellular beings was inevitable. That is why, having ascertained that the mobile cells of the lower Metazoa absorbed foreign bodies by inclusion, he naturally concluded that that phenomenon was similar to digestion in unicellular beings. Having established the fact of intracellular digestion in lower animals, he extended it to certain cells of the higher animals; thus his phagocyte theory was born. Seeing that unicellular beings, like the mobile cells of Metazoa, englobe, not only food, but foreign bodies, he asked himself whether this was not at the same time a defensive action. Such a possibility brought no surprise to a zoologist, accustomed to see that, in the struggle for existence, animals often devoured their enemies. All the materials for the building up of the phagocyte theory were therefore ready in Metchnikoff's mind when he asked himself, as by an intuition, whether the white globules of our blood, globules so similar to amoebæ, do not play the part of a defensive army in our organism when they envelope in accumulated masses intrusive bodies injurious to the organism. The thought was but the result of a preparatory work already accomplished; it was the butterfly escaping out of the chrysalis. Metchnikoff had recourse to his method of simplification in order to solve the question. The organism of the higher animals being extremely complicated, he went down as far as the transparent larva of the starfish (bipinnaria) in order to watch with his own eyes the phenomena which take place within it. He introduced a rose-thorn into the transparent body of the larva, and noted the next day that the mobile cells in the latter had crowded towards the splinter, like an army rushing to meet a foe. The analogy of this phenomenon with inflammation and the formation of an abscess was striking. Metchnikoff said to himself that since most diseases in the higher animals are accompanied by inflammation and provoked by microbes, it was chiefly against these microbes that our defensive cells had to struggle. He named the defensive cells _phagocytes_. He confirmed his hypothesis by another observation, equally simple. In a little transparent crustacean (Daphnia) infected by a small parasitic fungus, (_Monospora bicuspidata_), he was easily able to observe the struggle between the animal's mobile cells and its parasites.