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, amoeboid, occupying the centre of the colony and capable of digesting.
It may be interesting to mention here that, in this hypothetical description, Metchnikoff foresaw the existence of similar, but real, beings discovered a year later by Saville Kent, namely, the flagellated colonies of _Protospongia_.
Thus the link between the unicellular and the multicellular beings could be constituted through the intermediary of flagellated colonies on the one hand and, on the other hand, of beings similar to a _phagocytella_. The _indivisible colony_ became the _multicellular individual_.
While studying the genealogy of beings, Metchnikoff continued his researches on intracellular digestion. In 1879, at Naples and at Messina, he was able to establish the fact that the mesodermic cells of many larvæ of Echinodermata and Coelenterata, endowed with a digestive tube, nevertheless contained strange bodies. Therefore, even complicated organisms with a differentiated digestive system could still contain at the same time some primitive cells with an autonomous digestion.
All these researches on the unity of the origin of multicellular beings and their morphological elements, and also those concerning intracellular digestion, were gradually preparing Metchnikoff's mind for the conception of the phagocyte theory.
We spent the summer of 1880 with my family in the country. The cereals were invaded by a harmful beetle, the _Anisoplia austriaca_, which was devastating the country. Metchnikoff took the study of this scourge to heart and tried to find a remedy. He had, the preceding year, observed a dead fly enveloped with a sort of fungus which had evidently been the cause of its death. Hence he conceived the idea that it might be possible to combat harmful insects by provoking epidemics among them. He now returned to this idea; on dead bodies of _Anisoplia_ he found a small fungus, the _muscardine_, which was invading the insects by means of filaments, and he succeeded in infecting healthy beetles.