His robust nature, however, triumphed over all these grave complications, and, during his convalescence, he was filled with a joy of living such as he had never experienced before; from that moment his moral and physical balance was completely restored. There was one unpleasant sequel to his illness, an acute affection of the sight (choroiditis), but it fortunately disappeared without leaving any traces, and, in fact, he never suffered again from his eyes, in spite of his constant use of the microscope.
After his recovery he had a renascence of vital intensity; the life instinct developed in him in a high degree; his health became flourishing, his energy and power for work greater than ever, and the pessimism of his youth began to pale before the optimistic dawn of his maturity. However, the relapsing fever had very probably increased, if not started, the cardiac trouble which eventually caused his death.
During the time when Metchnikoff was forbidden the use of the microscope on account of his eye weakness, he studied Ephemeridæ from the point of view of natural selection. He wished to elucidate the manner in which this selection operates during the very short life of those insects: the rudimentary structure of their buccal organs does not allow them to feed themselves, and they have no time to adapt themselves to external conditions.
During the 1875 holidays, at Gmunden and on the Danube, he observed the nuptial flight of the mayflies, a phenomenon which constitutes their short adult existence, preceded by a long period in the larval state. Thousands of these diaphanous, ephemeral insects swarm above the water in a compact cloud; now and then, dead Ephemeridæ fall like snow-flakes, and that is the final and tragic completion of the nuptial flight. Metchnikoff wished to unveil the mechanism of this sudden death, evidently due to a physiological cause; but he obtained no definite results either that year or the following, when he continued his observations in the Caucasus. He realised that the life of these insects was too short to allow him to solve the problems which interested him, and, his eyes now being cured, he went back to his studies on the origin of multicellular beings or metazoa.
He studied the development of inferior sponges and ascertained that they possess the three embryonic layers which correspond to those of other animal types, but that these layers have not the same degree of independence or differentiation. He found that in certain inferior sponges the mesoderm develops before the endoderm and gives birth to it. These two layers, born one from the other, manifest common primordial characters. Therefore he was in no wise surprised to discover that, in these inferior sponges, the amœboid and mobile cells of the mesoderm fulfil digestive functions equally with, and even more than those of the endoderm; in fact, with primitive beings, functional characters are not more strictly delimitated than morphological characters. It is only a more advanced differentiation which separates them.
He connected these new facts with that which he had observed in 1865 in one of the lower worms, the earth planarian Geodesmus bilineatus. This worm is actually without a digestive cavity, for the latter is entirely filled by parenchymatous cells inside which digestion takes place.
By their primitive structure, lower sponges and worms come near the higher Infusoria, to which they are even more closely related by this intercellular digestion which is common to them.
This led Metchnikoff to ask himself whether this was not, generally speaking, the primitive mode of digestion. He carried out numerous researches on this point during the following years, and found the same intercellular digestion in other lower worms, such as the Mesostoma and aquatic planarians, and afterwards in some lower Cœlentera and some Echinoderma. He was thus enabled to establish definitely that the primitive mode of digestion was really intercellular, for the lower multicellular animals either do not possess any digestive cavity or else their digestive cavity develops late, as for instance with lower jelly-fish or with hydropolypi. Even when the cavity is developed in these inferior animals, the digestive functions are fulfilled by the mesodermic cells.
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.