Every animal begins by being _unicellular_, for the egg-cell, the reproducing cell, common to all, corresponds to a unicellular being. It is only after fecundation, when it has become an ovum, that this first cell evolves by dividing itself into consecutive segments, each of which is a new cell. This phenomenon is analogous with the multiplication of unicellular beings through division; only, those segments of the ovum do not separate but constitute a whole under the aspect of a hollow sphere, called a _blastula_, which is the first manifestation of a multicellular being. This blastula is formed of superposed layers, each of which gives birth to specialised organs in the embryo. The outside layer, or _ectoderm_, produces teguments and the nervous system; the internal layer, or _endoderm_, gives birth to endothelial cells, the digestive and internal organs; between those two layers comes a third, intermediary layer, the _mesoderm_, from which the skeleton is developed and also the muscle and blood tissues. The evolution of these layers in Vertebrates was well known, but very little so in Invertebrates, though it is only through the development of inferior forms that the origin and general evolution of living beings can be elucidated. That is why, during many years, the principal theme of Metchnikoff's researches was the comparative study of the embryonic layers of inferior animals and the ulterior fate of their constituting elements. By following this train of thought, he was able to demonstrate that the development of lower animals takes place on the same plan and follows the same laws as that of higher animals; thus, that there is a real communion between all living beings, which is the concrete confirmation of the theory of evolution. By their work, Kovalevsky and Metchnikoff contributed to the foundation of Comparative Embryology. The comparative study of cells produced from the divers embryonic layers, and observations on the ulterior development of the functions of those cells, gradually led Metchnikoff to his theory of phagocytes and to pathological biology. An uninterrupted thread can be followed right through his life-work, from the beginning until the end. In spite of his absorbing work he took great interest in his surroundings, and during this first stay in Italy he became acquainted with two interesting personalities, Bakounine the anarchist and the celebrated physiologist Setchénoff. Both resided at Sorrento. Kovalevsky and Metchnikoff, who greatly desired to know them, decided to call on them, after much hesitation.