I was too excited to sleep that night in the expectation of the result of my experiment, and very early the next morning I ascertained that it had fully succeeded. That experiment formed the basis of the phagocyte theory, to the development of which I devoted the next twenty-five years of my life. This very simple experiment struck Metchnikoff by its intimate similarity with the phenomenon which takes place in the formation of pus, the diapedesis[15] of inflammation in man and the higher animals. The white blood corpuscles, or _leucocytes_, which constitute pus, are mobile mesodermic cells. But, while with higher animals the phenomenon is complicated by the existence of blood-vessels and a nervous system, in a star-fish larva, devoid of those organs, the same phenomenon is reduced to the accumulation of mobile cells around the splinter. This proves that the essence of inflammation consists in the reaction of the mobile cells, whilst vascular and nervous intervention has but a secondary significance. Therefore, if the phenomenon is considered in its simplest expression, inflammation is merely _a reaction of the mesodermic cells against an external agent_. [15] Migration of the white blood corpuscles (leucocytes) through the walls of blood-vessels. Metchnikoff then reasoned as follows: In man, microbes are usually the cause which provokes inflammation; therefore it is against those intruders that the mobile mesodermic cells have to strive. These mobile cells must destroy the microbes by digesting them and thus bring about a cure. Inflammation is thus a _curative reaction_ of the organism, and morbid symptoms are no other than the signs of the struggle between the mesodermic cells and the microbes. In order to verify these conjectures, he started studying the englobing of microbes by mesodermic cells in larvæ and in other marine invertebrates which he inoculated. At that time, a well-known German scientist, Kleinenberg, was Professor of Zoology at Messina. Metchnikoff imparted his ideas to him and showed him his experiments. Kleinenberg encouraged him very much; he looked upon his theory as "an Hippocratic thought" and advised him to publish it at once. Metchnikoff was also greatly encouraged by Virchow, who happened to pass through Messina and came to see his preparations and his experiments, which seemed to him conclusive. However, Virchow advised him to proceed with the greatest prudence in their interpretation, as, he said, the theory of inflammation admitted in contemporary medicine was exactly contrary to Metchnikoff's. It was believed that the leucocytes, far from destroying microbes, spread them by carrying them and by forming a medium favourable to their growth. Metchnikoff always preserved a deep gratitude towards Virchow and Kleinenberg for the moral support which they gave him at that time. * * * * *