The first eighteen years of Metschnikoff’s career, after his undergraduate course, were devoted to zoological and embryological investigations. He discovered many important facts, such as the alternation of generations in the parasitic worm of the frog’s lung--_Ascaris nigrovenosa_--and the history of the growth from the egg of sponges and medusæ. In these latter researches he came into contact with the wonderfully active cells, or living corpuscles, which in many low forms of life can be seen by transparency in the living animal. He saw that these corpuscles (as was indeed already known) resemble the well-known amœba, and can take into their soft substance (protoplasm) at all parts of their surface any minute particles and digest them, thus destroying them. In a transparent water-flea Metschnikoff saw these amœba-like, colourless, floating blood-corpuscles swallowing and digesting the spores of a parasitic fungus which had attacked the water-fleas and was causing their death. He came to the conclusion that this is the chief, if not the whole, value of these corpuscles in higher as well as lower animals, in all of which they are very abundant. It was known that when a wound bringing in foreign matter is inflicted on a vertebrate animal the blood-vessels became gorged in the neighbourhood and the colourless corpuscles escape through the walls of the vessels in crowds. Their business in so doing, Metschnikoff showed, is to eat up the foreign matter, and also to eat up and remove the dead, wounded tissue. He therefore called these white or colourless corpuscles ‘phagocytes,’ the eater-cells, and in his beautiful book on Inflammation, published twenty years ago, proved the extreme importance of their activity. At the same time he had shown that they eat up intrusive bacteria and other germs (see figs. 37 to 43); and his work for the last twenty years has mainly consisted in demonstrating that they are the chief, and probably the only, agents at work in either ridding the human body of an attack of disease-causing germs or in warding off even the commencement of an attack, so that the man or animal in which they are fully efficient is ‘immune’--that is to say, cannot be effectively attacked by disease-germs. [Illustration: FIG. 40. FIG. 41. FIG. 42. Fig. 40.--Phagocyte of a guinea-pig in the course of engulphing a very mobile undulating spirillum. Fig. 41.--The same, forty minutes later. Fig. 42.--The same taken half an hour after Fig. 41. (_From_ Metschnikoff’s “Immunity.”)] [Illustration: FIG. 43. A large kind of phagocyte of the guinea-pig, killed and stained for microscopic examination. It shows the large spherical nucleus and three specimens of the spirillum of relapsing-fever which have been engulphed, and are lying within its protoplasm. They would have been slowly digested--that is to say, dissolved by the digestive juices within the phagocyte. (_From_ Metschnikoff’s “Immunity.”)]