microscopic constituents of the animal body. Besides the cells which constitute the animal tissues, there are in the body of man and all vertebrates a number of free cells the white corpuscles of blood and lymph and the wandering cells of the tissues which exhibit all the characters of real amoebæ. Four different varieties of these amœboid cells, usually known under the general name of leucocytes, have been described the distinctions between them being chiefly based upon the shape and the numbers of their nuclei; but the commonest form is that of a speck of protoplasm containing several nuclei which are connected by filaments of nuclear substance, as well as a little radiated sphere which plays such an important part in the bipartition of cells.
The leucocytes of both the higher and the lowest animals have all the distinctive features of simple amoeba?. They protrude pseudopodia, and move about like amoeba? (only the smaller ones, usually described as lymphocytes, possessing this capacity to a smaller extent), and, like amoeba?, they are endowed to a high degree with the capacity of ingesting all kinds of small granules which they find in their way, such as grains of coloring matter suspended in water, and various smaller micro-organisms. It is very easy to observe how leucocytes of the frog, the pigeon, the guinea-pig, and so on, ingest bacilli by surrounding them with their protoplasm; and an immense literature, with illustrations by photographs and correct drawings, has already been published in order to show how various bacteria and micrococci are ingested by leucocytes. In some cases the thus ingested bacilli are digested—that is, transformed into a soluble matter which is assimilated by the protoplasm of the leucocyte, exactly in the same way as an amoeba digests a diatom. In other cases the bacteria are for some time kept alive within the leucocytes, and if the leucocytes have been put into conditions which are unfavorable for themselves but favorable for bacteria, the latter develop, and are set free. It has also been seen pretty often that some bacilli propagate, by means of spores, within the leucocytes, or that the spores which have been kept for some time seemingly without life, begin to develop and give origin to a new generation of bacilli.
- See his paper Immunity, in British Medical Journal, January 31, 1891. Also his last most attractive and profusely illustrated work, Leçons sur la Pathologie comparée de l'lnflammation, Paris, 1802, which can be safely recommended to the general reader, notwithstanding its rather technical title. Its subject is the struggle for life carried on within organisms by the amœboid cells against the microbes.
- See Recent Science in the Nineteenth Century, May, 1892, p. 758. The best morphological description of leucocytes is to be found in Ehrlich's Farbenanalytische Untersuchungen zur Histologic und Klinik des Blutes, Berlin, 1891, quoted by Metchnikoff.
- P. Netschajeff, Ueber die Bedeutung der Leucocyten bei Infection der Organismen, in Archiv für pathologische Anatomie, 1891, Bd. exxv, p. 415.