Such is the result to which have converged the researches of the biologists who have examined plants or the lower animals, as well as of the anatomists who have been more especially occupied with the vertebrates and with man. All their researches have brought them to the same conclusion—the cellular theory. Either living beings are composed of a single cell—as is the case with the microscopic animals called _protozoa_, and the microscopic vegetables called _protophytes_—or, they are cellular complexes, _metazoa_ or _metaphytes_—that is to say, associations of these microscopic organic units which are called cells. _The Law of the Composition of Organisms._—The law of the composition of organisms was discovered in 1838 by Schleiden and Schwann. From that time up to 1875 it may be said that micrographers have spent their time in examining every organ and every tissue, muscular, glandular, conjunctive, nervous, etc., and in showing that in spite of their varieties of aspect and form, of the complexity of structures due to cohesion and fusion, they all resolve into the common element, the cell. Contemporary anatomists, Koelliker, Max Schultze, and Ranvier, have thus established the generality of the cellular constitution, while zoologists and botanists confirm the same law for all animals and vegetables, and exhibit them all as either unicellular or multicellular. _The Cellular Origin of Complex Beings._—At the same time embryogenic researches showed that all beings spring from a corpuscle of the same type. Going back in the history of their development to the most remote period, we find a cell of very constant constitution—namely, the _ovule_. This truth may be expressed by changing a word in Harvey’s celebrated aphorism—_omne vivum ex ovo_; we now say omne _vivum e cellula_. The myriads of differentiated anatomical elements whose association forms complex beings are the posterity of a cell, of the _primordial ovule_, unless they are the posterity of another equivalent cell. The second task of histology in the latter half of the nineteenth century consisted in following up the filiation of each anatomical element from the cell-egg to its state of complete development. The whole cellular theory is contained in the two following statements, which establish the morphological unity of living beings:—_Everything is a cell, everything comes from an initial cell_; the cell being defined as a mass of substance, protoplasm or protoplasms, of an average diameter of a few microns. § 2. THE SECOND PERIOD: THE DIVISION OF THE CELL.