really a step in advance, since it brought a process, so highly peculiar according to existing ideas, under the general conception of cell-formation. The systematic elaboration of the theory of cells, founded on careful observation and mature reflection, began with the year 1844. Almost at the same time in this year appeared Nägeli’s detailed enquiries into the occurrence of the cell-nucleus and into cell-division, von Mohl’s observations on the primordial utricle and its behaviour in the process of cell-division in young tissue, and lastly those of Unger on merismatic cell-formation (cell-division) as a general mode of proceeding in the growth of organs. As these observers were chiefly concerned to test the correctness and general applicability of Schleiden’s theory, they necessarily paid special attention to the general occurrence of the cell-nucleus and to its position on the side of the cell-wall, for these were the points most accessible to observation and criticism. The discussion of these observations disclosed a defect in the current phraseology, in which the word cell was commonly understood to mean only the cell-membrane, but sometimes included everything belonging to and contained in the cell; hitherto moreover the protoplasm of the cell had not been sufficiently distinguished from the rest of the cell-contents. Nägeli and von Mohl arrived simultaneously at a clearer understanding of these points; von Mohl recognised the primordial utricle (1844) as a component part of the cell-contents and not belonging to the cell-wall, and explained the part which it plays in cell-division; in 1846 he arrived at a clear conception of the protoplasm as a peculiar substance distinct from the other contents of the cell and gave it the name it still bears. Meanwhile Nägeli had also distinguished the protoplasm from everything else in the cell, and noticed its pre-eminent importance in cell-formation and its nitrogenous character. We must not omit to mention here, that investigations into the processes of cell-formation compelled observers to search for the spots where cell-formation actually takes place, and thus the fact was ascertained, that cells in statu nascendi are not to be found in all parts, not even in all growing parts of the plant, but that we must look for them in the so-called puncta vegetationis in the stem and root, in the youngest lateral organs, and between the bark and the wood in woody plants. About this time a new idea began to be attached to the word cambium, which Mirbel had used in the sense of a nourishing juice saturating the plant; it was now applied to the tissue-masses in which the formation of new cells takes place, and specially to the very thin layer of tissue lying between the wood and the rind, from which new layers of wood and rind in woody plants are formed—a layer, which according to Mirbel’s theory had been a mass of sappy matter, in which new cells arise as vacuoles.