cell-germs in the spores of Marchantia usually only from two to four serve to form cells, the rest becoming overlaid with chlorophyll, and being consequently withdrawn from the vital process. He who is acquainted with the modern view of the processes of free cell-formation founded on the numerous and careful investigations of later times will scarcely discover in the above account of Schleiden’s theory a single correct observation. Soon after, von Mohl published in ‘Linnaea,’ 1839, p. 272, his observations on the division of the mother-cells of the spores of Anthoceros; these were carefully made and were correct in all the main points; and in opposition to Mirbel’s former statements they establish the fact, that the division is effected by the mucilaginous contents of the cell, and consequently that it is not a passive division of the contents of the mother-cell produced by the growth inwards of projections of the cell-wall. Unger[87] was the first to declare distinctly against Schleiden’s doctrine, and his observations on the punctum vegetationis appeared in the ‘Linnaea’ of 1841, p. 389; from the size and position of the cells he concluded that the tissue-cells in this case are formed by division, and not in the manner alleged by Schleiden. Soon after Nägeli also (‘Linnaea,’ 1842, p. 252) observed the processes of cell-formation in the extremities of roots, but he did not conceive them to be cases of division; he saw two nuclei form in each mother-cell, and a new cell form round each nucleus, and explained the origin of the dividing wall as due to the meeting together of the two new cells; he thought that a similar process takes place in stomata and in the mother-cells of pollen; this conception was not absolutely incompatible with Schleiden’s theory, but there was this difference, that in Nägeli’s case essential processes were correctly observed, but were to some extent incorrectly interpreted. In the same year appeared the first edition of Schleiden’s ‘Grundzüge der wissenschaftlichen Botanik,’ in which his theory of cell-formation was repeated in a more precise form. That he was thoroughly in earnest to maintain it is shown by the fact that he gave still another exposition of it in his ‘Beiträge zur Botanik’ in 1844, where he insists that his method of cell-formation is the general one, though it has been distinctly ascertained in the Phanerogams only. But how completely an observer may be led captive by a preconceived opinion may be learnt from Schleiden’s suggestion, that the formation of zygospores in Spirogyra is in accordance with his views, though it is impossible to conceive of a case of cell-formation more easy to observe, or less reconcilable with Schleiden’s theory. It was mentioned in the first book, that Hedwig and Vaucher were acquainted with the remarkable process of the formation of zygospores in the alga-genus Spirogyra; but this as late as Schleiden’s time was not regarded as an example of cell-formation, and his view was