CHAPTER IV.
History of Development of the Cell, Formation of Tissues, Molecular Structure of Organised Forms.
1840-1860.
In the period between 1830 and 1840 it had come to be understood, that the old theories of cell-formation of Wolff, Sprengel, Mirbel, and others, resting on indistinct perceptions and not on direct and exact observation, could only give an approximate idea of the formation of cells. But in the course of that time really different cases of formation of new cells were accurately observed by Mirbel, and more especially by von Mohl, who described different modes of formation of spores, and in 1 835 the first case of vegetative cell-division. Unfortunately these observations, excellent in themselves, applied to cases of cell-formation which do not occur in the ordinary multiplication of cells in growing organs, and von Mohl guarded himself from founding a general theory of cell-formation on his observations on cells of reproduction and on a growing filamentous Alga. Mirbel also cautiously regarded the formation of pollen-cells and that which he supposed to be the process in the germination of spores as cases of a peculiar kind, adhering to his old theory of the origin of ordinary tissue-cells.
Schleiden's behaviour was different. Having somewhat hastily observed the free cell-formation in the embryo-sac of Phanerogams in 1838, he proceeded at once to frame a theory upon it which was to apply to all cases of cell-formation, and especially to that in growing organs. The very positive way
