As usually happens in the inductive sciences, the period of strict inductive investigation into cell-formation was preceded by a still longer time, during which botanists ventured to put forward general theories in reliance on highly imperfect observations. We have already seen how Caspar Friedrich Wolff in 1759 made cells originate as vacuoles in a homogeneous jelly, and how this view was adopted in all essential points by Mirbel at a late period in the 18th century; how Kurt Sprengel, and with him a number of phytotomists, among them Treviranus as late as 1830, supposed cells to be formed from granules and vesicles in the cell-contents, an idea which Link it is true opposed in 1807, but afterwards accepted to a great extent. Though Moldenhawer as early as 1812 (‘Beiträge,’ p. 70) distinctly rejected these theories, and published observations which if followed up would have led to the right path, yet the botanists above-named and others with them, long continued to adhere to the earlier views. Kieser, for example (‘Mémoire sur l’organisation des plantes,’ 1812) further developed Treviranus’ theory, that the fine granules in the latex of plants are cell-germs which are afterwards hatched in the intercellular spaces. Schultz-Schultzenstein in his work ‘Die Natur der lebenden Pflanze,’ 1823-28, i, p. 607 rejected this view and adopted that of Wolff and Mirbel. Scarcely better than the notion of cell-germs represented by Sprengel, Treviranus, and Kieser was the theory propounded by Karsten soon after 1840; that of the French botanists Raspail and Turpin[85] (1820-1830), though conveyed in a different terminology, corresponded in its main points with the views of Sprengel. It had been the good fortune of Mirbel at the beginning of the century, and again thirty years later, to promote the advance of phytotomy by means of important observations, though he may have interpreted some of them incorrectly; the same thing happened again thirty years later, and it was a German enquirer, von Mohl, who corrected his observations and views on both occasions. In his famous treatise on Marchantia polymorpha, which appeared in 1835 in the Memoirs of the French Institute, the first part having been laid before the Paris Academy in 1831-32, Mirbel distinguished three modes of cell-formation; in the germination of the spores of Marchantia new cells are formed from the germ-tube and new cells again from these by a similar process, much in the same way therefore as that which actually occurs in the germination of Yeast-fungi; he found a second kind of cell-formation in the production of the gemmae of Marchantia, where he distinctly observed the successive appearance of the dividing walls, but formed an erroneous idea of the proceeding on the whole; in the further development of the gemmae and in other cases of growth he considered that new cells are formed between those that are already present in the manner supposed in his earlier theory.