322 Theory of Cell-formation [BOOK u.

Dumortier had observed the division of cells as early as 1832 l , and Morren had seen it in Closterium in 1836, but had not given the needful details. Finally, von Mohl applied the experience which he had gained from Cladophora to other filamentous Algae, and pointed out the similarity between these processes and the division of Diatoms, which he con- sequently claimed as plants in opposition to Ehrenberg, who considered them to be animals ( 'Flora,' 1836, p. 492).

Meyen next, relying on von Mohl's observations on Clado- phora, declared in the second volume of his ' Neues System ' that cell-division was a very common occurrence in Algae, Fila- mentous Fungi and the Characeae, but he neglected any closer investigation of the processes by which the division is intro- duced and completed. His comparison of these cases of cell- formation with the formation of spores, pollen-grains, and endo- sperm-cells is moreover noticeable as the first attempt to distin- guish what is now known as free cell-formation from cell-division ; it was obviously the want of this distinction which long pre- vented clearer views on the whole of this field of observation. The due separation of these two modes of cell-formation was a short step after the observations that had been already made ; and if that step had been taken, Schleiden's theory would have been impossible, and the development of the cell-theory would not have been prejudiced by the mistake, introduced by Schleiden after 1838, of applying the mode of free cell-form- ation, which he believed he had observed in the embryo-sac of Phanerogams, to the multiplication of vegetative cells in grow- ing organs, and regarding it as the only mode of cell- formation. This would have been the more impossible, since von Mohl in the same year gave an excellent description of the development of stomata by division of a young epidermis-cell and the later separation of the dividing wall into two laminae. But von Mohl in the years immediately following was over-cautious in

1 See Meyen, ' Neues System,' ii. 344.