been, in fact, since the beginning. The kernel of the new protoplasm theory, which, as before stated, has since dominated research, is contained in Schulze's definition of a cell as "a lump of protoplasm endowed with the properties of life," Yet for a few years some botanists found it difficult to give up the old idea of the active participation of the cell wall in the life of the cell, until Sachs's classic studies removed the last basis for such belief.
We have seen that Schleiden and Schwann recognized that it is only cells and their products which make up the substance of all organisms, but that their ideas of how cells arise were quite erroneous. It was soon observed by von Mohl and Naegeli that cells multiply by the division of those already present, and botanists soon came to the conclusion that plant cells can only come from previously existing ones. This conclusion was reached much more slowly for animals, since it presented many difficulties in the field of pathology, especially in connection with such processes as the formation of pus. But gradually the objections were shown to be of no weight, and the great pathologist Virchow, in 1858, gave expression to the result in the aphorism, "Omnis cellula e cellula."
It will be noticed that the view of the cell current thirty years ago laid less stress upon the nucleus than that of twenty-five years earlier; and we shall see that more importance is attached to it to-day than ever before. Yet, so far as it went, Schulze's view of the nucleus was better than Schleiden's, for it recognized it as a specially differentiated organ of the protoplasm, though knowledge of its particular relations to the activity of the cell was very meager. Up to about 1875 it was generally thought by zoölogists that, before the cell divides, the nucleus is constricted into two portions, one of which forms the nucleus of each of the new cells. The botanists, on the contrary, generally believed that the nucleus disappears before cell division, after which a new one appears in each new cell. These conclusions had been reached by studies of living dividing cells. Practically nothing had been done with preserved material, since no one trusted results so obtained or believed it possible to guard against artificial appearances due to the action of the preservative medium. The introduction of alcohol by Strasburger for killing and preserving tissues, and the proof by comparison with fresh material that no destructive or misleading changes are produced by it, mark the beginning of the epoch of cell studies, which has been characterized by a most astounding development of technical methods for killing, preserving, staining, and sectioning tissues of every sort with the least possible alteration in their living structure. The results of the first profound studies of the nucleus are contained in two volumes which laid the foundations for all future
