6o6 POPULAR SCIENCE MONTHLY. 



been, in fact, since the beginning. The kernel of the new proto- 

 plasm theory, which, as before stated, has since dominated research, 

 is contained in Schulze's definition of a cell as " a lump of proto- 

 plasm 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 ISTaegeli that 

 cells multiply by the division of those already present, and bota- 

 nists 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 difficul- 

 ties 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 Vir- 

 chow, in 1858, gave expression to the result in the aphorism, " Om- 

 nis 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 

 zoologists 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 ob- 

 tained or believed it possible to guard against artificial appear- 

 ances due to the action of the preservative medium. The intro- 

 duction 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 charac- 

 terized 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 con- 

 tained in two volumes which laid the foundations for all future 



