gg z Theory of Cell-formation [Book ii. 



von Mohl had observed with accuracy a number of important 

 facts, but that Nageli added largely to them, and, which is the 

 main point, elaborated them into a comprehensive theory em- 

 bracing all kinds of cell-formation. How important the correct 

 distinction of the protoplasm from the rest of the cell-contents 

 was for the perfecting of the theory of cells is seen from Nageli's 

 declaration, that he retracts his former view which rested on 

 the authority of Schleiden, because it sprang from a time when 

 he was ignorant of the significance of the mucilage-layer (the 

 protoplasm\ though it is true that he indicates at the same 

 time other points and new considerations which definitively set 

 aside Schleiden's theory. After investigating the different 

 modes of free cell-formation and finding the processes there 

 quite different from Schleiden's account of them, he proceeded 

 to search for free cell-formation where Schleiden had affirmed 

 that it invariably occurs, namely in growing vegetative organs 

 in the higher plants. But this investigation led him to the 

 conclusion that all vegetative cell-formation is true cell-division, 

 and that even the reproductive cell-formation in some Algae 

 and Fungi is effected by division ; the reproductive cells of 

 most plants are the result of free cell-formation, but it should 

 be observed that the term free cell-formation is here used not 

 exactly in the modern sense, inasmuch as Nageli included in it 

 the formation of four-fold grains (tetrads) in spores and pollen. 

 If the distinction between cell-division and free cell-formation 

 had often been suggested by former observers, Nageli was the 

 first who distinctly defined it, though not exactly as it is now 

 defined. ' In cell-division the contents of the mother-cell 

 separate into two or more portions ; a perfect membrane forms 

 round each of these portions, which at the moment of its 

 appearance rests partly on the wall of the mother-cell and 

 partly on the adjacent walls of the sister-cells. In free cell- 

 formation a smaller or larger part of the contents of a cell, or 

 even the whole of them becomes isolated. On its surface is 

 formed a perfect membrane, which is everywhere free on its 



