396 THE HISTORY OF BIOLOGY 



contrast to Miiller he found no further or greater finality in living nature 

 than existed in inanimate nature; the same purely mechanical forces shape 

 both the cell and the crystal. 



Further development of the cell theory 

 The cell-theory which has just been described, and which has always been 

 called the Schleiden-Schwann theory, after its founders, was adopted and 

 at once followed up by other investigators, while, as mentioned above, the 

 two pioneers withdrew from the field. The most important contributions 

 made during the next few years were those of Mohl, who published a series 

 of new observations regarding the role of the cell in the vegetable kingdom. 

 In these brief but weighty papers he analyses the different components of 

 the cell. To him the cell is still "a vesicle formed of a fixed membrane and 

 containing a moisture"; the character of the membrane is the essential thing, 

 and the shape, consistency, and interrelation of the cellular walls are de- 

 scribed before anything else. Moreover, an account is also given of the con- 

 tents of the cell: the "viscid moisture" that forms its fundamental constituent 

 is carefully described; its currents, which were discovered by the Italian 

 CoRTi and rediscovered by Treviranus, are depicted in detail in various plant- 

 forms — inter alia in those, since then, classical objects of demonstration, 

 the Tradescantia hairs — similarly, the evolution of the cell-content is 

 followed through its different stages of growth, and the secondary forma- 

 tions that accompany it — vacuoles, chlorophyll- and starch-granules — 

 are described. The fundamental substance in the cell Mohl calls protoplasm; 

 he thereby establishes the fact that the cell-content is an element by itself 

 and not merely "slime" of some indeterminate kind, as Schleiden supposed. 

 The name, which in spite of its clumsiness has come into permanent use, is, 

 as a matter of fact, based on the false assumption that all the component parts 

 of the cell, even (and above all) the nucleus, originate in this element, the 

 "primal slime." The nucleus is described by Mohl in greater detail than by 

 his predecessors; true, it is still stated to be, as mentioned above, a deriva- 

 tive of protoplasm come into being through an accumulation of a granulate 

 substance in young cells and disappearing in the older ones, but the grossly 

 mechanical precipitation-theory that Schleiden and Schwann held was ac- 

 cepted with reserve. And, above all, division is mentioned as being the nor- 

 mal method of cell-reproduction; independent cell-formation is confined to 

 the embryo-sac alone. In regard also to the alimental physiology of the cell, 

 Mohl offers some interesting observations, but they must be passed over 

 here. 



At the same time, valuable contributions to the evolution of the cell 

 were made by Karl Nageli, an investigator whose far-reaching activities 

 will be described later on. In an essay on the pollen-formation in the phan- 

 erogams, particularly in the Liliaceas, he describes the cell-divisions (in 1841) 



