A HISTORY OF SCIENCE 



the banishment of the cell that Schwann had defined. 

 The word cell was retained, it is true, but it no longer 

 signified a minute cavity. It now implied, as Schultze 

 defined it, "a small mass of protoplasm endowed with 

 the attributes of life." This definition was destined 

 presently to meet with yet another modification, as we 

 shall see ; but the conception of the protoplasmic mass 

 as the essential ultimate structure, which might or 

 might not surround itself with a protective covering, 

 was a permanent addition to physiological knowledge. 

 The earlier idea had, in effect, declared the shell the 

 most important part of the egg; this developed view 

 assigned to the yolk its true position. 



In one other important regard the theory of Schleiden 

 and Schwann now became modified. This referred to 

 the origin of the cell. Schwann had regarded cell 

 growth as a kind of crystallization, beginning with the 

 deposit of a nucleus about a granule in the intercellular 

 substance the cytoblastema, as Schleiden called it. 

 But Von Mohl, as early as 1835, had called attention to 

 the formation of new vegetable cells through the divi- 

 sion of a pre-existing cell. Ehrenberg, another high 

 authority of the time, contended that no such division 

 occurs, and the matter was still in dispute when Schlei- 

 den came forward with his discovery of so-called free 

 cell-formation within the parent cell, and this for a long 

 time diverted attention from the process of division 

 which Von Mohl had described. All manner of schemes 

 of cell-formation were put forward during the ensuing 

 years by a multitude of observers, and gained currency 

 notwithstanding Von Mohl's reiterated contention that 

 there are really but two ways in which the formation 



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