PROGRESS IN ANATOMY AND PHYSIOLOGY 



alike. This amounted to the logical extension and cul- 

 mination of Schwanu's doctrine as to the similarity of 

 development of the two animate kingdoms. Yet at the 

 same time it was in effect 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 an- 

 other modification, as we shall see ; but the conception 

 of the protoplasmic mass as the essential ultimate struct- 

 ure, which might or might not surround itself with a 

 protective covering, was a permanent addition to physi- 

 ological knowledge. The earlier idea had, in effect, de- 

 clared the shell the most important part of the egg; 

 this developed view assigned to the yolk its true posi- 

 tion. 



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 divis- 

 ion of a pre-existing cell. Ehrenberg, another high au- 

 thorit} 7 of the time, contended that no such division oc- 

 curs, and the matter was still in dispute when Schleiden 

 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 



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