ANATOMY AND PHYSIOLOGY 



of new cells takes place namely, "first, through divi- 

 sion of older cells ; secondly, through the formation of 

 secondary cells lying free in the cavity of a cell." 



But gradually the researches of such accurate ob- 

 servers as Unger, Nageli, Kolliker, Reichart, and Re- 

 mak tended to confirm the opinion of Von Mohl that 

 cells spring only from cells, and finally Rudolf Virchow 

 brought the matter to demonstration about 1860. His 

 Omnis cellula e cellula became from that time one of 

 the accepted data of physiology. This was supple- 

 mented a little later by Fleming's Omnis nucleus e 

 nucleo, when still more refined methods of observation 

 had shown that the part of the cell which always first 

 undergoes change preparatory to new cell-formation is 

 the all-essential nucleus. Thus the nucleus was re- 

 stored to the important position which Schwann and 

 Schleiden had given it, but with greatly altered signifi- 

 cance. Instead of being a structure generated de novo 

 from non-cellular substance, and disappearing as soon 

 as its function of cell-formation was accomplished, the 

 nucleus was now known as the central and permanent 

 feature of every cell, indestructible while the cell lives, 

 itself the division-product of a pre-existing nucleus, 

 and the parent, by division of its substance, of other 

 generations of nuclei. The word cell received a final 

 definition as " a small mass of protoplasm supplied with 

 a nucleus." 



In this widened and culminating general view of the 

 cell theory it became clear that every animate organ- 

 ism, animal or vegetable, is but a cluster of nucleated 

 cells, all of which, in each individual case, are the direct 

 descendants of a single primordial cell of the ovum. In 



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