PROGRESS IN ANATOMY AND PHYSIOLOGY 



Omnis cellula e cellula became from that time one of 

 the accepted data of physiology. This was supplement- 

 ed 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 under- 

 goes change preparatory to new cell formation is the all- 

 essential nucleus. Thus the nucleus was restored to the 

 important position which Schwann and Schleiden had 

 given it, but with greatly altered significance. 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 divis- 

 ion-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 organism, 

 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 the devel- 

 oped individuals of higher organisms the successive gen- 

 erations of cells become marvellously diversified in form 

 and in specific functions; there is a wonderful division 

 of labor, special functions being chiefly relegated to defi- 

 nite groups of cells; but from first to last there is no 

 function developed that is not present, in a primitive 

 way, in every cell, however isolated; nor does the de- 

 veloped cell, however specialized, ever forget altogether 

 any one of its primordial functions or capacities. All 

 physiology, then, properly interpreted, becomes merely 



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