THE CELL DOCTRINE. 37 



fail to realize how comparatively few have been the 

 changes necessitated in his descriptions, or the method 

 of application of his theory to the formation of the dif- 

 ferent tissues. Indeed, the portion of the theory of 

 Schleiden and Schwann which does not accord with 

 the latest expression of the cell doctrine, is not so 

 much that which pertains to the formation of tissues 

 from existing cells, as that which relates to the 



O ' 



method in which they supposed the cells to origi- 

 nate; which, it will be recollected, was by a species 

 of spontaneous generation of the essential parts of the 

 cell, in a homogeneous cytoblastema. 



A difference in the anatomy of the cell as given by 

 Schwann, and physiologists of the present day is 

 seen in the location of the nucleus by the latter, who 

 places it not merely eccentrically, but actually "sepa- 

 rated from the surface only by the thickness of 

 the assumed cell-wall;"* though an inspection of 

 Schwann's drawings would not convey this impres- 

 sion. At the present day, the situation of the nu- 

 cleus, though usually central, is known to be not 

 unvarying. Again, the primary and absolutely es- 

 sential presence of the nucleolus, as well as the uni- 

 versal presence of the cell-wall, may be considered 

 characteristics of Schleiden and Schwann's idea of 

 the cell, which are now no longer insisted upon. 



As already stated (p. 33), Schwann would seem 

 to have admitted also, the formation of cells by 

 division, though with some hesitation. Thus he 

 writes :f "A mode of formation of new cells, differ- 



* Schwann, op. citat., p. 37, a. f. 



f Schwann, op. citat., Introduction, p. 4. 



