42 THE CELL DOCTRINE. 



researches cannot but be surprised at the accuracy of 

 the observations of this histologist, nor can he fail to 

 realize how comparatively few have been the changes 

 necessitated in his descriptions, or the method of ap- 

 plication of his theory to the formation of the differ- 

 ent tissues ; while 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 

 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 former, 

 who places it not merely eccentrically, but actually 

 "separated from the surface only by the thickness of 

 Ihe assumed cell-wall."* At the present day, the 

 situation of the nucleus, though usually central, is 

 known to be not unvarying. Again, the primary 

 J and absolutely essential presence of the nucleolus, as 

 well as the universal 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. 38), Schwann would seem 

 to have admitted also, the formation of cells by di- 

 vision, though with some hesitation. Thushewrites:t 



* Schwann, op. citat., p. 87, a. f. 



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



