74 THE GENETIC AND 



explanatory hypotheses raise those dead masses of facts 

 to the place of true and living sciences these must 

 not be taught at all. And how then do matters stand 

 with regard to the cell-theory, that fundamental theory 

 on which every element of our morphology and physio- 

 logy depends, and by applying which Virchow himself 

 reached his grandest results ? 



Since Schleiden in Jena, forty years ago, first put 

 forward the cell-theory, and Schwann immediately 

 after applied it to the animal kingdom and so 

 to the whole organic world, this fundamental doc- 

 trine has undergone very important modifications, for 

 it is indeed a biological theory, but not a fact. We 

 may recollect under what different aspects its main 

 principles have appeared in the course of these four 

 decades : what changes have taken place in the con- 

 ception of the cell itself. After the organic cell had 

 originally been conceived of as a vesicle, consisting of a 

 firm capsule and a fluid content, we subsequently dis- 

 cerned it to be composed of a glutinous semi-fluid cell- 

 substance, the protoplasm, and convinced ourselves that 

 this protoplasm and the cell-core or nucleus enclosed in 

 it are the most important and indispensable constituent 

 parts of the cell, while the external firm capsule, the 

 cell-membrane, is not essential and very frequently 

 wanting. But even now opinions widely differ as to 

 how the conception of a cell should be precisely defined, 



