OUTSTANDING BIOLOGICAL ADVANCES 19 



formation of all organic products, and that this 

 principle is the formation of cells, as well as the con- 

 clusions which may be drawn from this proposition, 

 may be comprised under the term cell-theory, using 

 it hi its more extended signification, while, in a more 

 limited sense, by the theory of cells we understand 

 whatever may be inf erred from this proposition with 

 respect to the powers from which these phenomena 

 result." 



Schleiden had not used the term cell-theory, nor 

 developed the ideas at the basis of it, accordingly it is 

 more proper to give the credit to Schwann for this 

 great generalization and to speak of it as the cell- 

 theory of Schwann rather than the cell-theory of 

 Schleiden and Schwann. 



There were serious defects in the theory as it 

 existed in the minds of the early observers. Both 

 Schleiden and Schwann attached importance to the 

 cell-wall and had a wrong idea of the nucleus (called 

 cytoplast by Schleiden) and of the formation of cells. 

 But, as researches progressed, the theory was greatly 

 expanded and unproved. 



One of the most helpful steps was the full recogni- 

 tion of the origin of cells in multicellular organisms. 

 The many-celled animals and plants begin then- 



