THE PRESENT POSITION OF CELL-THEORY, in 



leading an independent existence. This is so important a 

 part of the cell-theory that I may again quote in his own 

 words Schwann's reasons for calling the cell an elementary 

 unit of life. " Now if we find that some of these elementary 

 parts not differing from the others are capable of separating 

 themselves from the organism and pursuing an independent 

 growth, we may thence conclude that each of the other 

 elementary parts is already possessed of the power to take 

 up fresh molecules and grow, and that therefore each 

 elementary part possesses a power of its own, an indepen- 

 dent life." 



In the case which we are considering the very faculty 

 which was so powerful a reason for regarding cells as 

 independent units is wanting. Nobody has ever observed 

 a nucleus or a centrosome or even a chromatophore to 

 separate itself from the cell and pursue an independent 

 existence. And not only is there no recorded case of the 

 constituent particles of cells separating themselves spon- 

 taneously from the cell, but experiments which have been 

 made with the express purpose of determining whether 

 these particles can live apart from the cell to which they 

 belong have in every case given a negative result. Even 

 the nucleus, highly complicated as it is, and itself composed 

 of smaller particles which may easily be demonstrated, 

 perishes when removed from the cell body. The chroma- 

 tophores similarly perish, and so no doubt would the 

 centrosomes if it were possible to isolate such very minute 

 particles. Many instances might be cited in proof of this, 

 but it is scarcely necessary to bring forward the details ; 

 the reader can obtain them by reference to the works of 

 Nussbaum, 1 A. Gruber and Verworn. 2 



It is of some interest in this connection to contrast the 

 process of reproduction in unicellular and multicellular or- 

 ganisms. In the latter reproduction is effected by the 

 separation of a single unit, a cell, from the aggregate, and 



1 M. Nussbaum, Biol. Centralblatt, vol. iv. 



2 Max Verworn, " Die physiologische Bedeutung des Zellkerns," 

 Pfliiger s Archiv, vol. li., 1892. 



