GROWTH 17 



tificial cell. It consists of a membrane 

 in the form of a rounded sack which com- 

 pletely incloses a drop of the solution A 

 and which is surrounded by the solution 

 B. If now solution A is more concen- 

 trated than solution B water will be at- 

 tracted by solution A and will pass into 

 the artificial cell which in consequence 

 will expand and stretch the membrane. 

 Under the proper experimental condi- 

 tions this may continue for a long time. 

 We may employ for such experiments 

 a great variety of materials, as copper 

 salts in a solution of potassium ferro- 

 cyanide, metallic salts of various kinds 

 in a solution of water glass, or tannic 

 acid in a solution of gelatin. In some 

 cases the artificial membrane expands 

 by repeated rupture and repair, in others 

 it is steadily stretched without rupture, 

 and at the same time strengthened by 

 the deposit of new material. The pro- 

 toplasmic membrane might conceivably 

 expand in either way. It is not certain 

 which method is followed. 



