PHTSIOLOGT 13 



tween them, so that it is impossible to say what 

 actually penetrates the protoplasm. Others have 

 sought to analyze the cell sap. Plant cells are 

 most favorable for this purpose, since, as a rule, 

 they contain vacuoles filled with sap. In general 

 the method has been to crush the tissues and ex- 

 press the sap, but this procedure involves many 

 possibilities of error, such as contamination of 

 the cell sap by substances present in the cell walls 

 or intercellular spaces and chemical reaction be- 

 tween the cell sap and the crushed protoplasm or 

 the cell walls. (The degree of pressure used in 

 expressing has a marked influence on the concen- 

 tration of the sap.) The investigation of blood 

 and other body fluids is open to the objection 

 that we do not know to what extent substances 

 penetrate between the cells in reaching these 

 fluids. In many of these cases penetration seems 

 to present very special features. 



The taking up of dyes has been extensively in- 

 vestigated but this method is beset by many pit- 

 falls. To a great extent the coloration of the cell 

 by a dye shows the extent to which the dye can 

 combine with the substances within the cell 





