564 SCIENCE PROGRESS 



medium itself may bring about changes in the permeability. 

 Many of these are reversible and can be repeated over and over 

 again without causing any injury to the cell. The work of 

 Osterhout upon selective permeability, true and false plas- 

 molysis, and the quantitative measurement of the antagonistic 

 action of different ions is of especial interest in this connection. 

 It is, however, too large a subject to be discussed in any detail 

 here. 



For these reasons the cryoscopic method is to be preferred 

 when sufficient quantities of the tissue fluid can be obtained 

 unaltered in composition, and when the latter does not 

 undergo any appreciable change during the interval that 

 elapses between expression and the actual determination of 

 freezing-point . 



It has already been mentioned that in plasmolysis the pro- 

 toplasmic layer is forced back from the cellulose wall. Under 

 normal conditions it is this cellulose wall that takes up the 

 hydrostatic pressure or osmotic pressure of the cell sap and sets 

 a limit to the distention of the protoplasm. Without this sup- 

 port the latter would expand owing to the intake of water 

 through it into the vacuole, and would blow out like a soap 

 bubble until it met with the usual fate of bubbles. The analogy 

 is not at all a superficial one, for in each case the enlargement is 

 resisted by the force required to do work, in increasing the 

 surface, against the surface tension of the soap-film in contact 

 with air or of the protoplasm in contact with the external solu- 

 tion. It is precisely the possession of a cellulose wall setting a 

 limit to expansion that constitutes such a profound distinction 

 between plants and animals. For animal cells constancy of the 

 osmotic pressure of the external medium and of the internal 

 fluids is of paramount importance. For plant cells such 

 constancy is quite unnecessary provided always that the 

 osmotic pressure of the external medium is not raised above that 

 of the cell sap to too great an extent or too rapidly. 



Consider, for instance, the fate of a protozoan placed in a 

 hypotonic medium ; water is absorbed continually till the 

 cell bursts . The same result might be effected by the accumula- 

 tion of urea or other katabolic products by increasing the 

 internal osmotic pressure, and so causing a relative change, 

 hence the need for an elementary nephric system, such as a 

 contractile vacuole as has been pointed out by Dixon. 



