THE BIOLOGY OF THE CELL SURFACE 



means if one knows definitely that this is on the cytoplasm 

 alone, or on one or another of the inclusions or on the whole 

 of the cell-contents. Finally, the effect of experimental 

 treatment should be known to be reversible, and coagula- 

 tive death-changes should not be confused with what 

 is denominated as the normal viscosity-changes in normal 

 and viable cells. Until the viscosity studies have been 

 refined, changes in this physical property cannot be 

 regarded as an infallible sign of the state of being alive. 



A leading characteristic of biological researches of the 

 last twenty-five years lies in the emphasis which came to be 

 placed upon the investigation of the physical properties of 

 protoplasm — an emphasis so great that it has virtually 

 created another department in an already heavily depart- 

 mentalized science. Everywhere nowadays bio-physics, 

 the physical chemistry of the cell, and the colloid chemistry 

 of protoplasm are in evidence. Many an interesting new 

 datum concerning electrical conductivity, hydrogen-ion- 

 concentration, temperature-coefficients, etc., has been 

 accumulated, whilst the older conceptions of osmotic pres- 

 sure, surface tension and the like have been refined by 

 more exact mathematical treatment. If from all these 

 studies far less has come than had been hoped, one benefit 

 of them deserves emphasis. No matter how refined a study 

 on a physical property of the living substance may be, its 

 value for the understanding of vital activities is determined 

 by its relation to these vital activities and by its correlation 

 with known form-changes and normal processes in viable 

 cells. Modern studies in bio-physics and the like have 

 served to indicate anew how necessary is adequate knowl- 

 edge of the visible structure and the form-changes of cells, 

 and how much we still need to extend this knowledge. 

 What is called the morphology of a cell — its visible form 

 and its visible form-changes — still remains the basis of 

 biological investigation both for its own sake and as the 



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