260 PHYSIOLOGICAL EEGULATIONS 



In order that an effective ''osmotic" pressure gradient shall be 

 proportional to a concentration gradient, some solutes either do 

 not penetrate the surface of the living unit, or do so very slowly as 

 compared to the water. How fast any solutes pass is not always 

 ascertained, and sometimes perhaps cannot be wholly ascertained, 

 thus giving rise to permeability coefficients bearing diverse tenta- 

 tive definitions. 



In a study of any permeability in which "osmotic" pressure is 

 believed to be a factor, two further assumptions are introduced. 

 (l) The concentration inside, if measured at all, is measured as one 

 of the colligative properties of a hypothetical solution prevailing 

 in the organism. (2) The "effective" volume of this solution con- 

 tained within the organism or tissue is not the entire volume of the 

 body. It is usually estimated by finding the relative volumes when 

 organisms or cells are believed to have concentrations equal to their 

 media. It is the volume of distribution of an increment of water. 

 Then the osmotically effective volume is the measured volume (V) 

 minus the ' ' non-solvent ' ' volume (b). In some instances b occupies 

 about the same space as the total solids of the tissue, or the fats 

 plus proteins of the tissue, or the cells of the tissue. There is no 

 predictable relationship of b to any one colligative that holds for 

 a variety of organisms and tissues, and "independent" methods of 

 measuring it {e.g., Leitch, '34, '36) deal with diverse definitions 

 otb. 



Permeability coefficients are often expected to be constant. But 

 as I scan the values reported for water in living tissues I find, first, 

 that few of them have been tested under many conditions, and 

 second, that all which have been so tested are not constant. Choos- 

 ing dissimilar conditions within one species at a time, I find the 

 following significant contrasts of coefficients : 



(a) Different in two directions with like gradient of concentra- 

 tions (differential permeability). Phascolosoma; leucocyte 

 of rabbit. 



(b) Different with time. Phascolosoma in exosmosis; Echino- 

 metra egg. 



(c) Different in media of diverse ranges of concentrations but 

 of supposedly equal gradients of concentration. Cerato- 

 cephale egg. 



(d) Different in media containing diverse non-penetrating 

 solutes. Arbacia egg in various calcium concentrations. 



