3 ANNALS NEW YORK ACADEMY OF SCIENCES 



Introduction 



Differences in osmotic pressure have been held to explain many physio- 

 logical processes. It is a great temptation, for example, to ascribe the 

 passage of materials into and out of the cell to differences in molecular 

 concentration between the cell contents and the circulating medium ; and 

 yet, such a process as the secretion of urine is not satisfactorily explained 

 by the physical theories of osmosis and diffusion. It does not necessarily 

 follow, however, that, because these theories in our present state of knowl- 

 edge fall short of a complete explanation of physiological processes, they 

 should on that account be altogether discarded, nor should this be taken 

 as an argument for vitalism. The full understanding of both the physi- 

 cal process and the associated chemical process would make clear the 

 physiological process. So that everything that can be ascertained with 

 regard to the passage of materials through membranes is of value to the 

 science of physiology. 



A common method of determining the osmotic pressure of a solution 

 is by means of a determination of its freezing point. The degree of de- 

 pression of the freezing point of the solution below that of pure water is 

 proportional to the osmotic pressure of the solution. The amount of the 

 depression of the freezing point, or A, is usually obtained by the use of 

 the Beckmann apparatus. The form of apparatus that was used in the 

 determinations described in this paper was made by Goetz of Leipzig. 

 When in constant use at low temperature, there is a tendency toward a 

 slight re-arrangement of the molecules of the glass tube, and this results 

 in a contraction that is sufficient to introduce a slight error in the read- 

 ings. It is therefore advisable for any one working with this instrument 

 to make frequent determinations of the freezing point of pure water. In 

 the experiments here described, this procedure was followed and the 

 proper correction was always made in the calculations. 



Various aspects of the osmotic relations of the body fluids of aquatic 

 animals to the surrounding medium have been fruitfully investigated. 

 The waters of the earth differ in their molecular concentration. Fresh 

 water contains but a very small amount of salts in solution. The water of 

 the ocean, with a specific gravity of 1.025, contains about 3.0 per cent of 

 salts in solution. The water of the Black Sea and the Baltic Sea contain 

 less salts than the water of the ocean because of the great influx into 

 them of river water. The Mediterranean and Red Seas on the other hand 

 contain more salts in solution than ocean water because of the excess of 

 evaporation over the inflow of fresh water. There is considerable varia- 



