400 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 31 



the variations of osmotic pressure in the localities from which they 

 were taken covered as wide a range as 64 per cent. 



The same data which have been used to show that the proportions 

 between some of the elements in the bodies of living organisms are 

 perpetuations of the proportions to which these organisms were 

 exposed at certain stages of their development, have been used to 

 show that the osmotic pressure of the body fluids of the higher 

 animals may also be a perpetuation of the conditions of an early 

 stage of their development. The sea water at the time when these 

 animals were developing closed circulations must have been much 

 more dilute than it is to-day; its osmotic pressure has been assessed 

 at a value only about one-third of what it is at present. Such a 

 value would be very close to the osmotic pressure of the body fluids 

 of the higher animals. 



EXCHANGE OF MATERIAL BETWEEN ORGANISM AND ENVIRONMENT 



The mechanism for the control of the proportions between certain 

 of the constituents of the living organism must be of considerably 

 more ancient origin than the mechanism for the control of the total 

 concentrations of these constituents. Even the simplest unicellular 

 organisms must be possessed to some extent of the former mechanism. 

 The means for controlling the concentration of materials, as ex- 

 pressed by their osmotic pressure, has, on the other hand, only been 

 developed in the most complex metazoa. 



It has already been indicated that the action of the former mech- 

 anism is a result of the control which the barriers between the cell 

 and its environment are able to exert over the passage of materials 

 into and out of the interior of the cell. It will also be dependent 

 to some extent upon what may for the moment be termed the 

 " affinity " of the cell contents for certain of these materials. 



Very little is yet known about the conditions governing the passage 

 of different substances across the cell membrane into the body of the 

 cell. The properties of the surface presented by the cells to their 

 immediate environment must be the most important factors in the 

 process. These properties are determined largely by the degree of 

 dispersion of protein and phospholipide colloids of which the cell 

 membrane largely consists. The proportions of the ions adsorbed 

 to these colloids exercise an important influence on their condition. 

 In particular, they affect the distribution of the dispersant medium 

 (water) between their sol and gel phases, and so can vary the area 

 of the portion of the surface through which water soluble substances 

 would be able to pass. 



The importance of maintaining the correct balance between the 

 proportions of the ions in contact with living tissue was fully 



