204 SURFACES AND MEMBRANES 



magnesium between that of the jellyfish and the mammal. Macallum 

 [1917] suggested that these and a mass of other data point to the pos- 

 sibility that an animal not possessed of a kidney, or analogous organ, 

 cannot accurately regulate the concentration of ions in its body fluids, 

 and must through osmosis approximate the total ionic concentrations 

 of its surrounding medium. 



It is possible, for instance, to introduce a spider crab (Maia verrucosa) 

 into diluted or concentrated sea water and find that the crab's body fluid 

 can adjust itself through osmosis to its new environment. The fiddler 

 crab (Portunus depurator), commonly found on our southern beaches, 

 also has no regulative osmotic pressure capacity; hence, the osmotic 

 pressure of its body fluid is that of normal sea water (A = 2.30). As we 

 ascend the animal scale and examine the migratory forms, such as the 

 salmon, which leaves the deep sea to go up into fresh water, or the eel, 

 which migrates thousands of miles into the Sargasso Sea in order to 

 spawn, it is found that the osmotic pressure of their blood is only slightly 

 modified. The blood of the sand shark has a freezing point whose 

 A value is slightly below that of sea water. The blood of whales and 

 other marine mammals, on the other hand, has the same freezing point 

 as the blood of land mammals. Apparently, the more specialized the 

 animal form, the more perfectly is it adapted to regulate the osmotic 

 pressure of its fluids. 



In the higher animal forms the kidney plays a very important role in 

 regulating the osmotic pressure of the blood. The theories of kidney 

 secretion, however, are outside the scope of this subject and can be 

 reviewed in any textbook on general physiology, such as that by Mitchell 

 [1938]. 



Model of a Cell Membrane 



Despite the extensive research of the past years, the constitution and 

 functions of the limiting surfaces of cells remain on the whole unsolved. 

 As a rule, the limiting surfaces are ill defined, probably highly variable in 

 structure and properties, and so closely bound up with the protoplasm 

 of which they form a part that they cannot be clearly recognized as mem- 

 branes. There is a body of evidence suggesting that the limiting layer 

 of the protoplast is a thin membrane composed largely of fatty sub- 

 stances containing submicroscopic compartments composed of an aque- 

 ous phase which is chiefly responsible for determining the entrance or 

 exclusion of substances into the cell. Another well-supported view is 

 that the cell envelope is a fluid bimolecular layer of lipoid (fatlike) mole- 

 cules. This membrane, however, cannot be homogeneous, since water 

 penetrates with ease into cells. 



