THE CONTRACTILE VACUOLE 415 



morphological membranes with which they are often associated in liv- 

 ing material. Semipermeability, or more properly selective permeability, 

 is a property of all living physiological membranes. Colloid-chemists, 

 as well as many physiologists, are agreed that a physiological membrane 

 is simply a phase boundary, an interface between two different fluids. 

 In order that such phase boundary may be more or less permanent, it 

 is necessary that the two phases be only slightly miscible at most — the 

 more complete the immiscibility the more nearly perfect and permanent 

 the membrane. A very wide variety of molecules show polar phenomena; 

 that is, the two ends of the molecules are electrically and chemically 

 different. This results in orientation of molecules with respect to one 

 another and to various other molecules, in much the same manner that 

 a compass needle becomes oriented with respect to the magnetic poles of 

 the earth. This phenomenon of orientation is associated with organic 

 acids, alcohols, aldehydes, lipoids, fats, proteins, and many other so- 

 called "physiological" compounds. Thus at the interface of the two- 

 phase system, oil-water, the oil molecules (glycerol-esters of fatty acids) 

 become oriented in such a manner that the hydrocarbon ends of the 

 molecules project into the oil phase, whereas the glycerol ends project 

 into the water. Fat molecules undergo much more nearly perfect orienta- 

 tion than water molecules, although with the latter there appears to be 

 a certain degree of orientation. Such an aggregation and packing to- 

 gether of oriented polar molecules at an interface represents a physio- 

 logical membrane. Since protoplasm contains a variety of polar molecules, 

 the membrane formed between protoplasm and water is composed of 

 various types of molecules apparently arranged in the form of a mosaic. 

 The thickness of such a membrane has not been definitely established. 

 Some authors maintain that it is only a single molecule thick, or at most 

 only one or two milli-micra thick, but at least one author (Peters, see 

 Clark, 1933, p. 40) has advanced a theory according to which the cell 

 is composed of a three-dimensional protein mosaic, with the molecules 

 in the interior of the cell oriented on the surface film. Since the interior 

 of many cells is known to be fluid, the structure must be regarded as an 

 orientation rather than as an anatomical skeleton. This theory of Peters's 

 agrees fairly well with certain evidence concerning the action of drugs 

 on cells. Without entering into the question as to how far orientation 

 extends beneath the surface layer, suffice it to say that it is well established 



