THE CONTRACTILE VACUOLE 421 



of membranes around contractile vacuoles is exceedingly unsatisfactory. 

 Aside from the fact that many investigators admit the probability of, 

 or in some instances the necessity for, a physiological membrane of the 

 type described above as composed of molecules oriented and more closely 

 packed together at the phase boundary, very little of a positive nature is 

 known. It is difficult to understand how a vacuole without any sort of 

 membrane can retain its identity in cytoplasm with which its contents ap- 

 pear to be freely miscible. That most of the cellular contents are freely 

 miscible with water is indicated by the fact that discharge of the cell 

 contents into the surrounding water, following rupture of the cell wall, 

 is soon followed by dispersion of most of the cytoplasm into the sur- 

 rounding medium; usually only granules, of one sort or another, re- 

 main to indicate the original position of the extruded material. If the 

 vacuole content is mostly water, a belief quite generally if not universally 

 held, how can a vacuole ever be formed in the absence of any kind of 

 membrane to prevent this water from flowing back into the cytoplasm as 

 rapidly as it is mobilized? 



As indicated above, a great many investigators have demonstrated 

 structures which were interpreted as vacuole "walls." In some material 

 these walls were visible in living organisms as layers of substance, opti- 

 cally different from substances on either side of it. In other material the 

 walls were visible only after fixation and staining. One is more or less 

 obliged to accept an author's description of structures in material ex- 

 amined by him; but an observation sometimes is subject to two entirely 

 different interpretations. This is clearly shown by MacLennan (1933) in 

 his work on the Ophryoscolecidae, as pointed out earlier. In several in- 

 stances the presence of morphological membranes is claimed by various 

 authors, on the basis of observations on what may have been overim- 

 pregnated material. Because of the extreme thinness of physiological 

 membranes, it is doubtful if they ever can be demonstrated visually, but 

 evidence obtained from the study of other colloidal systems indicates that 

 they almost certainly exist. 



The Function of Contractile Vacuoles 



During the years that have intervened since the discovery of the con- 

 tractile vacuole, an extensive literature concerning its function (or func- 

 tions) has accumulated. Excellent reviews of this literature have been 



