430 THE CONTRACTILE VACUOLE 



increase in concentration is necessary to cause shrinkage, indicating an 

 osmotic value for the cytoplasm, after adjustment, only slightly above 

 that of the medium. Hopkins concludes from these observations that 

 when the medium is either diluted or concentrated, the organism auto- 

 matically loses or gains osmotically active substances to or from the 

 medium respectively, in such proportion that when adjustment is com- 

 pleted the osmotic value of the cytoplasm is but slightly higher than 

 that of the medium, and that this is accomplished independently of the 

 action of vacuoles. Herfs, whose observations on Opalina have been 

 described previously, may find such an explanation applicable to the 

 unexpected behavior of this organism. It is interesting to speculate as 

 to whether or not such adjustment to external osmotic-pressure differences 

 as postulated for Flabe/iula, and possibly Opalinct, represents the most 

 primitive type of mechanism for this type of adjustment with Noct'duca 

 (see Harvey 1917), which develops contractile vacuoles when the tonic- 

 ity of the external medium is greatly reduced, occupying a position inter- 

 mediate between Flabellula and those forms which possess vacuole sys- 

 tems. 



In spite of the quite extensive literature dealing with the question, 

 one is obliged to admit that virtually nothing has been proved beyond 

 question concerning the function or functions of contractile vacuoles. Car- 

 bon dioxide and nitrogenous wastes of one sort or another are undoubtedly 

 excreted by Protozoa. It is reasonable to suppose that at least a part of these 

 highly soluble wastes finds its way into the fluid of the vacuole and is 

 excreted in this manner. Many authors hold that it is not only reasonable 

 to suppose this, but that it is unreasonable to suppose that it does not 

 occur. But, be that as it may, the contractile vacuole certainly has not 

 been proved an organelle whose main function is excretion of metabolic 

 wastes. Likewise, there is indisputable evidence that many fresh-water 

 Protozoa show a decreased pulsation frequency when the tonicity of the 

 exterior medium is increased; and there is equally valid evidence indicat- 

 ing that the reverse occurs when many marine and parasitic Protozoa 

 ' are transferred to a medium having a decreased tonicity. One may regret 

 the fact, but it is none the less true, that these observations prove 

 nothing more than the bare statement which describes the observations. 

 They strongly suggest that the vacuole operates to prevent excessive 

 dilution of the cytoplasm, or to regulate osmotic pressure within the 



