THE CONTRACTILE VACUOLE 427 



cate that a decreased permeability of the cell wall is partly responsible. 



Herfs (1922) investigated the effects of changes in tonicity of the 

 external medium on several kinds of organisms, both free-living fresh- 

 water forms and parasitic forms. He found the pulsation frequency in 

 Paramecium to be decreased to about one-fourth the normal when the 

 organism is transferred from fresh water to 0.75 percent NaCl solution. 

 Lower concentrations of salt produce less marked changes. Gastrostyla 

 steinii showed essentially the same reaction, except that organisms kept 

 for about fourteen days in one-percent NaCl solution were found to 

 contain no contracting vacuoles. With Gastrostyla the vacuole seems to 

 disappear at a NaCl concentration of 1.1 percent to 1.3 percent, and 

 to reappear at a concentration of about 0.5 percent. The pulsation fre- 

 quency of Nyctotherus cordijormis. an intestinal parasite of the frog, 

 was found to vary between wide limits, presumably because of cor- 

 responding variations in the water content of the medium. Graded pulsa- 

 tion frequencies were observed in vitro when the exterior medium varied 

 from tap water to one-percent NaCl solution. Opalina ranarum, which 

 possesses no contractile vacuole, can adapt itself to relatively wide varia- 

 tions in tonicity of the exterior medium without developing a vacuole, 

 if the changes are made gradually. From this Herfs was led to doubt 

 whether or not a vacuole is necessary for the prevention of overdilution , 

 of the cytoplasm. He noted further that the lack of a vacuole in Opalina 

 goes hand-in-hand with the lack of a cell mouth; whereas Nyctotherus 

 possesses both a cell mouth and a vacuole. From this he assumes that it 

 is the water taken in through the mouth that is pumped out by the 

 vacuole. Herfs does not seem to be altogether consistent in this idea, 

 since he further states, as his opinion, that in ordinary cases at least 

 the water taken in through the entire cell surface is of decisive signifi- 

 cance for the appearance of the vacuole. With respect to the adaptation 

 of organisms possessing no vacuoles to variations in tonicity of the ex- 

 terior medium, Herfs seems to have overlooked as an explanation, the 

 possibility of an interchange of salts between cell and medium, a pos- 

 sibility which will be mentioned again later. 



Eisenberg (1926), assuming the volume of Paramecium to be ap- 

 proximately equal to that of an ellipsoid of rotation having the same 

 dimensions, found that the two vacuoles discharge a volume of liquid 

 equal to that of the organism in 20 minutes, 51 seconds. The average 



