422 THE CONTRACTILE VACUOLE 



published from time to time (Howland, 1924b; Day, 1927; Lloyd, 

 1928). Therefore no attempt will be made to present another review 

 at this time, except in so far as the works to be mentioned have a direct 

 bearing on one or the other of the two functions generally conceded to 

 be most probable. 



Of the various functions assigned to the contractile vacuole those of 

 excretion of metabolic waste products and regulation of hydrostatic pres- 

 sure within the cell have received most frequent support. Some authors 

 prefer to limit "metabolic waste products" to nitrogenous substances, 

 although others include carbon dioxide as well. In view of the scarcity 

 of evidence bearing directly on the subject, it hardly seems advisable at 

 this time to distinguish between different kinds of metabolic wastes. On 

 the other hand, if one is to understand excretion to mean the expulsion of 

 any sort of waste material from the organism, then the function was defi- 

 nitely established as excretory when Stokes (1893), and later Jennings 

 (1904) proved the discharge of the vacuole to the exterior. But such a 

 generalization offers little satisfaction. 



Probably the earliest suggestion that the vacuole is an excretory organ- 

 elle was made by Stein and Schmidt (see Kent, 1880, p. 69), who 

 stated that "the functions discharged by the contractile vacuole are ex- 

 cretory and correspond most nearly with that of the renal organs of the 

 higher animals." Griffiths (1888) made the statement, based on his 

 own experiments, that the vacuole performs the function of a kidney, 

 and that its secretions are "capable of yielding microscopic crystals of 

 uric acid." As material for these experiments he used Amoeba, Parame- 

 cium, and Vorticella. In describing these experiments, Griffiths says (p. 

 132): 



After the addition of alcohol minute flakes could be distinctly seen floating 

 in the fluid of certain vacuoles. Bearing in mind the murexide reaction, there 

 is every reason to believe that these flakes are nothing more or less than 

 minute crystals of uric acid. 



These experiments were repeated many times, generally with positive 

 results, indicating the presence of uric acid. At times, however, the vacuole 

 was found not to contain the slightest trace of uric acid. Howland 

 (1924b) repeated these experiments using Paramecium, Cent ro pyxis, 

 and Amoeba, but always with negative results. However, uric acid was 

 found in cultures of Paramecium and Amoeba by Howland, and the 



