THE CONTRACTILE VACUOLE 423 



concentration was observed to be roughly proportional to the age of the 

 culture. From this she concludes that uric acid is excreted by these forms, 

 though probably not by the vacuole. 



Experiments of Nowikoff (1908), Shumway (1917), and Riddle 

 and Torrey (1923), in which the effects of thyroid feeding and the re- 

 sponse of 'Pavaniecium to thyroxin were observed, offer further though 

 indirect evidence in favor of the excretory function. Flather (1919) 

 found that epinephrine, posterior pituitary extract, and pineal gland ex- 

 tract produce similar results — an acceleration in pulsation frequency, and 

 a dilatation of the vacuole. Since these drugs cause diuresis in vertebrates, 

 the action on vacuoles may be interpreted as resembling stimulation of 

 excretion. 



Weatherby (1927) found that urea is excreted by 'Paramecium cauda- 

 tum, but was unable to detect urea in the fluid of the contractile vacuole ' 

 by means of the micro-injection of his own modification of the xanthydrol 

 reagent of Fosse (1913). This reagent yields positive results with dilu- 

 tions of urea as great as one part in 12,000. Calculations based on the 

 volume of fluid eliminated by vacuoles and the quantity of urea excreted 

 by known numbers of organisms in mass cultures indicate that the con- 

 centration in fluid of the vacuole would be of the order of one part in 

 2,000 or 3,000, if all the urea were excreted via this route. It therefore 

 appears that at most only a small part of the total urea is excreted in this 

 manner. After removal of the fluid from the contractile vacuole of 

 Spirostomum by means of micro-manipulation apparatus, and subsequent 

 hydrolysis with urease, Weatherby (1929) found urea to be present in 

 the vacuolar fluid in a concentration of about one part in 100,000. Cal- 

 culations of the rate of excretion of urea by known numbers of Spirosto- 

 mum in mass cultures indicate that this amount of urea accounts for only 

 about one percent of the total urea excreted. 



Parnas (1926) concludes from observed differences in pulsation fre- 

 quency that the vacuole is mainly excretory in marine Protozoa, and both 

 excretory and osmotic-pressure-regulatory in fresh-water forms. The ex- 

 cretory function is accepted apparently without reservation by von Gelei 

 (1925, 1928), who homologizes the various parts of the vacuole system 

 in Paramecium with the vertebrate kidney, ureter, bladder, and urethra, 

 although he admits the possibility that this system may aid in removing 

 excess water from within the organism. In Paramecium, von Gelei states 



