CONTRACTILE VACUOLE IN PARAMECIUM CAUDATUM. 2OQ 



is widely accepted, and is supported to some extent by experi- 

 mental evidence. In certain instances, the term excretion is 

 limited to mean only the expulsion of a fluid from the cell, as in 

 Carter's (1861) observations. Generally, however, this theory 

 assigns to the vacuole the function of a renal organ of some 

 kind. 



Hartog (1888), Calkins (1901), Ziilzer (1910), Doflein (1911), 

 and others maintain that the vacuole is an organelle for regulating 

 the hydrostatic pressure within the cell, or a mechanism for 

 removing the excess water which is taken into the body in 

 feeding and through the cell membrane by osmosis. Stempell 

 (1914) constructed a mechanical system which shows clearly 

 that osmosis can be made the causal agent for producing inter- 

 mittent discharge of a fluid from such a system, but this system 

 was doubtless not intended to be compared with the vacuole in 

 any respect other than the pulsating effect. 



The literature concerned with the function of the contractile 

 vacuole consequently reveals no conclusive evidence in support 

 of one theory to the exclusion of all others. The consensus of 

 the evidence, however, seems to indicate that the vacuole is an 

 organelle concerned either with the removal of waste products 

 of metabolism, or with the removal of excess water which 

 accumulates in the organism as a result of endosmosis and 

 feeding. The experiments described in the following pages have 

 a direct bearing on the question as to which of these obtains. 

 These experiments consist of attempts to ascertain the nature 

 of the nitrogenous excretion products of metabolism in Para- 

 mecium caudatum, and whether or not they are excreted through 

 the contractile vacuole. 



THE NATURE OF THE NITROGENOUS END PRODUCTS IN 



Paramecium. 



The nitrogenous end products of metabolism in organisms 

 vary in their nature according to the type of organism. For 

 example, in man the bulk of the nitrogen is eliminated as urea, 

 considerably less as uric acid and amino acids, and a very small 

 part as free ammonia; while in the birds and reptiles the bulk 

 of the nitrogen is eliminated as uric acid. 



