THE VITAL PROPERTIES OF THE CELL 87 



selves in turn, with the result that the water is discharged as 

 regularly as possible. The frequency with which these evacuations 

 take place varies considerably in different species. According to the 

 observation of Schwalbe (III. 21) the following law may be stated : 

 that the smaller the vacuoles are, the more frequently are they 

 emptied. For instance, in Chilodon cucullulus they contract about 

 13 to 14 times in two minutes, in Paramtecium aurelia, only 10 or 

 11 times in the same period, whilst in Vorticella microstoma, only 

 once or twice. In Stentor and Spirostomum the contractions occur 

 less frequently still. Of all the above-mentioned animals, the two 

 last have the largest contractile vacuoles, next comes Vorticella^ 

 then Paramtecium aurelia, and lastly Chilodon cucullulus, whose 

 vacuoles are only half as large in diameter as those of Paramtec- 

 ium, where the diameter is about '0127 mm. ; in Vorticella it is 

 0236 mm (Schwalbe). 



The interval which elapses between the two evacuations is very 

 regular at the same temperature ; it is, however, considerably 

 affected if the temperature is raised or lowered (Rossbach III. 19, 

 Maupas). For instance, with Euplotes charon, the interval between 

 the contractions is 61 seconds; at 30 Celsius, it has diminished 

 to 23 seconds (Rossbach); that is tosay, the frequency has become 

 nearly trebled. 



The amount of water which in this manner passes through the 

 animal is extremely great. According to the computations of 

 Maupas, Param&cium aurelia, for example, evacuates, in 46 

 minutes at 27 Celsius, a volume of water equal to its own 

 volume. 



From the above-mentioned observations, it may be concluded 

 that contractile vacuoles are not merely simple variable drops of water 

 in the plasma, but that they are permanent morphological differentia- 

 tions in the body of the Protozoon ; that is to say, true cell organs, 

 which appear to perform an important function in the carrying on of 

 breathing and excretion. The energy with which the vacuole dis- 

 charges its contents, so that it completely disappears, indicates that 

 its walls, which consist of hyaline substance resembling the flagel- 

 lum substance, must be contractile to an exceptional degree, and 

 by means of this property are to be distinguished from the endo- 

 plasm of the infusorian body. It must, however, be admitted that 

 no special membrane, clearly denned from the remainder of the 

 body mass, can be seen microscopically, just as with smooth muscle 

 fibres the contractile substance and the protoplasm are not sharply 



