CONTRACTILE VACUOLE IN PARAMECIUM CAUDATUM. 215 



was finally overcome by drawing into the pipette a very small 

 amount of paraffine oil after the pipette had been filled with the 

 reagent. The oil is chemically inert under ordinary conditions 

 and served the purpose very well. 



The effect of the reagent on the five paramecia successfully 

 injected was quite striking. The animal was fixed immediately. 

 It assumed an almost hyaline appearance with the exception of 

 the nucleus, food granules, and numerous short, thick crystals 

 which are normally found throughout the body. The contractile 

 vacuole disappeared completely. No trace of the characteristic 

 needle-like crystals of di-xanthyl urea, which are precipitated by 

 xanthydrol in the presence of urea, were found either in that 

 part of the organism in which the vacuole is usually situated, 

 or in the liquid surrounding the organism. Some of these 

 observations were made under an apochromatic oil immersion 

 lens system. It seems from this, then, that if urea is present in 

 the fluid of the vacuole its concentration is too low to be detected 

 with the reagent used, that is, one part in 12,000. 



Now the question arises as to whether or not all of the urea 

 excreted could be eliminated by the contractile vacuole if the 

 concentration is as low as this. If not, then it is evident that 

 the contractile vacuole does not function specifically in the 

 excretion of nitrogen, and if this is true it is not an excretory 

 organelle in the ordinary sense of the term. 



The concentration of urea that should be in the fluid of the 

 vacuole, if all of it is eliminated through it, was ascertained in 

 the following manner. Maupas (1883) found that the vacuoles 

 of Paramecium aurelia evacuate a quantity of water equal to 

 the volume of the entire organism in forty-six minutes at twenty- 

 seven degrees. It was assumed in making these calculations 

 that the relative quantity of water evacuated by Paramecium 

 caudatum is approximately equal to that evacuated by Para- 

 mecium aurelia during the same period and at the same tempera- 

 ture. The average volume of Paramecium caudatum was as- 

 sumed to be that of a cylinder 150 microns long and 35 microns 

 in diameter, and the diameter of the vacuole when distended 

 10 microns. 



On the basis of these assumptions and the observations of 



15 



