1923] Contractile Vacuoles in Amoebae 53 



The amoebae were grown on Petri dishes which were inoculated 

 with a loop full of amoebae in the center of the dish. As the amoebae 

 and the bacteria which served as their food multiplied, they spread 

 out over the Petri dish in ever ^yidening circles. The amoebae at the 

 outer edge of the growth were always the largest and apparently in 

 the most healthy condition. It was these which were used each day. 

 The same method of observation as noted above was used. Five read- 

 ings from five different amoebae were made and the averages 

 calculated. 



The temperature varied from 22° to 25.5° C. as the experiments 

 were performed in July and August at Woods Hole. The Petri dishes 

 were kept at room temperature which is not constant. However the 

 slight variations did not seem to appreciably affect the experiments. 



In tables II and III there is considerable variation but one 

 notices a steady slowing down of the rate of contraction of the vacuole 

 as the age of the amoeba culture increases. This is especially notice- 

 able in table III wiiere observations were made until the day the 

 amoebae encysted. 



Looking at the contractile vacuole as an excretory organ, as the cul- 

 ture ages, waste products accumulate in the medium, the metabolic 

 processes are slower and the contractile vacuole does not pulsate so 

 rapidly. 



The difference in the vitality of the amoebae used can be observed 

 in table III where A W encysted on the ninth day while A P.„ growing 

 on the same kind of medium, under the same climatic conditions, did 

 not encyst until the fourteenth day. It is of course possible that aside 

 from the individual peculiarities of the amoebae there may have been 

 great differences in the condition of the bacteria which surrounded 

 them and served as their food. 



It was also noticed that when an amoeba is at rest the rate of con- 

 traction of the vacuole is much slower than when the amoeba is mov- 

 ing. One amoeba was watched, which when it was quiet had a con- 

 tractile vacuole pulsating at the following intervals in seconds: 70, 

 145, 105, 95. When it began to move, the rate of contraction immedi- 

 ately changed to every 60 and then every 50 seconds. This seems 

 further evidence that the contractile vacuole functions also as an 

 excretory organ. The faster the amoeba moves, the faster are the 

 waste products formed and in order to expell them the contractile 

 vacuole pulsates more rapidly. 



