AMOEBA PROTEUS 49 



plasm and are due to enzymes which probably come from the 

 cell nucleus. They are oxidizing enzymes which bring about the 

 union of oxygen with receptors of the protoplasmic molecules. 

 The ultimate result is the formation of simple compounds, the 

 hydrogen leaving the protoplasm molecule to unite with the 

 oxygen forming water; carbon with oxygen forming carbon 

 dioxide and the ammonium combination (NH 3 ) forming with 

 carbon and oxygen the compound urea (NH 2 ) 2 CO, which still 

 contains some energy. This urea, however, cannot be used by 

 the animal protoplasm as a further source of energy but is 

 voided to the outside as waste matter. The energy, however, 

 is not wasted in nature for as we have seen bacteria have the 

 power of breaking urea into free ammonia, carbon dioxide and 

 water and of converting the contained energy into the energy 

 of their own vital processes. 



As urea is of no use to the animal but rather a menace in case 

 of its undue accumulation, it is necessary for the animal to get 

 rid of it. In all animals this is accomplished by means of 

 special organs which form the excretory systems. In mammals 

 and vertebrates generally, the kidney and associated organs 

 are set apart for this purpose; in lower animals like worms, 

 Crustacea, etc., special funnel-like organs termed "nephridia" 

 perform this function. In all types of animals in short, there 

 is some structure or structures of more or less complicated type 

 which are devoted almost exclusively to this end. 



In Amoeba proteus there is a special organ which has the func- 

 tion of disposing of waste matters and is analogous therefore, 

 with the kidney of higher types. This is the " con tractile 

 vacuole" which pulsates with a regular or rhythmic contraction, 

 the rate of pulsation varying with the temperature. It is not 

 improbable that the contractile vacuole has other functions than 

 that of urea excretion, but this has never been demonstrated 

 while the presence of uric acid crystals has been shown in the 

 fluids of the vacuole. After a vacuole has contracted the new 

 one is formed in the vicinity of the nucleus by union of small and 

 at first unnoticeable vesicles, one or more of which may be left 

 over by the incomplete emptying of the preceding one. This 



