236 AMCEBA CHAP. 



spoken of collectively as metabolism constructive and 

 destructive (p. 149). Living protoplasm is thus the most 

 unstable of substances ; it is never precisely the same 

 thing for two consecutive seconds ; its existence, like 

 that of a waterfall or a fountain, depends upon the 

 constant flow of matter into it and away from it. 



It follows from what has been said that if the income 

 of an Amoeba, i.e., the total weight of substances taken 

 in (food plus oxygen plus water), is greater than its ex- 

 penditure or the total weight of substances given out 

 (faeces plus excreta proper plus carbon dioxide), the 

 animalcule will grow : if less it will dwindle away : if the 

 two are equal it will remain of the same weight or in a 

 state of physiological equilibrium. 



It is evident that Amoeba must also be able to 

 perform the function of reproduction. You have learnt 

 that the cells of the frog multiply by simple or binary 

 fission (p. 106) : the nucleus first divides into two, and 

 then the surrounding protoplasm ; and precisely the 

 same thing occurs in Amoeba, the reproduction of 

 which therefore takes place by the simplest method 

 known, without any special reproductive organs. The 

 animalcule simply divides into two Amoebae, each exactly 

 like itself, and in doing so ceases to exist as a distinct in- 

 dividual. Instead of the successive production of 

 offspring from an ultimately dying parent, we have 

 the simultaneous production of offspring by the division 

 of the parent, which does not die, but becomes simply 

 merged in its progeny. There can be no better instance 

 of the fact that reproduction is discontinuous growth. 



From this it seems that an Amoeba, unless suffering a 

 violent death, is practically " immortal," since it divides 

 into two completely organised individuals, each of which 

 begins life with half of the entire body of its parent, 

 there being therefore nothing left of the latter to die : 



