THE ANIMAL A MACHINE 161 



into the interior. Here it is surrounded by a drop of watery 

 fluid, and by degrees disappears. It is digested which 

 means transformed into the body substance of the Amoeba 

 by the enzymes present. Along with this digestible matter, 

 useless inorganic matter is occasionally taken, but this re- 

 mains unchanged and is soon thrown out again. 



The digested matter which thus disappears obviously 

 mixes with the liquid matter of the Amoeba and adds to its 

 bulk. When food is abundant and the bulk of the animal 

 has considerably increased, another striking change appears; 

 a fissure appears at the outer border of the Amoeba, divid- 

 ing it into two parts. This fissure grows inwards, the two 

 parts separate more and more, until finally the separation 

 becomes complete, and we have two distinct Amoebae from 

 the division of the one. This is an example of a "cell divi- 

 sion"; it represents the primitive mode of propagation of 

 these one-celled animals. (Fig. 62.) 



3. AMOEBOID MOVEMENT OF OIL-DROPS 



In former centuries, an animal as primitive as the Amoeba 

 would hardly have been looked upon as alive, since it alto- 

 gether lacks such differentiated organs as muscles for mov- 

 ing, a stomach and intestine for digestion, and genital 

 organs for reproduction, which are elaborately developed in 

 the higher animals. The Amoeba performs all these func- 

 tions — and still looks like a simple oil-drop. 



The question presents itself whether any plain oil-drop 

 is capable of producing similar movements. An answer 

 would obviously enable us to judge whether forces of the 

 non-living world determine the behavior of the Amoeba. 



Experience shows that a simple oil-drop does act like 

 Amoeba; it can perform movements which resemble those 

 of the animal in almost every detail, even some of the com- 

 plicated processes, such as digestion and extrusion (excre- 



