ffifi RABBIT. 41 



the side of katabolisrn, the amoeba will dwindle and 

 eventually die. If it is on the other side, it will grow in 

 size. 



5. Reproduction of Amoeba. Growth does not con- 

 tinue indefinitely; at some point an increase in numbers 

 takes the place of increase in size : in other words, repro- 

 duction is effected. This process in amoeba is very simple ; 

 first, the nucleus divides into two and the two move apart ; 

 then, gradually, the protoplasm becomes separated into two 

 approximately equal parts, one around each nucleus. 

 Thus we have two small amoebae instead of one large one. 

 Each one goes on feeding and growing and eventually 

 divides again. The tendency of this process is to in- 

 definitely over-populate the world with amoebae ; but it is 

 held in check by the enormous number of accidents to 

 which these creatures are liable. Thus the swallowing of 

 a spoonful of pond-mud by a fish or duck might kill 

 amoebae by the hundred, and against such misadventures 

 they are perfectly helpless. 



6. Encystment. The greatest of all dangers, how- 

 ever, is the drying-up of the pond. Against this danger 

 the amoeba has a slight protection ; under certain conditions 

 amoebae assume a spherical shape, and cease forming pseudo- 

 podia. Presently a change appears at the surface of 

 the protoplasm a firm external membrane is formed, 

 called a cyst. In this encysted condition the protoplasm is 

 preserved from destruction by drought, for the cyst pre- 

 vents evaporation of the water which forms so large a 

 portion of protoplasm. When the pond is again filled with 

 water, some of the encysted amoebae may still be among the 

 dried mud : their cysts will be burst, and the protoplasm 

 will flow out again and resume its ordinary mode of life. 

 But while the mud is dry, the wind blowing over it will 

 carry particles away as "dust," and among these will 

 probably be some encysted amoebae which may thus be 

 blown through the air some, doubtless, to perish, but 

 some to reach other ponds and begin life again there. In 

 this way among others, amoebae (like many other minute 



