i 4 AMOEBA 



less layer will be deposited round the coloured crystal : if 

 growth took place by intussusception we should have a 

 gradual weakening of the tint as the crystal increased in size. 

 This mode of growth by the deposition of successive layers 

 -is called growth by accretion. 



It is probable that the cyst of Amoeba referred to above 

 (p. n) grows by accretion. Judging from the analogy of 

 other organisms it would seem that, after rounding itself off, 

 the surface of the sphere of protoplasm undergoes a 

 chemical change resulting in the formation of a thin super- 

 ficial layer of non-protoplasmic substance. The process is 

 repeated, new layers being continually deposited within the 

 old ones until the cell-wall attains its full thickness. The 

 cyst is therefore a substance separated or secreted from 

 the protoplasm ; it is the first instance we have met with of 

 a product of secretion. 



From the fact that Amoeba rarely attains a greater dia- 

 meter than \ mm., it follows that something must happen to 

 counteract the constant tendency to grow, which is one of 

 the results of assimilation. We all know what happens in 

 our own case : if we take a certain amount of exercise 

 walk ten miles or lift a series of heavy weights we undergo 

 a loss of substance manifested by a diminution in weight 

 and by the sensation of hunger. Our bodies have done a 

 certain amount of work, and have undergone a proportional 

 amount of waste, just as a fire every time it blazes up 

 consumes a certain weight of coal. 



Precisely the same thing happens on a small scale with 

 Amoeba. Every time it thrusts out or withdraws a pseudo- 

 pod, every time it contracts its vacuole it does a certain 

 amount of work moves a definite weight of protoplasm 

 through a given space. And every movement, however 

 slight, is accompanied by a proportional waste of substance, 



