1^ AMCEBA LESS. 



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. ii) 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 chemi- 

 cal change resulting in the formation of a thin superficial 

 layer of non-i)rotoplasmic substance. The process is re- 

 peated, 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 proportiona' 

 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, a cer- 

 tain fraction of the protoplasm becoming oxidized, or in other 

 words undergoing a process of low temperature combustion. 



