158 life's beginning on the earth 



to the naked eye. Even with a microscope it is not easy to 

 distinguish it from microscopic bits of non-living matter 

 such as sand particles, which are likewise glassy looking. 

 Because it constantly changes its shape, an Amoeba can be 

 discriminated from these particles. This change is effected 

 by projections pushed out from the soft mass of which the 

 Amoeba consists; the projections are called pseudopodia. 

 They undergo the most diversified alterations of size and 

 shape. At times they may be withdrawn, while other 

 processes are slowly pushed out at other places. (Fig. 60.) 



At the same time careful watching shows that the Amoeba 

 is also changing its position, although this is done with 

 extreme slowness. It moves by a kind of streaming motion. 

 A projection forms itself on one side, and the entire sub- 

 stance of the Amoeba gradually streams into it. A fresh 

 projection appears toward the same side; the streaming 

 movement is repeated. The Amoeba moves along very 

 slowly, so slowly in fact that it takes close watching to 

 detect it. (Fig. 61.) 



The essential feature of the movement of Amoeba is a 

 streaming of its internal liquid substance. No continued 

 locomotion can be observed unless accompanied by this 

 streaming, the main streams being invariably in the direc- 

 tion in which the Amoeba is moving. Wherever pseu- 

 dopodia are formed, the mass in the center of the pseu- 

 dopodium rapidly flows forward. It then turns back at the 

 periphery, but flows at a much slower rate. On account of 

 the greater rapidity of the forward movement, the pseu- 

 dopodium extends. 



Judging from the nature of the movements, we are obliged 

 to infer that the substance of which Amoeba is composed 

 must be soft and semi-fluid. Yet it preserves a sharp con- 

 tour; consequently, it must be either immiscible in water, 

 or surrounded by a membrane or skin. Further observa- 

 tions show that there is a membrane since the outer portion 



