54 BIOLOGY 



However, the production of these pseudopodia cannot be 

 satisfactorily explained by any of these means; enough careful 

 study of the Amceba in motion has been made to show that the 

 pseudopodia may be thrust out in any direction, either horizon- 

 tally or vertically; and when thrust out vertically they may be 

 bent forward until they come in contact with the surface on 

 which the animal rests and then become attached. Their motion 

 has to be explained by an active power of the living substance. 

 This power on the part of the living substance has been called 

 contractility, and it cannot be explained as due to any physical 

 force like surface tension, adhesion, or chemical attraction, but 

 is due rather to active contraction which must be regarded as 

 a general function of the protoplasm of a living cell. 



Structure. The body of Amoeba is made up of a transparent 

 mass of protoplasm, in which there may be distinguished an 

 outer clearer layer, called ectoplasm (Gr. edos = outside + 

 plasma), and an inner, more granular mass called endoplasm 

 (Gr. endon = within + plasma). No very definite line can be 

 drawn between them, the difference being due chiefly to the 

 presence of granules in the interior and their absence from the 

 outer layer. These granules are in motion, slowly circulating 

 within the animal, and thus showing the existence of currents 

 in the protoplasm. When the pseudopodia are protruded, the 

 first change is the protrusion of a lobe of the ectoplasm; after 

 which the granules can be seen flowing into the lobe until 

 finally the whole of the endoplasm may flow into the extruded 

 lobe. Many of these granules represent food in various stages 

 of digestion, some of them being digested food and others un- 

 digested refuse. Among them may be found drops of clear 

 liquid with a bit of digested food in their center. 



Besides these granules, two more definite bodies are always 

 found. One (Fig. 19 ri), the nucleus, is a small rounded body 

 near the center of the animal, but not fixed in position, since it 

 moves with the protoplasmic current. This is one of the struc- 

 tural parts of the animal, not, like most of the granules, merely 



