THE PHYSICAL BASIS OF LIFE 



27 



or mechanical, but the nature of the response is determined rather 

 by the fundamental character of protoplasm itself than by the 

 nature of the stimulus. Thus muscle cells respond by contracting, 

 regardless of the nature of the stimulus. 



The reactions of organisms usually result in movement, one of 

 the most obvious manifestations of life. Movement depends in 

 every instance upon tumultuous ultramicroscopic physico-chemical 

 changes of protoplasm itself. Thus it is to these changes that, in 

 the last analysis, we must turn for the energy which brings about 

 the visible movements in animals and plants. 



Fig. 11. — Amoeboid movement. Successive changes in form assumed by 

 an Amoeba. The clear ectoplasm flows forward, followed by the granular 

 endoplasm with the nucleus (darker) and contractile vacuole (lighter). See 

 Fig. 6. (Modified, after Verworn.) 



The obvious movement of the higher animals is, of course, the 

 result of the contraction (shortening and broadening) of the indi- 

 vidual muscle cells forming the muscular system, but movements 

 of other cells of the body occur as, for example, the flowing move- 

 ment of the white blood cells which is similar to that of certain 

 unicellular animals, the Amoebae, and so is known as amoeboid 

 movement. The protoplasm of an active Amoeba is one of the 

 most striking and beautiful sights under the microscope; the cell 

 ceaselessly changing its form as one outflowing, or pseudopodium, 

 follows another and the whole cell advances in the direction of the 

 main stream. When particles of food are met, the pseudopodia 



