THE PHYSICAL BASIS OF LIFE 9 



granules as well as clear spaces or vacuoles. (Fig. 2.) 

 If it is examined in water it exhibits no tendency to mix with 

 the surrounding medium, though investigations show that 

 osmotic interchanges are constantly going on. For this 

 reason it is impossible to consider protoplasm except in 

 connection with its surroundings whatever they may be 



FIG. 2. A simple animal (Amoeba proteus) which consists ot a single unit mass 

 of protoplasm (highly magnified). 1, nucleus; 2, contractile vacuole; 3, pseudopodia; 

 4, food material in process of digestion (food vacuole); 5, sand particle or other indi- 

 gestible inclusion. (From Shipley and McBride, after Gruber.) 



variations in its environment and variations in its activities 

 being reflected directly or indirectly in its appearance. 

 Under the highest magnifications, not only does the finer 

 structure of protoplasm differ in various specimens, but also 

 in the same living unit mass under slightly different physi- 

 ological conditions. At one time it presents the appearance 

 of a fairly definite net-like structure, or reliculum, the meshes 

 of which enclose a more fluid substance; at another, a frothy 



