UNICELLULAR ORGANISMS 



53 



There has been considerable speculation as to the forces which 

 produce pseudopodia, and various attempts have been made to 

 explain them by purely physical forces. It has been suggested 

 that they are due to the adhesion of the sticky substance of 

 which the animal is made, to the object upon which it rests. 



FIG. 19. AMCEBA PROTEUS 



A, the animal in its natural condition; B, an animal that has swallowed a long filamentous 

 plant; C, the animal in the state of division. 



cv, contractile vacuole; ex, remains of undigested food; 



ec, ectoplasm; p, protoplasm. 



en, endoplasm; 



Another suggestion is, that the pseudopodia are due to changes 

 in surface tension produced by the currents in the body as they 

 flow to and fro. Still another theory seeks to explain the forma- 

 tion of pseudopodia by stereotropism (Gr. stereos = a solid -f- 

 trope =a turning), the attraction of a solid body for living 

 tissue, which is supposed to cause the body of the animal to 

 flow from one point to another of the surface upon which it 

 rests., There is also the theory of chemical attraction. 



