172 THE BEHAVIOR OF LOWER ORGANISMS. 



fasten the stretched sheet of India rubber by means of a strong cement 

 to a plate of glass. Upon being released the tense layer of protoplasm 

 contracts again, just as the rubber sheet would do. In the protoplasm 

 the contraction takes place rather slowly, causing the steady pulling 

 forward of the posterior half of the body, as exhibited by objects attached 

 to its lower surface. 



In connection with the contraction of the lower surface as just de- 

 scribed we must consider also certain properties of the upper surface. 

 When this is pulled upon by the advancing anterior wave it does not 

 respond like an inelastic membrane, but like an elastic, contractile one. 

 If it were inelastic its motion would follow that of the anterior end 

 exactly, and thus take place in a series of jerks. But this does not 

 occur. When the anterior wave* pushes forward, thus extending the 

 upper surface, there is no immediate increase in the movement of this 

 surface, nor of the posterior end ; the movement forward is a steady 

 one. The property of contractility is further shown very directly in the 

 phenomena which take place when a large portion of the lower surface 

 of the Amoeba is suddenly released, as described on page 167. It seems 

 clear that the entire surface of the Amoeba is in a state of tension and that 

 this tension is directed toward the advancing anterior end. The condi- 

 tion would be imitated by partly rilling a rubber sack with a heavy fluid, 

 causing one surface to adhere to the substratum and pulling on one side. 



As a result of this tension on the surface the internal contents of the 

 Amoeba must, of course, be under a certain slight amount of pressure. 

 As we have seen (p. 171) even without this pressure the internal con- 

 tents must flow forward, since the posterior surface, against which 

 they are resting, is moved forward. But the pressure accounts for 

 certain details of the movements of the endosarc. Thus, when a pseudo- 

 podium is sent forth, or one of the anterior waves moves forward, it is 

 usually soon filled by endosarc. In its pushing forward the pseudo- 

 podium forms a region where the tension is relieved ; the fluid contents, 

 under pressure elsewhere, therefore flow into it. 



It is to be noted that this pressure is a mere consequence of the tension 

 due to the pushing forward of the anterior edge, and is by no means a 

 cause of the pushing forward ; it is always, therefore, subordinate to and 

 dependent upon the latter, and is not a matter of primary significance. 



Altogether, then, our results lead us to look upon Amoeba as an elastic 

 and contractile sac, containing fluid. In locomotion one side of this sac 

 actively stretches out, becomes attached to the substratum, and draws 

 the remainder of the sac after it in a rolling movement. The primary 

 phenomena are the stretching out of one side, the elasticity, and the 

 contractility of the outer layer. 



Whether this elasticity and contractility should not be considered 



