122 MOVEMENT 



in many muscles, etc. It was formerly thought that amoeboid 

 movement took place owing to local differences of surface- 

 tension at the surface of the Amoeba. If the surface-tension 

 was weakened at one spot the protoplasm would bulge out 

 there just as a rubber balloon bulges at a place where the 

 rubber is weakened. But the problem is not so simple. Sur- 

 face-tension alone is probably not powerful enough. The 

 mechanism of amoeboid movement is still a mystery, but it 

 is becoming more and more probable that it is bound up with 

 the colloidal nature of protoplasm which turns readily from 

 a more solid gel state to a more liquid sol state and vice versa. 

 I have not been able to find any record of the rate of pro- 

 gression of an Amoeba, so I asked a friend of mine to determine 

 its average speed. He timed four separate Amoebae, very large 

 Amoebae, for they measured when fully extended 0-5 mm. 

 Altogether he made ten different observations, and he recorded 

 the time the Amoebae took to pass over a measured distance 

 of 0-2 mm. The fastest Amoeba covered the distance in 

 60 seconds : the slowest took 80 seconds. On the whole they 

 averaged 70 seconds. At this rate their speed amounted to 

 10-3 mm. an hour. Unlike Browning's hero who "never 

 turned his back, but marched breast forward," they sauntered 

 leisurely along, pausing here and there, sometimes to throw 

 out a pseudopodium on one side or the other, and then with- 

 drawing it they moved forward again, like a man dawdling 

 down a village street, stopping a moment to look at some object 

 in a shop window, and a few minutes later to exchange the 

 time of day with a neighbour. Their progress was discon- 

 tinuous, so that the actual rate of their fastest movement is 

 quicker than the above figures indicate. On a perfectly clean 

 slide, however, they do not dawdle. 



Movement of Animals 



The great majority of animals can and do move from 

 place to place; and even when they are fixed or, as it is 

 termed, sessile, their larvae are capable of very active move- 

 ment. Like all young animals they wish to scatter and see 

 the world. They swim away from their mother and seek 

 "fresh woods and pastures new." 



