62 



ELEMENTS OF BIOLOGY 



podia is brought about. More recent work indicates, however, that 

 the process is much more comphcated. Apparently internal changes 

 in the protoplasmic structure are constantly going on, so that from 

 a central core of liquefied protoplasm materials are flowing in the 

 direction of the forming pseudopodium. Near the tip these ma- 

 terials are constantly being added to a distinctly less liquid rind 



WATER 



CHLOROFORM 



ALCOHOL 



Fig. 14. — Diagram to show how a chloroform drop in water may be made to 

 exhibit extrusions resembling pseudopodia by the local application of alcohol. The 

 alcohol from the mouth of the pipette causes a local lowering of the surface tension 

 of the chloroform. The surface tension of the remainder of the drop is unaltered and, 

 being greater than that of the region affected by the alcohol, causes the chloroform 

 drop to bulge at the weakened area. 



and there jelled, while at the opposite extremity of the animal the 

 gel is being liquefied and flowing into the central core. The pellicle 

 appears to behave somewhat like the continuous tread of a tractor, 

 the living organism moving forward on the pellicle as a fulcrum 

 (Fig. 15). Just what the physico-chemical events are that bring 

 about this phenomenon is unknown. 



Irritability in Amoeba. Any object or condition in the en- 

 vironment of the amceba that will afifect it first encounters its sur- 

 face. So the first response of an amceba, or of any other cell, is the 

 response of its surface. In amoebae the type of response is necessarily 



