PHYSIOLOGY 



105 



the ectoplasmic tube forces the endoplasmic streaming to the 

 front. This observation is in agreement vnth that of Mast (1923, 

 1926, 1931) who after a series of carefully conducted observations 

 on Amoeba proteus came to hold that the amoeboid movement is 

 brought about by ''four primary processes; namely, attachment to 

 the substratum, gelation of plasmasol at the anterior end, solation 

 of plasmagel at the posterior end and the contraction of the plas- 

 magel at the posterior end" (Fig. 42). As to how these processes 

 work, Mast states: "The gelation of the plasmasol at the anterior 



Fig. 43. Diagrams of varied cytoplasmic movements at the tip of a 

 pseudopodium in Amoeba proteus (Mast), g, plasmagel; he, hyaline 

 cap; hi, hyaline layer; pi, plasmalemma; s, plasmasol. 



end extends ordinarily the plasmagel tube forward as rapidly as it 

 is broken down at the posterior end by solation and the contrac- 

 tion of the plasmagel tube at the posterior end drives the plas- 

 masol forward. The plasmagel tube is sometimes open at the 

 anterior end and the plasmasol extends forward and comes in con- 

 tact with the plasmalemma at this end (Fig. 43, a), but at other 

 times it is closed by a thin sheet of gel which prevents the plas- 

 masol from reaching the anterior end (6). This gel sheet at times 

 persists intact for considerable periods, being built up by gelation 

 as rapidly as it is broken down by stretching, owing to the pres- 

 sure of the plasmagel against it. Usually it breaks periodically at 

 various places. Sometimes the breaks are small and only a few 

 granules of plasmasol pass through and these gelate immediately 

 and close the openings (d). At other times the breaks are large and 

 plasmasol streams through, filling the hyaline cap (c), after which 

 the sol adjoining the plasmalemma gelates forming a new gel 

 sheet. An amoeba is a turgid system, and the plasmagel is under 

 continuous tension. The plasmagel is elastic and, consequently, is 



