490 Physiology 



end, remains on the lower surface for a time as the body rolls forward, 

 and then passes upward at the posterior end to repeat the cycle (248). 

 Locomotion in Amoeba Umax has been interpreted as "fountain stream- 

 ing," in which there is a forward streaming of endoplasm through a 

 tubular layer of ectoplasm. During movement, endoplasm is continually 

 converted into ectoplasm at the anterior end, and ectoplasm into endo- 

 plasm posteriorly. According to this interpretation, the flow on the upper 

 surface is backward, instead of forward as in rolling movement (480), 



Mast (382, 384) has resolved the ectoplasm of A. proteus into a thin 

 elastic pJasmalemma, or surface layer, and a thicker plnsmagel. Between 

 the two there is usually a hyaline fluid, except where the plasmalemma 

 is attached to the substratum. During locomotion the plasmalemma flows 

 forward, as in rolling movement. The plasmagel remains a tube which 

 is converted into endoplasm (plasmasol) posteriorly and is formed from 

 plasmasol anteriorly as a pseudopodium grows. Locomotion is attributed 

 to several processes: (1) The plasmalemma becomes attached to the sub- 

 stratum. (2) There is a local, partial liquefaction of the plasmagel. (3) 

 The rest of the plasmagel, which is under tension, forces the plasmasol 

 against this weakened area to produce a bulge, the beginning of a pseudo- 

 podium. (4) Posteriorly, the inner surface of the contracting plasmagel is 

 converted into plasmasol. (5) Anteriorly, the plasmagel tube is continu- 

 ously regenerated by gelation of the plasmasol as the pseudopodium grows. 

 The major factor is thus assvmied to be a contraction of the ectoplasm, or 

 plasmagel of Mast. The nature of this contraction remains uncertain, 

 although it has been suggested that contraction represents the elastic 

 recoil of a plasmagel under continuous tension (382), that syneresis of 

 the plasmagel causes contraction (383, 433), and that the process of 

 gelation involves or causes a contraction (231, 316). 



In locomotion of shelled rhizopods, such as Arcella and Diffliigia, a 

 developing pseudopodium extends from the mouth of the shell and 

 swings about freely until it makes contact with the substratum and ad- 

 heres to it. Contraction of the pseudopodium then pulls the body forward 

 (91, 383). If movement is to continue, a new pseudopodium is extended to 

 repeat the process. Locomotion of creeping Foraminiferida, by means of 

 myxopodia, is similar. The myxopodia are extended, become attached 

 to the substratum, and then contract to pull the organism toward the 

 point of attachment. Axopodia also may function to a limited extent in 

 movement along a substratum. The mechanism in Acanthocystis (434) 

 apparently involves terminal adhesion of axopodia, followed by a con- 

 traction which rolls the body toward the point of attachment. 



FLAGELLAR LOCOMOTION 



The mechanical aspects of flagellar activity have been disputed 

 and various explanations have been suggested for locomotion in flagel- 



