552 ROBERT D. ALLEN 



ectoplasm so that it can be returned to the endoplasmic stream. If the 

 contraction part of the contractility cycle involves syneresis, then this fluid 

 must be resorbed in whatever part of the cell the relaxation part of the 

 cycle occurs. It is also possible that some of the hyaline fluid is squeezed 

 backward from the fountain zone as a counter-current. The theory further 

 requires that the contraction remain localized in the fountain zone ; hence, 

 this front contraction theory has been named the "fountain zone contrac- 

 tion theory". In order to remain localized in the fountain zone, the 

 contraction itself must be propagated posteriorly toward the axial endo- 

 plasm at approximately the velocity of forward endoplasmic displacement 

 relative to the advancing tip. 



The theory so far explains only endoplasmic displacement with respect 

 to the ectoplasm. Locomotion can occur only if the ectoplasmic tube is 

 attached at certain points, by means of the plasmalemma, to the sub- 

 stratum. The larger species of amoebae, such as Amoeba proteiis, are 

 attached near the middle of the cell at from one to several points [8]. If 

 attachment does not take place, the ectoplasm and endoplasm are indeed 

 displaced in opposite directions, a situation which has been called "foun- 

 tain streaming" [3, 14]. 



The fact that we are now faced with two opposing contraction theories 

 which postulate contractions localized at opposite poles of the cell makes 

 it imperative to examine the question of exactly what constitutes evidence 

 of an active contraction. In muscle, the measurement of tension developed 

 or work performed removes all doubt as to whether an observed shortening 

 is passive or active. In the amoeba, the situation is not so simple. The 

 fundamental observational basis for the tail contraction theory is that the 

 tail shortens [11]; many authors have uncritically accepted this as con- 

 clusive evidence for tail contraction. Actually, this shortening is compatible 

 with the tail contraction hypothesis, but is also compatible with the 

 hypothesis that the tail is "pumped out" as the new theory proposes. 

 Even the measurement of tension between two points in the shortening 

 tail would not settle the question, for some of the work done in the fountain 

 zone contraction would appear as tension between two points in the 

 shortening tail ectoplasm. 



It seems reasonable to propose that only the development of tension or 

 the production of large amounts of heat in a localized shortening region of 

 cytoplasm should be considered conclusive evidence of an active contrac- 

 tion. Localized syneresis is probably also conclusive evidence of an active 

 contraction, since syneresis is well known to result from increased cross- 

 bonding in gels. Simultaneous shortening and thickening of a body of 

 cytoplasm, however, is by itself not much more than suggestive evidence 

 of active contraction unless it is accompanied by localized syneresis, 

 tension development, or heat production. 



