STRUCTURE AND FUNCTION IN AMOEBOID MOVEMENT 553 



The visible events in amoeboid movement are perfectly compatible 

 with the fountain zone contraction theory but pro\ide no conclusive 

 evidence for it. On theoretical grounds it would be expected that a cylin- 

 drical block of endoplasm should widen in the fountain zone, and that in 

 becoming ectoplasmic tube it should increase in cross-sectional area and 

 shorten (Fig. 2). In fact, it does increase in cross-sectional area by a factor 

 of 2 to 3 [is]» depending on en^■ironmental conditions; a compensatory 

 shortening also occurs [5]. The hyaline cap fluid, which is known to be 

 produced bv svneresis [3, 5, 6], appears only when there is forward flow 

 of cvtoplasm through the fountain zone. Hyaline caps erupt in diflFerent 

 pseudopodia in the same cell at difli"erent frequencies, suggesting that 

 dift'erent contractions in diflFerent pseudopodia are the sources of this 

 fluid, rather than a contracting tail common to all of the pseudopodia. The 

 existence of a fluid channel in the hvaline ectoplasmic region is shown bv 

 the fact that the plasmalemma is free to slide over the ectoplasmic tube in 

 most parts of the pseudopod except at limited points of attachment [10]. 

 Entry of the hyaline cap fluid into the tail region has not been demonstrated 

 but can be perhaps inferred from the "softening" of tail ectoplasmic 

 structure observed in the centrifuge microscope [2]. The development of 

 tension in the axial endoplasm would be diflicult to demonstrate directly 

 by physical methods, but the development of tension would be perhaps 

 the best simple explanation for the fact that birefringence is highest in the 

 tail endoplasm, despite the modest velocity gradients developed there [3]. 

 So far, none of these observations excludes the tail contraction theory 

 completely. 



The most direct evidence in support of the fountain zone contraction 

 theory comes from a recent study of streaming in cytoplasm dissociated 

 from the giant amoeba. Chaos chaos. About 6 years ago while working in 

 Professor Runnstrom's laboratory at the Wenner-Grens Institute, I 

 discovered that amoeba cytoplasm could continue to stream for periods 

 of up to I hr. after it had been dissociated from the intact cell [i]. At that 

 time, it was apparent that this phenomenon was neither predicted nor 

 explained by the tail contraction theory, but there was then no alternative 

 mechanism that offered an explanation. It seemed clear, however, that the 

 cytoplasm possessed more structure than prevailing concepts allowed, and 

 that streaming endoplasm might somehow be "self-propelled". 



We have recently re-examined this phenomenon in the light of the 

 concept of amoeba structure illustrated in Fig. i and in the light of the 

 predictions offered by the fountain zone contraction theory [5]. Our data 

 not only strengthen this concept of amoeba structure (Fig. i) but also 

 provide positive indications that the fountain zone contraction theory 

 satisfactorily explains streaming in dissociated cytoplasm. 



When the plasmalemma of an amoeba is ruptured while the amoeba 



