STRUCTURE AND FUNCTION IN AMOEBOID MOVEMENT 55 1 



presence of this weak gel structure in the axial portion of the endoplasm. 

 This region of the cell was found to offer \isible resistance to the displace- 

 ment of accelerated cytoplasmic inclusions [2]. These centrifugation 

 experiments have confirmed the generally held concept of a more rigid 

 consistency for the ectoplasmic tube as a whole (Mast's plasmagel [14]), 

 but have revealed an unexpected gradient of rigidity in both the ectoplasm 

 and axial endoplasm from a high at the front of the cell to a low in the tail. 



The results of these and other rheological experiments, which have been 

 summarized elsewhere [3], are presented schematically in Fig. i. It is this 

 concept of amoeba cytoplasmic structure which led to the new front 

 contraction theory. It is perhaps simplest to outline the theorv first, and 

 then point out some of the experimental evidence which supports it. 



First let us assume that the endoplasm (especially the axial portion) is 

 uncontracted or relaxed cvtoplasm. According to the theory, a given 

 portion of this material begins to contract just before it splits and becomes 

 everted to form the continually advancing ectoplasmic tube. The contrac- 

 tion is completed by the time this material has become incorporated into 



(a) (b) (c) 



Fig. 2. A diagram to illustrate the fate of a cylindrical block of axial endoplasm 

 as it contracts at the part of the cell as proposed by the fountain zone contraction 

 theory [4]. 



the ectoplasmic region. During its passage through the region of the cell 

 which we have termed the fountain zone (Fig. i), this given portion of 

 cytoplasm shortens (along the axis of the pseudopod) and thickens 

 (radially) (Fig. 2). At the same time it develops tension, which is trans- 

 mitted posteriorly through the axial endoplasm to "pump out" the tail. 

 Increased cross-bonding during contraction in the fountain zone region 

 causes localized syneresis, the fluid from which appears periodically in the 

 hvaline cap. The increase in rigidity in the fountain zone accounts for the 

 difference in consistency between the ectoplasmic tube and the endoplasm ; 

 this change is probably analogous to the increased rigidity which accom- 

 panies muscular contraction. The dilute fluid of the hyaline cap, which 

 has been pressed out of the cytoplasm contracting in the fountain zone, is 

 pumped tailward by the advance of the granular cytoplasm of the pseudo- 

 pod within the loosely fitting plasmalemma. The hyaline ectoplasm serves 

 as a channel between the plasmalemma and ectoplasmic tube through 

 which the hyaline fluid travels to the tail region, where this fluid is returned 

 eventually to the endoplasmic stream. The pull exerted on the axial endo- 

 plasm from the front may in part draw some of this fluid through the tail 



