Structure and Function in Amoeboid Movement* 



Robert D. Allen 



Departvient of Bio/ogy, Princeton Unirers/tv, 

 Princeton, 'XJ., U.S.A. 



Amoeboid movement is a process with which every biologist and 

 student of biologv is familiar. Yet, despite its fundamental importance, 

 this form of cellular motility has been one of the most poorly understood 

 phenomena in cellular biology. Nearly every generation of biologists over 

 the last centurv and a quarter has produced a new explanatory theory, only 

 to have it supplanted in the next generation by a totally different one. 

 Theories of amoeboid movement seem now to have passed through a 

 complete cvcle, for the front- and tail-contraction theories, which I shall 

 discuss here, both represent a return to the general idea, first expressed by 

 Dujardin [9], that amoeboid mo\'ement is basically a contractility phe- 

 nomenon. If one accepts this idea, then obviously the most fundamental 

 question is the location of active contraction (i.e. the "engine") in the 

 moving cell. 



The streaming endoplasm has in the past been excluded as a possible 

 site for this "engine" because this region of the cell has been assumed to 

 have the phvsical properties of a Newtonian sol [12, 14]. Since structureless 

 fluids can neither develop nor transm.it tension, this concept of endoplasmic 

 consistencv led inexorably to the tail contraction theory [10, 13], according 

 to which the endoplasm is moved passively by a pressure gradient generated 

 bv an activelv contracting ectoplasmic tube. The concept of the endoplasm 

 as a structureless sol also excluded any consideration of an alternative 

 mechanism such as will be proposed below. 



Several recent developments ha\e led us to propose such an alternati\e 

 mechanism. First, it has been pointed out elsewhere [3, 4] that many of 

 the behavioural aspects of amoeboid movement are incompatible with the 

 tail contraction theory in its present form. Second, it has been shown that 

 amoeba cytoplasm will continue to stream after it has been dissociated from 

 the cell [i, 5]. This capability was neither predicted nor explained by the 

 tail contraction theory. 



* Supported by Research Grant C-3022(Ci-C4) from the U.S. Public Health 

 Service. 



