J. theor. Biul. (1970) 28, 11 l-120 Hydrodynamic Model of Amoeboid Movement JUAN A. SUBIRANA Escuela T&mica Superior de lngenieros Industriales, Barcelona (14), Spain (Received 29 July 1969) A hydrodynamic theory of the flow produced by active shear in amoebae is developed. Active shear is defined as the movement induced in a fluid at the interface with a resting solid.The fluid acquires a velocity at the interface, which is transmitted to the rest of the liquid through the shearing forces due to viscosity. The monopodial movement of amoebae can be explained by this mechanism, if it is assumed that active shear appears at the surface of the gel tube. According to this model, the production of motive force in amoebae is located in the central region of the cell, whereas other theories consider it to be located in the front or tail. A similar mechanism may originate other types of movement in different biological systems. In this paper, their relative efficiency is quantitatively compared. 1. Introduction Several authors [see reviews by Kamiya (1959) and Rebhun (1967)] have pre- sented evidence showing that cell movement may be produced by active shearing forces. This mechanism of movement is particularly evident in the case of protoplasmic streaming (Kamiya & Kuroda, 1966) and cyclosis in plant cells (Kamiya, 1959). In the latter case, the active structure has been tentatively identified by Nagai & Rebhun (1966) as a bundle of 50 to 60 A microfilaments located on the chloroplast surface, next to the cytoplasm in movement. It has also been suggested that microtubules, besides their obvious struc- tural role (Porter, 1966), are causally and actively involved in movement within the cell (Rebhun, 1967; Roth, Jenkins, Allenspach, Thomas & Wilson, 1965; O’Brien & Tbimann, 1966; Schmitt, 1968; Subirana, 1968), through the production of active shearing forces at their surface. In particular, the rele- vance of this hypothesis for an understanding of the movement of chromo- somes and other particles during mitosis has been recently discussed (Subi- rana, 1968). In amoeboid cells, thin filaments (40 to 100 A) and shorter spindle-like filaments (160 to 220 A) are also present. Their structure and distribution has 111