534 Comparative Animal Physiology 



In feeding, those amoebae which travel by small pseudopods, as A. pro- 

 tens, form food cups consisting of lateral and dorsal pseudopods flowing 

 around a food mass which may itself be motile. Food particles do not ad- 

 here to the plasmalemma. The pseudopods of the cup meet distally, they 

 may or may not touch the food, and the food mass is thus incorporated in 

 a food vacuole. In amoebae with a single broad pseudopod, as in A. Umax, 

 there appears to be some adhesion of food masses to the plasmalemma and 

 the cell surface indents, enclosing the food in a vacuole.^o js^\^q f^g surface 

 of reticulopods and axopods are sticky, and food adheres to their surface. 



THEORIES OF AMOEBOID MOVEMENT 



Several theories have been proposed to account for amoeboid movement. 

 The fascinating history of this subject has been well reviewed.^' ^^ The first 

 of these theories was popular with various modifications from 1835 (Du- 

 jardin) until 1875 (Schulze). It postulated contraction in elements in vari- 

 ous parts of the cell, particularly the pseudopods, which pulled the proto- 

 plasmic mass ahead. The second theory involved surface tension forces and 

 was based on the analogy of amoeboid movement with the movement of a 

 mercury drop toward some salt, such as potassium dichromate, or with the 

 currents in a drop of oil in contact with a soap. This theory was developed 

 with many variations by Berthold (1886), Biitschli (1892), and Rhumbler 

 (1898), and still persisted in a paper by Tiegs in 1928;^- it postulates that 

 some organic solute lowers the surface tension at some point on the amoeba, 

 and a pseudopod is there extended. That such action can occur is shown 

 by the modern observation^- ^^ that a drop of one of various paraffin oils 

 can form a cap, a pseudopod forms beneath it, and fountain streaming, often 

 without forward locomotion, can continue beneath the cap for many hours. 



There is abundant evidence against surface tension forces as being nor- 

 mally responsible for amoeboid movement. The plasmalemma is not a liquid 

 surface but an elastic membrane. The currents on the plasmalemma are 

 forward, not backward as on the surface of a drop of mercury or oil moving 

 by a "pseudopod." Also there is no reasonable suggestion regarding the 

 nature and method of secretion of any solute which might lower the surface 

 tension locally. 



The most reasonable current theory of amoeboid movement is based on 

 the reversible gel-sol transformation and assigns the propulsive force to con- 

 tractility of the gel. This notion was actually proposed in 1863 by Wallich 

 and was advanced by Jennings in 1902. The first discussion in terms of 

 colloidal properties of gels and sols was by Hyman;^^ the theory was ad- 

 vanced by Pantin,^-' ^^ and greatly extended by Mast and his associates.^^- 

 32, 34 Support has come from pressure experiments by Marsland and Brown,^® 

 and from studies of blood cells in tissue culture.'"'' ^^ 



The plasmagel is truly elastic. Measurements of the extensibility of strands 

 of myxomycete plasmodia gave a value of Young's modulus of 9 X 10* 

 dynes/cm.-, which is about one-thirtieth as elastic as muscle.^^ 



The tension at the cell periphery, as measured by the centrifugal force 

 to pull an oil droplet out through the surface, is 1 to 3 dynes/cm. or lower- 

 slightly more than the peripheral tension in marine eggs but much lower than 



