Viscosity Chaiiges of Protoplasm 165 



low viscosity of the interior of the hving cell is due to the absence 

 or scarcity of free calcium. Various other theories might be sug- 

 gested, such as increase in the acidity or of the COo at the surface. 

 Jacobs (1922, p. 29) states, "A short exposure to carbon dioxide of 

 various cells causes a decrease, and a longer exposure an increase in 

 the viscosity of the protoplasm. It is suggested that carbon dioxide 

 may be an important factor in producing many of the natural changes 

 in protoplasmic consistency which have hitherto been unexplained." 



We come now to our second important factor in ameboid loco- 

 motion, namely, the contractile tension which protoplasm (a colloid) 

 automatically exerts when it gels. This is a phenomenon common 

 to living and many nonliving colloids such as blood plasma, gelatin, 

 and silica gel. Some colloids exert enough tension when they gel 

 to flake off glass from the wall of the containing vessel. Mast (1926, 

 1931) believes that the contractile tension is due to an elastic recoil. 

 He states (1926. p. 404) that "there is an abundance of evidence 

 indicating that the plasmagel and the plasmalemma are elastic and 

 that they are usually somewhat stretched and consequently exert 

 inward pressure; that is, that amebae are usually turgid" (1926, 

 p. 405) . "The fact that amebae are usually turgid indicates that the 

 plasmagel, and possibly also the plasmalemma, is semipermeable 

 and that the plasmasol, and probably also the substance in the 

 vacuoles in the plasmagel, is hypertonic. If this obtains, it is plain 

 that there would be an excess inflow of water and a stretching of 

 the plasmagel and the plasmalemma until their combined strength 

 equals the diffusion pressure." (1931, p. 330) "An ameba is a 

 turgid system; owing to this the plasmagel is continuously under 

 tension. The plasmagel is elastic and consequently is pushed out at 

 the region where its elastic strength is lowest. This results in the 

 formation of pseudopods." 



My idea of the origin of the contractile tension of the gel layer 

 differs from that of Mast. He assumes that the plasmogel is elastic 

 and that it is stretched by the inflow of water into the plasmosol. 

 In this stretched condition it keeps the ameba turgid and drives the 

 plasmosol into the weakened areas to form pseudopods. My idea 

 is that protoplasm, like most colloids, automatically exerts contrac- 

 tile tension when it gels (Lewis, 1939) . 



I was led to adopt this view because Mast's theory did not seem 

 to offer an adequate explanation for the occurrence of constriction 

 rings. For example (Lewis, 1939, p. 415), "The contorted mitoses 

 observed in the division of the spindle cells of the spindle-cell 



