166 The Structure of Protoplasvi 



sarcoma C37 are explained by the development of changing con- 

 traction bands of the plasmagel layer which produce constrictions 

 that result in marked distortions and lobulations of the dividing and 

 young daughter cells." During anaphase, telophase, and early 

 daughter cell stages (Lewis, 1939, p. 409) , "Constrictions appear 

 and disappear in different regions, squeezing the cells into lobulated 

 and elongated forms." Constriction rings appear regularly on 

 migrating lymphocytes and neutrophiles, occasionally on fibroblasts 

 and sarcoma cells, and always in cell cleavage (Lewis, 1939) . It is 

 impossible to see how an elastic recoil theory can explain such con- 

 strictions. It is difficult to explain the retractions of cell processes 

 and pseudopodia on the elastic recoil theory or the retraction of gel 

 sheets and gel strands of the slime mold that are far away from 

 the streaming endoplasm, or the excessive contraction of an ameba 

 that squeezes out all the endoplasm through a break in the plasmogel 

 and plasmolemma. 



Presumably the contractile tension varies with the viscosity and 

 with the thickness of the gel layer. In the fluid state, it is practically 

 nil. In the gel state it is measurable. The "balance-pressure" of 

 Kamiya (1940) is probably a measure of the contractile tension of 

 the gel layer of the slime mold. 



What are the limits of the contractility of the gel layer? The 

 following quotations from Mast and Edwards seem to indicate that 

 the gel layer can contract until all the endoplasm is squeezed out of 

 an ameba through an artificial hole in the plasmolemma and plasmo- 

 gel. Nothing is left but a small mass of plasmogel covered by wrinkled 

 plasmolemma. In other words, the plasmogel can contract until it 

 meets plasmogel. This is of especial interest. In the first place, if 

 the contraction of the plasmogel is due to the elastic recoil after 

 stretching, then we must imagine that stretching began at the 

 instant plasmosol appeared, if there was ever such a time. This 

 rather counts against the elastic recoil idea. We have other evi- 

 dence of the great contractility of the gel layer, as for example on 

 the tail buds of the lymphocyte and the cleavage furrow of dividing 

 cells. Mast (1926, p. 404) states, "There is an abundance of evidence 

 indicating that the plasmagel and the plasmalemma are elastic and 

 they are usually somewhat stretched and consequently exert inward 

 pressure; that is, that amebae are usually turgid. If pressure is 

 brought to bear on the cover glass with an ameba under it, the 

 plasmagel and the plasmalemma break and the plasmasol flows out, 

 leaving nothing but a small mass of plasmagel covered with the 



