Viscosity Changes of Protoplasm 195 



If the elastic tension of the gel layer is due to a previous stretch- 

 ing by the osmotic pressure of the endoplasm, how can it contract 

 in the equatorial region until it meets there to form the connecting 

 stalk? Obviously, the gel layer could never have been stretched 

 that much in this region, so the elastic tension or recoil cannot account 

 for the constriction. The automatic contraction of a thickened or 

 of a more viscous equatorial band of the gel layer is probably respon- 

 sible for the cleavage of the ameba, as it is for the cleavage of cells. 

 The stretching and final rupture of the connecting gel stalk is 

 probably due to the ameboid movements of the daughter amebae in 

 opposite directions and in part, to the contraction and solation of 

 the stalk. 



BLEB FORMATION 



Most fibroblasts and sarcoma cells show bleb formation during 

 late anaphase and telophase (Fig. 7) . This has been noted by various 

 authors. No adequate explanation of this common, but rather 

 startling, phenomenon has been advanced. Perhaps they are analo- 

 gous to the hyaline caps on the pseudopods of the ameba (Mast) 

 and of the slime mold (Figs. 1-4) . 



Blebs arise rather suddenly during anaphase and telophase. 

 They are usually free of granules. Occasionally, a stream of endo- 

 plasmic granules pours in. They evidently consist of clear fluid, 

 which gathers under the interface membrane or a new membrane 

 which enlarges as the blelDS enlarge. The fluid does not appear to 

 escape to the outside. It seems quite probable that increased pres- 

 sure, exerted in the equatorial region by the contraction of the 

 equatorial band of the gel layer during cleavage, forces endoplasmic 

 fluid through the pores or interstices of local weak places in the gel 

 layer and that these pores are too small to permit granules to pass. 



Mast (1926, p. 420) states that "when a pseudopod is formed, 

 the inner layer of the plasmagel liquefies locally, and the remaining 

 portion stretches and bulges out, and at the same time becomes thin 

 and porous. Liquid from the plasmasol passes through this porous 

 layer of the plasmagel and collects under the plasmalemma forming 

 a hyaline cap." The pores are too small to permit the granules to 

 pass through. As already noted, almost precisely the same thing 

 frequently occurs at the advancing ends of the slime mold. The 

 hyaline caps are enormous as compared with the blebs of a fibro- 

 blast (Figs. 1 and 7), and consequently, offer better material for 

 observations. When the contractile pressure at the posterior end 



