PHYSIOLOGY OF CELL-DIVISION 373 



also agrees with the facts about to be described, indicating that 

 the properties of the plasma-membrane over the greater part 

 of the cell-surface undergo profound alteration at the time when 

 the cleavage-furrow is forming. ^'^ 



Changes in the permeability of the plasma-membiane to 

 diffusing substances imply changes in its general physical proper- 

 ties as well as in its osmotic behavior. In general the resistance 

 which a gel (in which class the plasma membrane belongs) 

 offers to diffusion, is a direct function of its density, i.e., colloid- 

 content ;i*' its mechanical properties (elasticity, tensile strength, 

 etc.) are similarly determined. Hence the more permeable 

 the plasma-membrane is to diffusion oi' to the passage of water, 

 the less resistance should the egg offer to osmotic distention 

 in dilute sea-water. The entrance of water into fertilized eggs 

 under such conditions is in fact several times more lapid than 

 into unfeitilized eggs.'^ This difference of behavior is undoubt- 

 edly an expression of the general increase of permeability which 

 results from fertilization. Correspondingly we should expect 

 to find an analogous difference between the behavior of eggs 

 placed in dilute sea-water during and previously to cleavage, 

 if the properties of the membrane do in fact undergo change 

 at this time. 



In the experiments about to be described I have investigated 

 the behavior of feitilized Arbacia eggs in dilute sea-water at 

 different periods of the cell-division cycle. A striking change 

 in the properties of the plasma-membrane at the time of cleavage 

 is in fact readily demonstrable. As soon as the cleavage-furrow 

 begins to form, or a little earlier, the membrane shows a marked 

 decrease in its extensibility, and the eggs undergo rapid cytolysis 

 in dilute sea-water to which previously they were resistant. 

 This unstable condition persists during the formation of the 

 furrow; afters its completion its original resistance returns. 

 The following section describes these phenomena in detail. 



1^ Other factors probably enter in the change of surface-tension; see the gen- 

 eral discussion below, p. 394, 



1^ Cf . Freundlich, Kapillarchemie, p. 515; Bechhold, Die Kolloide in Biologic 

 und Medizin, 1912, p. 48; Bechhold und Ziegler, Zeitschr. f. phj-sik. Chem., 

 1906, vol. 56, p. 105; Ruhland, Biochem. Zeitschr., 1913, vol. 54, p. 59. 



17 R. S. Lillie, Amer. Journ. Physiol., 1916, vol. 40, p. 249. 



