386 RALPH S. LILLIE 



required. Table 5 gives a detailed description of an experi- 

 ment of this kind (see table 5). 



The contrast between the resistance of the eggs at the two 

 stages is sufficiently evident from the table. The decline in 

 resistance during cleavage is such that many eggs undergo cytoly- 

 sis in sea-water of 40 per cent dilution; these are probably the 

 eggs that were introduced at the period of maximum suscepti- 

 bility. As the dilution increases the proportion of eggs under- 

 going cytolysis -also increases progressively. The progressive 

 character of the variation in susceptibility is well indicated by 

 these results. A dilution of 60 per cent, in which uncleaved 

 eggs remain intact, destroys all of the cleaving eggs in twenty 

 minutes or less. 



One remarkable difference between the cycle of susceptibility 

 to hypotonic sea-water and that to cytolytic substances and 

 cyanide, is that there is no period of increased susceptibility 

 to osmotic disruption immediately following fertilization. It 

 is known that there occurs at this time, together with the in- 

 crease of susceptibility to poisons, an increase of electrical con- 

 ductivity;-^ and other evidence indicates that the plasma- 

 membrane then undergoes an increase in its general permeability 

 to water-soluble substances and to water. ^^ Yet the membrane 

 appears to be no more readily broken down by osmotic disten- 

 tion than at any other time previously to cleavage. This differ- 

 ence is surprising, and not readily to be accounted for. The 

 lack of any change in the extensibility of the membrane is prob- 

 ably to be correlated with the absence of any change in the 

 form of the egg; apparently the loss of resistance to extension 

 occurs only at the time when the cell is undergoing, or is about 

 to undergo, active change of form. This would imply that the 

 membrane-change accompanying cleavage is of a different kind 

 from that immediately following fertilization, and involves a 

 loss of coherence or extensibility. The basis of this difference 



28 McClendon, Amer. Journ. Physiol., 1910, vol. 27, p. 240; Gray, Journ. Mar. 

 Biol. Assoc, 1913, vol. 10, p. 50. 



2' For a review of this evidence, and an account of experiments showing in- 

 creased permeability to water, cf., my recent paper in Amer. Journ. Physiol., 

 1916, vol. 40, p. 249. 



