MEMBRANE CHANGES DURING STIMULATION 363 



relations and a different velocity of propagation, accom- 

 panies stimulation in irritable cells and nerve fibers. 



During the formation of the cleavage-furrow in cell- 

 division a similar reversible change in the physical 

 consistency and coherence of the cell surface occurs in 

 echinoderm eggs;^ and there are many indications that 

 the same kind of change is of general occurrence in 

 dividing cells. The case of cell-division is of special 

 interest, since a rhythm of chemical change and of 

 susceptibility to physical and chemical injury is also 

 associated with the rhythm of the cleavage-process. 

 Lyon's experiments indicate that at the time when the 

 cleavage-furrow is forming in the Arbacia egg, the rate of 

 evolution of CO2 is several times greater than in the inter- 

 vals between cleavage;^ at this time the egg is also most 

 susceptible to injury by heat, ultra-violet radiation, 

 deprivation of oxygen, and poisons (KCN, acids, and 

 organic compounds). The time-relations of the accom- 

 panying change in the plasma membrane can be followed 

 readily and accurately by transferring successive portions 

 of a single lot of recently fertilized Arbacia eggs (in which 

 the cleavage-process is very regular and occurs simultane- 

 ously in all eggs) from normal sea water to dilute sea 

 water (50 to 60 volumes fresh water in 100 of the mixture) 

 at regular intervals before, during, and after the for- 

 mation of the cleavage-furrow.^ Eggs which are thus 

 treated some time before cleavage swell osmotically but 

 without undergoing evident increase of permeabihty 



^ R. S. Lillie, Journal of Experimental ZooL, XXI (1916), 369; 

 Herlant, Comp. rend. sac. bioL, LXXXI (1918), 151; Just, American 

 Journal of Physiology, LXI (1922), 505. 



2 Lyon, loc. cit. 3 R. S. Lillie, loc. cit. 



