PHYSIOLOGY OF CELL-DIVISION 371 



finds that the resistance of sea-urchin eggs to injury by hyper- 

 tonic sea- water is least "immediately before and during each 

 cytoplasmic division, and that the maximal resistance is shown 

 35 to 45 minutes after fertilization and just after each division."^ 

 It thus appears that the resistance to a variety of injurious 

 agencies is least at the time of cytoplasmic division, i.e., while 

 the form of the cell is undergoing rapid change. This change 

 of form indicates alteration of surface-tension, which again 

 suggests alteration of electrical surface-polarization, an effect 

 which would result from an increased permeability of the elec- 

 trically polarized plasma-membrane to electrolytes.^ Hence 

 it is possible to refer these several effects primarily to altera- 

 tions of the surface layer of the egg. The analogy between cell- 

 division, an essentially ihythmical process, and the phenomena 

 of rhythmical auto-stimulation in heart-muscle cells, cilia, or 

 respiratory nerve cells, favors such an interpretation, since there 

 is ample evidence that stimulation is associated with a temporary 

 increase of surface-permeability. Accordingly I have put for- 

 ward the hypothesis that these rhythmical variations in the 

 physiological state of the egg, during the cycle of cell division, 

 are essentially the result of variations in the physical condition, 

 especially the permeability, of the surface-film or plasma-mem- 

 brane, the latter undergoing a reversible increase in permeability 

 at the time of cleavage.'*^ ''A rhythm of alternate increase and 

 decrease of permeability thus accompanies the rhythm of the 

 mitotic process. "^^ Any temporary loss of semi-permeability 

 would be favorable to the entrance of poisons;'- it would also 

 decrease the electrical surface-polarization, and this change, 



«A. R. Moore, Biol. Bull., 1915, vol. 28, p. 253; cf. p. 257. 



9 Cf. my paper in Biol. Bull. 1909, vol. 17, p. 204. 



'"Cf. r". S. Lillie, Biol. Bull.. 1909, vol. 17, p. 188, cf. p. 207; Amer. Journ. 

 physiol., 1910, vol. 26, p. 126, and 1911, vo.l 27, p. 289; Jour. Morph., 1911, vol. 

 22, p. 711. 



" Cf. Amer. Journ. Physiol., 1910, vol. 26, p. 133. 



'^ This might account for the greater toxicity of certain substances at this 

 time,- but not of all, since lipoid-soluble substances, e.g., ether, appear to pene- 

 trate the cell equally readily at all times. A general decrease of stability, or of 

 resistance to abnormal conditions, (e.g., heat), seems to be associated with the 

 increase of permeability. 



