400 RALPH S. LILLIE 



diminish the P.D. of the double layer at that region, to a degree 

 which is greater, the nearer the astral area approaches the surface. 

 The effect of the approaching astral field upon the polarized 

 cell-surface is thus a local decrease of polarization, hence a local 

 increase of surface-tension and an increased curvature of the 

 surface. 



Whether this effect is adequate to produce the results observed 

 cannot be said definitely in the absence of quantitative data; 

 there seems, however, to be no doubt that, granting the exist- 

 ence of an externally positive surface-polarization and a cen- 

 trally negative astral diffusion-field, a qualitative effect of the 

 above kind would be produced. 



We may now briefly summarize the results of the foregoing 

 discussion of the conditions of protoplasmic change of form. 

 In normal mitosis cytoplasmic division is the consequence of 

 a definitely localized surface-depolarization resulting in in- 

 creased surface-tension. This effect is produced by the coopera- 

 tioti of two factors; one the general increase in the permeability 

 of the cell-surface (assumed to be greater about the circum- 

 polar or extra-equatorial areas), the other the depolarizing 

 effect of an electrolyte diffusing from the chemically active 

 astral centers. It is assumed that by the summation of these 

 two effects the surface-tension is increased over the greater part 

 of the area of each hemisphere sufficiently to account for- the 

 observed change of form. The relative importance of these 

 two factors cannot be decided at present; it is indeed difficult 

 to distinguish the parts played by each; probably some effect 

 analogous to auto-stimulation also enters, the initial surface- 

 change (due, e.g., to the spermatozoon) causing secondarily 

 astral activity, and this again causing further surface-change — 

 the two processes mutually influencing each other, like the 

 local change of permeability and the bioelectric variation in 

 the process of conduction of excitation.'''- Complete mitotic 



^^ I.e., in the conduction of the excitation-state the original stimulus causes 

 a certain local alteration in the surface-layer; apparently this originates a local 

 bioelectric variation which acts as stimulus on adjacent regions, the original 

 effect being thus extended and reinforced. For a fuller account of these condi- 

 tions, cf., Amer. Journ. Physiol., 1914, vol. 34, p. 414, 1915, vol. 37, p. 348, and 

 1916, vol. 41, p. 126. 



