326 ON THE INTERNAL FORM [ch. 



described under an electrical hypothesis; the lines of force will 

 have the same distribution, and such movements as the chromo- 

 somes undergo, and such symmetrical configurations as they assume, 

 may be accounted for under the one hypothesis pretty much as 

 under the other. There are however other phenomena accompanying 

 mitosis, such as Chambers's astral currents and certain local changes 

 in the viscosity of the egg, which are more easily explained by the 

 hydrodynamic theory. 



We may assume that the cytoplasm, however complex it may be, 

 is but a sort of microscopically homogeneous emulsion of high 

 dispersion, that is to say one in which the minute particles of one 

 phase are widely scattered throughout, and freely mobile in, the 

 other; and this indeed is what is meant by caUing it a hydrosol. 

 Let us assume also that the particles are a little less dense than the 

 continuous phase in which they are dispersed; and assume lastly 

 (it is not the easiest of our assumptions) that these ultra-minute 

 particles will be affected, just as are the grosser ones, by the forces 

 of the hydrodynamic field. 



All this being so, the disperse particles will be repelled from the 

 oscillating centrosome, with a force which falls off very rapidly, for 

 Bjerknes tells us that it varies inversely as the seventh power of 

 the distance; a round clear field, hke a drop or a bubble, will be 

 formed round the centrosome ; and the disperse particles, expelled 

 from this region, will tend to accumulate in a crowded spherica- 

 zone immediately beyond it. Outside of this again they will con- 

 tinue to be repulsed, but slowly, and we may expect a second and 

 lesser concentration at the periphery of the cell. A clear central 

 mass, or "centrosphere," will thus come into being; and the 

 surrounding cytoplasm will be rendered denser and more viscous, 

 especially close around the centrosphere and again peripherally, by 

 condensation of the disperse particles. Moreover, all outward 

 movements of these lighter particles entail inward movements of 

 the heavier, which (by hypothesis) are also the more fluid ; stream- 

 lines or visible currents will flow towards the centre, giving rise to 

 the star-shaped "aster," and the best accounts of the sea-urchin's 

 egg* tally well with what is thus deduced from the hydrodynamic 



* Cf. R. Chambers, in Journ. Exp. Zool. xxni, p. 483, 1917; Trans. R.S. Canada, 

 XII, 1918; Journ. Gen. Physiol, ii, 1919. 



