PO ON THE INTERNAL FORM [ch. 



turn, in great measure, upon whether or no we are justified in 

 assuming that, in the hquid surface-film of a minute spherical cell, 

 local and symmetrically localised differences of surface-tension are 

 likely to occur. If not, then changes in the conformation of the 

 cell such as lead immediately to its division must be ascribed not 

 to local changes in its surface-tension, but rather to direct changes 

 in internal pressure, or to mechanical forces due to an induced 

 surface-distribution of electrical potential. We have little reason to 

 be sceptical ; in fact we now know that the cell is so far from being 

 chemically and physically homogeneous that local variations in its 

 surface-tension are more than likely, they are certain to occur. 



Biitschh suggested more than sixty years ago that cell-division 

 was brought about by an increase of surface-tension in the equatorial 

 region of the cell ; and the suggestion was the more remarkable that 

 it was (I beheve) the very first attempt to invoke surface-tension 

 as a factor in the physical causation of a biological phenomenon*. 

 An increase of equatorial tension would cause the surface-area there 

 to diminish, and the equator to be pinched in, but the total surface- 

 area of the cell would be increased thereby, and the two effects 

 would strike a balance f. But, as Biitschh knew very well, the 

 surface-tension change would not stand alone; it would bring other 

 phenomena in its train, currents would tend to be set up, and 

 tangential strains would be imposed on the cell-membrane or cell- 

 surface as a whole. The secondary if not the direct effects of 

 increased equatorial tension might, after all, suffice for the division of 

 the cell. It was Loeb, in 1895, who first shewed that streaming went 

 on from the equator towards the divided nuclei. To the violence 

 of these streaming movements he attributed the phenomenon of 

 division, and many other physiologists have adopted this hypo- 

 thesis J. The currents of which Loeb spoke call for counter-currents 



* 0. Butschli, tJber die ersten Entwicklungsvorgange der Eizelle, Abh. 

 Senckenberg. naturf. Gesellsch. x, 1876; Uber Plasmastrqmungen bei der Zell- 

 theilung, Arch. f. Entw. Mech. x, p. 52, 1900. Ryder ascribed the earyokinetic 

 figures to surface-tension in his Dynamics in Evolution, 1894. 



t A relative, not positive, increase of surface-tension, was part of Giardina's 

 hypothesis: Note sul mecanismo della divisione cellulare, Anat. Anz. xxi. 1902. 



J J. Loeb, Amer. Journ. Physiol, vi, p. 432, 1902; E. G. Conklin, Protoplasmic 

 movements as a factor in differentiation, Wood's Hole Biol. Lectures, p. 69, etc., 

 1898-99; J. Spek, Oberflachenspannungsdifferenzen als eine Ursache der Zell- 

 teilung, Arch.f. Entw. Mech. xliv, pp. 54-73, 1918. 



