IV] AND STRUCTURE OF THE CELL 331 



towards the equator, in or near the surface of the cell; and theory 

 and observation both indicate that precisely such currents are bound 

 to be set up by the surface-energy involved in the increase of 

 equatorial tension. 



An opposite view has been held by some, and especially by 

 T. B. Robertson*. Quincke had shewn that the formation of soap 

 at the surface of an oil-droplet lowers the surface-tension of the 

 latter, and that if the saponification be local, that part of the surface 

 tends to enlarge and spread out accordingly. Robertson, in a very 

 curious experiment, found that by laying a thread, moistened with 

 dilute caustic alkali or merely smeared with soap, across a drop of 

 olive oil afloat in water, the drop at once divided into two. A 

 vast amount of controversy has arisen over this experiment, but 

 Spek seems to have shewn conclusively that it is an exceptional 

 case. 



In a drop of olive-oil, balanced in water f and touched anywhere 

 with an alkaH, there is so copious a formation of lighter soaps that 

 di^erences of density tend to drag the drop in two. But in the 

 case of other oils (and especially the thinner oils, such as oil of 

 bergamot) the saponified portion bulges, as theory directs; and 

 when the alkali is applied to two opposite poles the equatorial 

 region is pinched in, as McClendonJ, in opposition to Robertson, 

 had found it to do. Conversely, if an alkaline thread be looped 

 around the drop, the zone of contact bulges, and instead of dividing 

 at the equator the drop assumes a lens-like form. 



We may take it then as proven that a relative increase of equatorial 

 surface-tension, whether in oil-drops, mercury-globules or living 

 cells, does lead, or tend to lead, to an equatorial constriction. In 

 all cases a system of surface-currents is set up among the fluid drops 

 towards the zone of increased tension ; and an axial counter-current 

 flows towards the pole or poles of lowered tension. Precisely such 

 currents have been observed to run in various eggs (especially of 



♦ T. B. Robertson, Note on the chemical mechanics of cell-division, Arch. f. 

 Entw. Mech. xxvn, p. 29, 1909; xxxii, p. 308, 1911; xxxv, p. 402, 1913. Cf. 

 R. S. Lillie, Joum.-Exp. Zool. xxi, pp. 369^02, 1916; McClendon, loc. cit.; etc. 



t In these experiments, and in many of Quincke's, a little chloroform is added 

 to the oil, in order to bring its density as near as may be to that of water. 



X J. F. McClendon, Note on the mechanics of cell -division, Arch. f. Entw. Mech. 

 xxxrv, pp. 263-266, 1912. 



