IV] STRUCTURE OF THE CELL 191 



thus could we approach a comprehension of the balance of forces 

 which cohesion, friction, capillarity and electrical distribution 

 combine to set up. 



The manner in which we regard the phenomenon would seem 

 to 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 locahsed, 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. 



It has seemed otherwise to many writers, and we have a number 

 of theories of cell division which are all based directly on in- 

 equalities or asymmetry of surface-tension. For instance, Biitschli 

 suggested, some forty years ago*, that cell division is brought 

 about by an increase of surface-tension in the equatorial region 

 of the cell. This explanation, however, can scarcely hold; for 

 it would seem that such an increase of surface-tension in the 

 equatorial plane would lead to the cell becoming flattened out into 

 a disc, with a sharply curved equatorial edge, and to a streaming 

 of material towards the equator. In 1895, Loeb shewed that the 

 streaming went on from the equator towards the divided nuclei, 

 and he supposed that the violence of these streaming movements 

 brought about actual division of the cell : a hypothesis which was 

 adopted by many other physiologists f. This streaming move- 

 ment would suggest, as Robertson has pointed out, a diminution 

 of surface-tension in the region of the equator. Now Quincke has 

 shewn that the formation of soaps at the surface of an oil-droplet 

 results in a diminution of the surface-tension of the latter; and 

 that if the saponification be local, that part of the surface tetids to 

 spread. By laying a thread moistened with a dilute solution of 

 caustic alkali, or even merely smeared with soap, across a drop 

 of oil, Robertson has further shewn that the drop at once divides 

 into two: the edges of the drop, that is to say the ends of the 



* Cf. also Arch. f. Entw. Mech. x, p. 52, 1900. 



t Cf. Loeb, Am. J. of Physiol, vi, p. 432, 1902. ; Erlanger, Biol. Centralbl. 

 xvn, pp. 152, 339, 1897 ; Conklin, Biol. Lectures, Woods Holl, p. 69, etc. 1898-9. 



