466 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



THE CHEMICAL MECHANICS OF CELL-DIVISION. 



The essential mechanical resultant of cell-division is the increase of the 

 Protoplasmic Surfaces which is brought about. During the successive 

 divisions which an egg-cell undergoes in developing to the blastula- 

 stage the total area of the protoplasmic surfaces is enormously increased 

 even in attaining the thirty-two cell stage, for example, the total proto- 

 plasmic surface is increased by about three hundred per cent. Such 

 an increase in a fluid surface necessarily implies either the performance 

 of work by external forces or else a considerable reduction of Superficial 

 Tension. 



In 1876 it was suggested by Butschli that cell-division is brought 

 about through an increase of surface-tension, subsequent to nuclear 

 division, in the equatorial region of the egg. He pointed out that 

 substances diffusing from the nuclei or centrosomes must necessarily 

 reach their highest concentration in the equatorial plane and hence 

 assuming that these substances increase the surface-tension at the 

 periphery of the egg, the most marked increase would occur in the 

 equatorial region and, as a consequence, the surface-tension at the poles 

 of the cell would be less than that at the equator. 



Such a conception of cell-division is, however, manifestly erroneous 

 for an increase of interfacial tension at the equator, such as Butschli 

 imagines to occur, implies an increase in the molecular attractive forces 

 at the equator, and the fluids of the cell would not stream away from 

 the region of high attraction but would, on the contrary, stream toward 

 it. The result would be that the equatorial surface would tend to 

 become highly curved, as areas of high tension in a fluid always do, 

 and the surfaces at the poles would become relatively flattened. The 

 result would be the formation of a flattened disc with a highly curved 

 edge, the latter representing what was formerly the equatorial surface 

 of the egg. Such a process obviously could not lead to cell-division. 

 In fact, since the surfaces which Butschli imagines streaming from the 

 nuclei or centrosomes are supposed by him to raise the surface-tension 

 of the egg, their total effect could only be to diminish the surface of 

 the egg relatively to its volume, if that were possible, and not to increase 

 it, which is what the forces leading to cell-division actually accomplish. 

 In fact no model' can be imagined in a fluid which will accomplish 

 increase of surface by increase of superficial tension. Such a model 

 can be devised in a non-fluid system, as, for example a rubber balloon 

 subjected to compression by a rubber band around its equatorial 

 circumference. The equator of the balloon would by this means be 

 constricted and the single sphere would tend to divide into two, owing 

 to the application of additional tension at its equator. This model is, 

 however, in no way comparable to a fluid drop, for it is characteristic 

 of the superficial tension of liquids that it is not altered by diminution 

 or expansion of the surface, because it is really due to the unbalanced 

 attraction of the underlying molecules of the liquid for each other. If 

 a cleft is formed in a liquid drop the opposite walls of the cleft attract 



