August 5, 1922] 



NA TURE 



181 



that such mischievous doctrines should be suppressed 

 by the secular arm. 



Finally, is it not time that this old bogy of the 

 case of Galileo, as a proof that the Church was 

 opposed to scientific research, should be decently 

 buried ? " The Papal power," wrote De Morgan 

 (loc. cil.), " must on the whole have been moderately 

 used in matters of philosophy, if we may judge by 

 the great stress laid on this one case of Galileo." 

 Cardinal Newman dubbed it rightly as " the one 

 stock argument." A. L. Cortie, S.J. 



Stonyhurst College Observatory, Lanes. 

 July 17- 



Surface Tension and Cell-Division. 



In a recent paper on " Surface Tension and Cell- 

 Division " (Q.J.M.S. 66, pp. 235-245), Mr. J. Gray 

 maintains that " cell-division may be accounted for 

 by the movement of the two asters away from each 

 other," and that " there is no necessity to postulate 

 regions of differential surface tension at the poles or 

 equator of the cell." 



These conclusions are drawn chiefly from the 

 results of a series of experiments in which the effect 

 of " acid " sea-water on the cleaving eggs of Echinus 

 miliaris is interpreted in the light of the effect of 

 increased hydrogen-ion concentration on an oil-water 

 interface. Mr. Gray points out that there is un- 

 doubtedly a lipoid or oily phase in the protoplasmic 

 surface, but, if any value is to be placed on the 

 analogy between the effect of acid on cleaving eggs 

 and on an oil-water surface, one has to suppose that 

 the lipoid phase in the protoplasmic surface is a 

 continuous phase, otherwise two different types of 

 surface are being dealt with. This view, as pointed 

 out by Bayliss (2nd Brit. Assoc. Rep. Colloids, 1918), 

 would necessitate that the volume of the cell should 

 not have any definite relation to the osmotic pressure 

 of the external solution. 



In these experiments it was shown that if the egg, 

 in which a cleavage furrow had appeared, is placed 

 in " acid " sea-water, the furrow disappears and, 

 owing to the unknown force which elongates the 

 egg, equilibrium is attained when the egg has the 

 form of a cylinder with hemispherical ends. The 

 shape of the dividing egg is stated to be dependent 

 entirely on (a) the surface tension at the egg surface, 

 and (6) the other force acting against the surface 

 tension which produces elongation of the cell axis 

 and is " associated in some way with the elongation 

 of the astral figure." Now the pressure inside the 

 fluid egg at anv region due to surface tension can be 

 determined from the curvature of the egg surface 

 in that region. Since the egg is presumed to be in 

 equilibrium at any stage during mitosis, any in- 

 equalities in pressure due to the surface tension and 

 resulting from the varying curvature, must be 

 counteracted by the second " elongating " force. 

 In the case quoted above, in which the dividing egg 

 is in equilibrium when its shape is that of a cylinder 

 with hemispherical ends, it is easy to show that the 

 pressure inside the hemispherical caps due to surface 

 tension is exactly twice the pressure inside the 

 cylindrical portion due to surface tension. Thus the 

 elongating force, of which the astral figure is to some 

 degree an expression, must produce a differential 

 pressure on the inside of the egg surface, such that 

 the pressure exerted over the middle cylindrical 

 portion is uniform and equal to half the pressure 

 produced over the hemispherical ends. If the view 

 is accepted that the mitotic figure is in anv way 

 connected with the force that produces elongation 

 and cleavage of the egg, I think the hypothesis that 



NO. 2753, VOL. I IO] 



this force is essentially a polar force must be adopted, 

 and it is difficult to see how any polar force could 

 produce such a distribution of forces as is necessitated 

 to produce equilibrium in the fluid cylindrical egg 

 described by Mr. Gray. 



He further describes an experiment as due to 

 Plateau, on a drop of oil placed between two 

 metal rings "so as to form a complete cylinder " 

 (of course, with spherical end surfaces), which, 

 on moving the two rings apart, changed in 

 form, when the distance between the rings became 

 greater than J the diameter of the rings, in the 

 same way as a dividing egg. The exact reference to 

 this experiment is not given, but the experiment is 

 very similar to one that Plateau carried out on the 

 stability of a fluid catenoidal surface. According 

 to Mr. Gray a cleaving egg can be divided into three 

 parts : a middle cylindrical portion and two convex 

 ends to this cylinder. The middle cylinder is taken 

 to be analogous to the drop of oil in Plateau's experi- 

 ment. It is obvious, however, that the whole of this 

 experiment depends on the fact that the two rings, 

 as they are being moved apart, exert a lateral thrust 

 on the surface film of the drop. Is one, then, to 

 believe that the force which causes the egg to elongate 

 also exerts a lateral thrust in two parallel planes 

 corresponding to the two rings ? Actually, if there 

 is sufficient oil between the two rings they may be 

 moved apart to a distance equal to ?r times the 

 diameter of the rings before the central cylindrical 

 portion becomes unstable and divides, but this in no 

 way resembles the form of any cleaving egg, so far as 

 1 am aware. 



While I quite agree with Mr. Gray that the mitotic 

 force counteracts the effects due to surface tension, 

 it seems to me that one cannot account for the 

 stability of his cylindrical stage on the supposition 

 of uniform surface tension over the whole egg surface, 

 without ascribing to that mitotic force properties 

 that one would hesitate to ascribe to forces focussed 

 round two definite centres. Unless the view that 

 the forces which elongate the egg are correlated with 

 the spatial relations of the two centrosomes is adopted, 

 all the morphological evidence of the mechanism of 

 cell-division is neglected. 



In the introduction to his paper Mr. Gray says that 

 theories postulating a higher or lower surface tension 

 at the equator of a cleaving cell are of little value, as 

 " there is no apparent means of determining how 

 such a state of affairs could arise." In many cells 

 it has been shown that the cytoplasmic inclusions 

 are arranged around the centrosomes as foci, and 

 further, that the distribution of these elements may 

 be correlated with the apparent activity of the centro- 

 somes. From this it may be inferred that there are 

 substances in the egg which are repelled or attracted, 

 either directly or indirectly, by the centrosomes. 

 One of the first events in the cleavage of an Echinus 

 egg is the appearance of two " active " centrosomes. 

 These are approximately symmetrically placed in 

 the egg and move apart from one another. Sub- 

 stances repelled by the centrosomes will tend to gather 

 at the surface of the egg, but, as the centrosomes 

 move apart, these bodies, once having reached the 

 surface, will move towards that region of the egg 

 surface in the plane in between the two centrosomes, 

 which is termed the equator of the egg. If the pres- 

 ence of these substances in the surface film of the egg 

 affects in anv wav the surface tension of the proto- 

 plasmic surface, here is a mechanism whereby there 

 mav arise a differential surface tension over the surface 

 of the cleaving egg. H. Graham Cannon. 



I mperial College of Science and Technology, 

 South Kensington, London, S.W.7, 

 July 14- 



