PRESIDENTIAL ADDRESS. 753 



Tt does not follow from all this that surface tension has nothing to do with 

 cell division. If, as Brailsford Robertson holds, surface tension is lowered in tho 

 plane of division, then the internal streaming movement of the cytoplasm of each 

 half of the cell should be towards that plane, and, in consequence, not separation 

 but fusion of the two halves would result. The lipoids and soaps would indeed 

 spread superficially on the two parts from the equatorial plane towards the two 

 poles, and, according to the Gibbs-Thomson principle, they would not distributo 

 themselves through the cytoplasm in the plane of division, except as a result of tho 

 formation of a septum in that plane. In other words, the septum has first to 

 exist in order to allow the soaps and lipoids to distribute themselves in a 

 streaming movement over its two faces. In Brailsford Robertson's experiment 

 this septum is provided in the thread. If, on the other hand, surface tension is 

 higher about the nucleus in and immediately adjacent to the future plane of 

 division, then constriction of the nucleus in that plane will take place accom- 

 panied or preceded by an internal streaming movement in each half towards its 

 pole and a consequent traction effect on the chromosomes which are thus removed 

 from the equatorial plane. When nuclear division is complete then a higher 

 surface tension on the cell itself limited to the plane of division would bring 

 about there a separation of the two halves, a consequent condensation on each 

 side of that plane of the substances producing the low tension elsewhere, and 

 thereby also the formation of the two membranes in that plane. 



In support of this explanation of the action of surface tension as a factor in 

 division I have endeavoured to ascertain if, as a result of the Gibbs-Thomson 

 principle, there is a condensation of potassium salts in the cytoplasm at the poles 

 of a dividing cell, that is, where surface tension, according to my view, is low. 

 The difficulty one meets here is that, in the higher plant forms, cells preparing 

 to divide appear to be much less rich in potassium than those in the ' resting ' 

 stage, and under this condition it is not easy to get unambiguous results, while in 

 animal cells potassium may even in the resting cell be very minute in quantity, as, 

 for example, in Vorticella, in which, apart from the contractile stalk, it is limited 

 to one or two minute flecks in the cytoplasm. Instances of potassium-holding 

 cells undergoing division are, however, found in the spermatogonia of higher 

 vertebrates (rabbit, guinea-pig), and in these the potassium is gathered in tho 

 form of a minute, and thin caplike layers at each pole of the dividing cell. 



This of itself would appear to show that surface tension is lees in the neigh- 

 bourhood of the poles than at the equator of the dividing cell, but I am not 

 inclined to regard the fact as conclusive, and a very large number of observa- 

 tions to that end must be made before certainty can be attained. I am, 

 nevertheless, convinced that it is only in this way that we can finally determine 

 whether differences of surface tension in dividing cells account, as I believe 

 they do, for all the phenomena of cell division. The difficulties to be encoun- 

 tered in such an investigation are, as experience has shown me, much greater 

 than are to be overcome in efforts to study surface tension in cells under other 

 conditions, but I am in hopes that what I am now advancing will influence a 

 number of workers to take up research in microehemistiy along this line. 



I must now discuss surface tension in nerve cells and nerve fibres. I have 

 staled earlier in this address that I hold that the force concerned in the pro- 

 duction of the nerve impulse by the nerve cell is surface tension. The very fact 

 that in the repair of a divided nerve fibre the renewal of the peripheral portion 



of the axon occurs through a movement — a flowing outward, as it were of the 



soft colloid material from the central portion of the divided fibre is, in itself, 

 a strong indication that surface tension is low here and high on the cell body 

 itself. This fact does not stand alone. I pointed out six years ago that potas- 

 sium salt is abundant along the course of the axon and apparently on its exterior 

 surface, while it is present but in traces in the nerve cell itself. In the latter 

 chlorides also are present only in traces, and therefore sodium, if present, is 

 there in more minute quantities, while haloid chlorine is abundant in the axon. 

 Macdonald has also made observations as to the occurrence of potassium along 

 the course of the axon, and has in the main confirmed mine. We differ only 

 as to mode of the distribution of the element in the axon, and the manner in which 

 it is held in the substance of the latter; but, whichever of the two views may 

 be correct, it does not affect what I am now advancing. Extensive condensatioa 



