CROSSING-OVER 561 



1928 ; Astbury, 1933 ; Caspersson and Hammarsten, 1935 ; Wrinch, 



1936). 



Turning to the external movements of the chromosomes we 

 have already seen that they must be determined by surface charges 

 — or double electrical layers — of different intensities on different 

 cell-organs — chromosomes, centromeres, centrosomes, nuclear mem- 

 branes and cell-walls. Haldane (unpublished) calculates that two 

 spheres, like the centromeres, of diameter 2500 A° at a distance 

 apart of 10,000 A° (i jx) in a medium of dielectric constant 81 

 and at a potential difference from this medium of 100 mv. would 

 repel one another with a force equal to 67,000 times their weight. 

 Electrical potential differences of an order that is known in living 

 systems will therefore determine a force sufficient to affect chromo- 

 somes perceptibly at the distances at which they are seen to move 

 apart, an action that will be reinforced by the special dielectric 

 conditions that we have inferred from the properties of the spindle. 

 The spindle, as we saw, develops its molecular orientation under 

 the influence of an electrical field and has the character of a liquid 

 crystal. 



The existence of different and changing surface charges on 

 different cell-organs may be inferred in an entirely different way, 

 from their different and changing fixation-staining reaction. This 

 reaction is conditioned by the surface charge on the bodies 

 concerned, and, as we have seen, it passes through a cycle of 

 changes co-ordinated with the mitotic cycle {cf. Yamaha, 1935). 



The next question that arises is how a surface charge comes to 

 be developed on certain cell-structures. The reason is that any 

 amphoteric electrolyte, such as a protein, lying in a medium not 

 at its iso-electric point will develop a surface charge (or strictly, a 

 double electric layer). Different cell-organs have different surface- 

 charges because the charge is a function of the difference between 

 the pH of the medium and the iso-electric point of the ampholyte 

 concerned. Hence also the cycle of changes in surface charge 

 on each organ implies a cycle of changes in the pH of the medium. 

 And hence, again, comparable organs, the centromeres and centro- 

 somes, within and without the nucleus, must have a different cycle 

 of changes. Finally, we must recall that the pH cycle inferred from 



