SECT. 6] OF THE EMBRYO 869 



corresponding to the equilibrium between hydrogen and oxygen in 

 water vapour (rH 28), Rapkine concluded that the activation of 

 oxygen in the sense of Warburg no less than the mobilisation of 

 hydrogen, in the sense of Wieland, must be important here. Much 

 other work on plant cells, where actual bubbles of molecular oxygen 

 may be seen traversing a protoplasm of rH as low as 17, led to the 

 same conclusions, and what Clark, Cannan and Cohen call the 

 potential of "biological" oxygen must be much lower than that of 

 the same gas in the molecular condition. 



The colorimetric rH measurements of Needham & Needham on 

 marine egg-cells were afterwards confirmed by Vellinger, using a 

 direct electrode potential method on a thawing "puree" oi Strongplo- 

 centrotus eggs. The result so obtained, when corrected for temperature 

 and dilution, came to rH 20, that of sea water to rH 26. The fact 

 that our measurements and those of Vellinger corresponded so well 

 as regards rH and so badly as regards pH. might be explained by the 

 assumption that cytolysis does not affect the rH, but does the />H, 

 in this material. 



It then occurred to Reiss & VelUnger to ask whether developing 

 sea-urchin eggs could get their energy from hydrogen-acceptors, 

 and not from molecular oxygen, i.e. whether the eggs could carry 

 on their metabolism if put under anaerobic conditions. Eggs were 

 placed in anaerobic solutions poised at different points on the rH scale 

 by indicators, haemoglobin and other substances, and it was found 

 that if the potential was 200 mv. or over, i.e. rH 23 or above, the 

 cleavages would go on as usual, but if it was below that point, the 

 number of cells dividing fell off rapidly until at 150 mv., or rH 22, 

 no cells would divide. They concluded that the "rH d'arret" was 

 always a little above the rH of the cell-interior. The work was shortly 

 afterwards confirmed to some degree by Rapkine, and is very im- 

 portant in view of possible phases of " anaerobiosis " in embryonic 

 life^ (see pp. 700 and 742). The determination of the upper limit 

 of rH for cell-division was subsequently made by Reiss, using potassium 

 permanganate, sodium hypochlorite and ferricyanide. For sea-urchin 

 eggs it was 665-720 mv. and for those of Sabellaria 600-700 mv. 



1 There is no contradiction between these results and those of E. B. Harvey, who 

 studied the effects of anaerobiosis on the cleavage of echinoderm eggs. In her experi- 

 ments, the cells were merely stained with methylene blue as an indicator for the dis- 

 appearance of oxygen and when this was reduced, cell-division ceased. In Rapkine's 

 experiments an excess of reducible dye was present around the cells. 



