BIOELECTRIC MEASUREMENTS 



131 



and measuring the resultant ohmic drop of potential across the 

 plasma membrane with the other internal electrode. Such an 

 experiment would not be easy and might first be tried on unfer- 

 tilized trout eggs in oil, before contact with tap water. This would 

 avoid the complicating factor of the development of the chorion. 

 Cole & Guttman calculated from Holzer's data (1933) that the 

 trout egg membrane resistance was about 5,000 ohm-cm^. How- 

 ever, they failed to notice that Holzer's experiments were done on 

 trout eggs which had been cut in half. This treatment always kills 

 the eggs and entirely destroys the resistive properties of the 

 vitelline membrane; this hypothetical value for the trout egg 

 membrane resistance should not, therefore, be accepted. 



Lundberg (1956), using the 'ohmic drop' method with two 



TABLE 21 

 Egg membrane capacitance, Cm, and egg cytoplasm resistivity, rg 



♦ These measurements were on the inner contents of the egg, not the egg cytoplasm (see text). 

 U, unfertilized; F, fertilized. 



ultramicro-electrodes inside the egg (Psammechiniis miliaris), 

 managed to make a satisfactory measurement on one egg and ob- 

 tained the value 2,200 ohm-cm - for its membrane resistance. In 

 his other measurements, great difficulties were experienced, as 

 mentioned earlier, in establishing electrical contact between the 

 electrodes and the egg interior. 



Membrane capacitance. The most important work on this subject 

 is that of Cole and his co-workers. His results on egg membrane 

 capacitance and egg cytoplasm resistivity, together with a few 

 others, are given in Table 21. Cole made two important dis- 

 coveries in his capacitance measurements on eggs. First, that there 

 is a marked increase in membrane capacitance when the eggs of 

 Arbacia pnnctulata and of Tripneustes ventricosiis (Lamarck) are 

 fertilized (about 400% and 240%). This has been confirmed by 

 lida (i943«, 6), using the eggs of Pseudocentrotus depressus and 



