130 FERTILIZATION 



well known, indicators and dyes have been repeatedly injected into 

 sea-urchin eggs, I have confirmed the apparent lack of p.d. across 

 the sea-urchin egg plasma membrane, using ultramicro-electrodes, 

 and so have Scheer et al. (1954). Lundberg (1956) believes that 

 when such electrodes are used, attempts at insertion are often un- 

 successful because the electrode does cause an extended invagina- 

 tion of the plasma membrane. When, however, he did manage to 

 effect contact with the egg cytoplasm, it appeared to be 5-10 mV. 

 positive with respect to the external medium. In view of the size 

 of liquid junction potentials in systems of this sort, a resting 

 potential of 5-10 mV. is too close to zero to have much significance. 

 Moreover, it is very difficult to see how the potential of the inside 

 of the sea-urchin egg can be positive with respect to the outside, 

 unless the plasma membrane is impermeable to potassium ions 

 and permeable to sodium ions, the latter being actively pumped 

 out. 



To sum up this section: there is as yet no good evidence that 

 the irregular potential changes observed in eggs at fertilization 

 occur at the plasma membrane ; nor that they are propagated over 

 the egg surface and, therefore, connected with the block to poly- 

 spermy; nor that they are important in fertilization. 



Membrane resistance. Very few estimates of egg membrane 

 resistances have been made because of the technical difficulties 

 inherent in such measurements on small spherical cells. The 

 following calculation shows the origin of these difficulties, par- 

 ticularly if an a.c. method of measuring resistance is used, with 

 external electrodes. Suppose that the membrane resistance is 500 

 ohm-cm.^ (Davson, 195 1). The radius of a sea-urchin egg (Psam- 

 mechinus miliaris) is 50ju,, so that its surface area is about 3.10^* cm^. 

 The actual resistance to be measured will, therefore, be of the 

 order of 10^ ohms, in comparison with which the resistance of the 

 external medium is negligible. As a result, the only systematic 

 measurements of egg membrane resistance are those of Cole & 

 Guttman (1942), using the unfertilized egg of Rana pipiens 

 Schreber, They obtained a value of 170 ohm-cm.-, with an alternat- 

 ing current bridge method. If ultramicro-electrodes could be in- 

 serted into eggs without their tips being broken or clogged, the 

 most satisfactory method of measuring egg membrane resistances 

 would be by inserting two electrodes into the egg, flowing current 

 across the plasma membrane using one of the internal electrodes, 



