BIOELECTRIC MEASUREMENTS I33 



A = cross-sectional area of plates, and 1 = distance between the 

 plates. If 1 becomes smaller, i.e., the condenser becomes thinner, 

 the capacitance becomes larger. Chemical changes in the cell 

 surface may, of course, also be responsible for the observed change, 

 but with our present exiguous knowledge of the chemistry of the 

 cell surface, it is difficult even to speculate about the nature of 

 changes which might cause an increase in capacitance. 



It would be of great interest to try to find out at what rate this 

 capacitance increase, which presumably starts at the site of sperm 

 attachment, is propagated over the egg surface. Such an experi- 

 ment would again probably involve difficult experiments with 

 ultramicro-electrodes. 



The second discovery that Cole (1935) made in this field was 

 that the membrane capacitance of the eggs of Tripneustes ventri- 

 cosiis is inversely proportional to the surface area of the egg, when 

 this is varied by diluting the sea water around the eggs with dis- 

 tilled water. This also has been confirmed by lida (1943c), using 

 the eggs of Pseiidocentrotus depressus, in which the changes in 

 membrane capacitance associated with alterations in the hypo- 

 tonicity of the sea water were found to be reasonably reversible. 

 This shows that the unexpected sense of the capacitance change is 

 not due to irreversible injury of the cortex following stretching, 

 lida's results are given in Fig. 30. Unless an egg in hypotonic sea 

 water continually synthesizes new membrane material to maintain 

 its normal thickness (curve B, Fig. 30), a most improbable situa- 

 tion, the membrane capacitance should increase (curve A, Fig.30), 

 not decrease, when the egg swells. The reasons are clear, as before, 

 from a consideration of Equ. (3). These observations may well 

 have revealed a fundamental, but paradoxical property, of cell 

 membranes in general, quite apart from those of sea-urchin eggs. 

 They merit further investigation, particularly in conjunction with 

 the Elastimeter experiments of Mitchison & Swann (1954^), 

 discussed in chapter 8. When considering the interpretation 

 of these capacitance changes, lida (1943c, p. 171) says: 'If an 

 assumption is made that the membrane is of a mosaic structure 

 with two intermingling areas, of which one is "effective" and the 

 other is "ineffective" in manifesting measureable capacitance, and 

 if the latter area alone is extensible on mechanical stretching, the 

 capacitance will vary in a manner represented by C. A scheme like 

 this appears to be a little too artificial, but it may not be altogether 



