BIOELECTRIC MEASUREMENTS 



129 



changes may not be of much importance in fertilization. In spite of 

 the asymmetrical distribution of ions between the inside and out- 

 side of the sea-urchin egg (Table 20), which, unless the plasma 



TABLE 20 



Inorganic constituents of unfertilized eggs of Paracentrotus lividus 

 and of sea water {Rothschild & Barnes, 1953) 



In the case of eggs, mM means millimoles per kilogram of water in the eggs 

 (dry weight, 24%; density, 1-09); in the case of sea water, mM means milli- 

 moles per kilogram of water, chlorinity i9-21%o- The figure in square brackets 

 for total phosphorus is in mg./ml. eggs. 



membrane is impermeable to sodium and potassium, one would 

 have expected to cause a potential difference across the egg surface, 

 no such difference has been observed (Rothschild, 1938). In the 

 1938 experiments the terminal diameter of the electrode inserted 

 into the egg was 2-10 yc. On modern standards an electrode of this 

 size would be considered coarse and liable to tear the egg surface, 

 with consequent short-circuiting and failure to record any poten- 

 tial difference. Suppose that there is a potential difference across 

 the plasma membrane and that the insertion of a micro-pipette 

 causes such short-circuiting as to make the potential difference so 

 small as to be unmeasurable. The fact remains that eggs can be 

 fertilized after the insertion of two such electrodes, which means 

 that fertilization is not dependent upon the existence of a potential 

 difference across the egg surface. It is far from clear how an action 

 potential could be propagated over the egg surface, when, in the 

 resting state, there is no potential difference across the membrane 

 under consideration. The idea that no-one has ever got a micro- 

 electrode, as opposed to an ultramicro-electrode, into a sea-urchin 

 egg, and that the electrode merely causes an extended invagination 

 of the plasma membrane, is scarcely tenable. It is, for example, 

 possible to inject a live spermatozoon into the cytoplasm of a sea- 

 urchin egg, though fertilization does not occur. Similarly, as is 



