134 



FERTILIZATION 



physically implausible.' * An alternative interpretation of these 

 'anomalous' capacitance changes is that in the unswollen condition, 

 the sea-urchin egg membrane is folded, on a sub-microscopic 

 scale, and that when the egg swells, the membrane unfolds. Under 

 these conditions, calculation of the membrane capacitance per unit 

 area for the unswollen egg will produce too high a value, but the 

 calculation for the swollen egg will be more accurate. If, in fact, 

 the membrane is a 'reasonable' one (curve A, Fig. 30), it should be 







2-95 5-90 8-85 



FIG. 30. — Change in egg membrane capacitance per unit area with surface area of 

 egg, the latter being varied by immersing the eggs in sea water of different 

 degrees of hypotonicity. Fertilized eggs of Psendoccntrotus depressiis were 

 used. A, theoretical curve showing expected behaviour of the membrane 

 capacitance when the egg swells; B, theoretical curve if the membrane 

 thickness remains constant while the egg swells; C, variation in membrane 

 capacitance as an inverse function of the surface area. Adapted from lida 

 (1943c). 



possible to make an estimate of the minimum degree of folding 

 from capacitance measurements in normal and hypotonic sea water. 

 Cytoplasmic resistivity. Table 21 gives the results of experiments 

 in this field. The differences between unfertilized and fertilized 

 eggs may or may not be significant ; in any case, the most important 

 changes which occur in the early phases of fertilization, with which 

 we are concerned in this chapter, take place at the cell surface f 



* Unfortunately, a further paper dealing with the membrane capacitance of 

 sea-urchin eggs, in Japanese, by lida (1949) Zool. Mag. (Dobutsugaku Zasshi), 

 58, 122-125, is not available in the United Kingdom. 



t K. Dan (1947) reported a small change in i potential after fertilization of the 

 eggs oi Pseiidocentrotus depressus and Anthocidaris crassispina; but the change is 

 too small to be of much interest. 



