POLYSPERMY 107 



in the eggs oi Arbacia puuctidata at fertilization, observed by Moser 

 (i939«), and the similar phenomenon observed by Endo (1952) in 

 Japanese sea-urchins, are examples of such morphological changes. 

 Some workers, for example Runnstrom (1928, p. 395), have re- 

 ported that when the egg is examined with dark ground illumina- 

 tion, a colour change passes over the egg surface in four to six 

 seconds. But they did not realize that unless the egg is fertilized 

 at the equator, as seen by the observer who is looking down the 

 microscope, estimates of the time taken for the cortical change to 

 pass completely over the egg have no meaning. The observer, 

 whose eye is directly above the north pole of the egg, sees it in 

 optical section, the periphery of the section being the circum- 

 ference of an equatorial great circle. When a spermatozoon fer- 

 tilizes an egg, a cortical change spreads out in all directions over 

 the egg surface, from the point of attachment of the spermatozoon 



oo« 



FIG. 20. — Development of cortical change in an egg fertilized at 1.30 o'clock 



(Rothschild & Swann, 1949). 



(Fig. 20). If the spermatozoon fertilizes the egg exactly at the 

 north pole, the whole periphery of the optical section of the egg 

 will alter instantaneously, because the cortical change reaches all 

 points on the equatorial great circle simultaneously. This is shown 

 in Fig. 21, where the periphery of the alteration in surface structure 

 is shown as a series of 'isochrones' depicting the position of the 

 leading edge of the change at various times after fertilization 

 has started. If the spermatozoon fertilizes the egg at the equator, 

 the periphery of the optical section changes colour at the true 

 rate of propagation. At points intermediate between the north 

 (or south) pole and the equator, the propagation rate, as judged by 

 the time taken for the periphery of the optical section to change 

 colour, will seem to be faster than it actually is. The form of the 

 conduction-time curve will also be affected by these considerations. 

 The best way to make a serious examination of the conduction 

 time of the surface change is by taking dark-ground cinemicro- 

 graphs of eggs, immediately after insemination, and noting in 

 which eggs a fertilization cone can be seen at the equator. When 



