POLYSPERMY II3 



The cortical change which can be seen under the microscope is, 

 therefore, not the block to polyspermy, but a reflection or phase of 

 the slow part of it. The recent experiments of Nakano (1954) 

 support this analysis. 



Professor M. Sugiyama told me about an interesting experiment 

 he had done, the details of which will be published. It supports the 

 view that fertilization induces the propagation of an invisible 

 change, distinct from the cortical change, round the egg surface or 

 through the cytoplasm. He sucked an unfertilized sea-urchin egg 

 into a glass capillary whose diameter was slightly less than that of 



t=60 



FIG. 23. — Diagram of the block to polyspermy in a sea-urchin egg, showing rapid 

 partial block (grey), and slow complete block (black). Time, t, in seconds 

 (Rothschild, 1953). 



the egg. Part of the egg, the 'proximal' part, protruded from the 

 end of the capillary, while the other, 'distal', part was within the 

 capillary and in contact with sea water. When the capillary and egg 

 were placed in M-urea in distilled water, there was a breakdown of 

 granules in the cortex of the proximal part, but no membrane 

 formation, because of the absence of divalent cations in the medium. 

 A normal membrane formed round the distal part. Sugiyama con- 

 siders that, in these conditions, both the invisible and cortical 

 changes occur. When the capillary and egg were placed in sea 

 water containing butyric acid, cortical granules broke down and 

 a membrane formed at the distal end ; but no breakdown of cor- 

 tical granules occurred at the proximal end. Sugiyama's inter- 



