14 THE INITIATION OF DEVELOPMENT: 



described by Moser (1939). Starting from the point where the 

 sperm has entered, certain granules which are present in the 

 cortex of the unfertilised egg, vanish suddenly. This process 

 spreads over the egg surface in a wave-like progression, and 

 reaches the opposite pole within ten seconds. This reaction, too, 

 depends upon the presence of calcium ions; it fails to appear 

 in a calcium free medium. Runnstrom and collaborators (1946) 

 have shown that the granules which are expelled in the cortical 

 reaction, coalesce with the inside of the vitelline membrane. 

 Chemically, they consist largely of polysaccharides. They react 

 with the proteins of the vitelline membrane, thereby causing 

 a structural change which probably involves both interlinking 

 by side chains, and stretching of the protein molecules. This gives 

 the fertilisation membrane its very tough, horny consistency. 



In the marine polychaete worm Nereis, too, cortical granules 

 are extruded during fertilisation (Costello, 1949). Here, how- 

 ever, they coalesce into a substance, the greater part of which 

 passes through the vitelline membrane, and forms a thick 

 gelatinous mantle on the outside of the latter. This jelly is 

 again a polysaccharide. 



Apart from bringing about the formation of the fertilisation 

 membrane, the cortical reaction is very important in another 

 respect as well. In the great majority of animals, normally 

 only one sperm finds its way into the egg; penetration of more 

 than one sperm into the egg, so-called polyspermy, causes ab- 

 normal development. In a few groups only, e.g. in birds, we 

 find physiological polyspermy, i.e. penetration of several sperms 

 into one egg, is normal and does not lead to disturbances in 

 development. 



Where monospermy is normal, there must be a mechanism 

 that prevents the penetration of more than one sperm. Now it 

 has been shown that the cortical reaction which follows the 

 penetration of the first sperm, causes changes in the egg 

 surface which prevent the entrance of any further sperms. 

 Just, for instance, has demonstrated that in the sea urchin 

 Echinarachnius a "wave of negativity" spreads over the egg 

 surface from the spot where the sperm has entered, and that it 

 is impossible for other sperms to enter at any place which the 



