THE FERTILISATION OF THE EGG 



13 



the egg accumulate in the space between egg surface and 

 membrane, the so-called perivitelline space. These substances, 

 part of which are colloids, take up water from the environment 

 by osmosis. Consequently, the perivitelline space increases 



Fig. 3. Formation of the fertilisation membrane in the sea urchin 

 Strongylocentrotus purpuratus (the jelly layer around the egg has 

 been omitted), (a) sperm in contact with the egg, about 2 seconds 

 after insemination; (b-e) 30-50 seconds after insemination, formation 

 and coalescence of blebs at the egg surface; (/) 2-3 minutes after 

 insemination, the fertilisation membrane has been formed. After Chase. 



rapidly. The extrusion of these substances often results in a 

 considerable decrease in size of the egg. A few minutes after 

 its elevation, the fertilisation membrane undergoes a process of 

 consolidation, after which it resists further stretching. In sea 

 urchins, this consolidation occurs only in the presence of calcium 

 ions, or other bivalent cations. 



Usually the formation of the fertilisation membrane does not 

 set in simultaneously in all parts of the egg surface, but starts 

 from the spot where the sperm has penetrated (Fig. 3c), and 

 proceeds from there in all directions over the egg. Obviously, 

 the extrusion of the perivitelline substances, which causes the 

 elevation of the fertilisation membrane, must be due to a sudden 

 change in the outer layer of the egg, its so-called cortex. In 

 sea urchins, this cortical reaction is even visible, and has been 



