RE-FERTILIZATION OF THE FERTILIZED EGGS OF THE SEA URCHIN

1. If the fertilized eggs of sea urchins (Strongylocentrotus pulcherrimus, Pseudocentrotus depressus, Heliocidaris crassispina, Temnopleurus toreumaticus) are inseminated again after removal of the fertilization membrane and washing with a Ca-Mg-free medium for a few minutes, re-fertilization can take place. Spermatozoa not only penetrate into the previously fertilized eggs but also take part in the formation of the mitotic figure, so that the cleavage of re-fertilized eggs is irregular and characteristic of polyspermy. 2. Re-fertilization can occur without removing the membrane mechanically if the eggs have been treated with a Ca-Mg-free medium immediately after the first insemination. 3. Re-fertilization can take place at any stage of the first cell division and even at the 2-cell stage. 4. It is justifiable to consider that there is a sperm-excluding substance on the surface of fertilized eggs. This substance is stable in sea water but is easily lost in Ca-Mg-free media.