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MAGNESIUM ION‐REQUIRING STEP IN FERTILIZATION OF SEA URCHINS *

The magnesium ion‐requiring step in fertilization of sea urchins was investigated. When eggs were inseminated in Mg‐free sea water, several spermatozoa were found to bind to each egg surface with their reacted acrosomes without elevation of fertilization membrane. The number of binding jelly‐treated spermatozoa to an egg did not differ regardless of the presence or virtual absence of magnesium ions. Although fertilization did not occur in Ca, Mg‐deficient sea water (CM‐deficient SW) even when jelly‐treated spermatozoa were employed, some eggs could be fertilized by the addition of magnesium to the CM‐deficient SW 60 sec after insemination, when jelly‐treated spermatozoa had completely lost their fertilizing capacity in the CM‐deficient SW. The acrosomal process of jelly‐treated spermatozoa appeared to penetrate the vitelline layer in the CM‐deficient SW. DTT‐ or pancreatin‐treated eggs could not be fertilized in the virtual absence of magnesium. Re‐fertilization using the fertilized eggs deprived of fertilization membrane did not occur under conditions of magnesium deficiency. These results suggest that external magnesium ions are indispensable at least for the fertilization process following penetration of the vitelline layer by the spermatozoa, such as fusion of the plasma membrane between an egg and a reacted spermatozoon, or the subsequent step(s) such as sperm penetration into egg interior and egg activation which precedes the cortical reaction.