Uptake of Ca2+ is one of the earliest responses to fertilization of sea urchin eggs.

Sea urchin eggs take up Ca for 10 min following insemination (determined as uptake of 45Ca from the SW (3)). Although 90% of this uptake occurs after the beginning of the cortical reaction and may represent external binding of Ca to the egg surface coats, there is a brief phase of uptake (0–30 sec) which precedes the cortical reaction; this may represent a Ca flux into the eggs.