The slow block to polyspermy in the sea urchin embryo consists of a physical barrier to further sperm penetration into the egg. The physical barrier is the fertilization envelope, which lifts off of the egg surface first at the site of sperm entry. The basic sequence of events has been known for decades. One of the first careful studies of the dynaimcs of this process was carried by the African-American developmental biologist E. E. Just. Below is a reproduction of hand-drawn pictures of the process of fertilization envelope elevation drawn by Just (original digitization courtesy of Scott Gilbert , Swarthmore College). The arrow and asterisk in frame 2 shows the site of sperm entry. The asterisks (hard to see) in subsequent frames show the same sight at later times.