Figure 42.5 Fertilization of the Sea Urchin Egg (A) Sea urchin eggs are protected by a jelly layer and a proteinaceous vitelline envelope. Sperm must penetrate both to reach the egg cell membrane. Many sperm attach to the vitelline envelope, but only the first one to reach the egg cell membrane achieves fertilization. The numbers match structures with the events shown in (B). (B) The acrosomal reaction allows a sea urchin sperm to recognize an egg of the same species and pass through its protective layers. Enzymes from the egg’s cortical granules trigger the slow block to polyspermy.