Figure 1: Sea Urchin embryo 20 hours after fertilization under control conditions. The embryo is in late gastrulation stage.
Figure 2: Control embryo after 7 hours of development	Figure 3: Embryo in calcium-free seawater from fertilization after 7 hours of development
Figure 4: Embryo placed in calcium-free seawater at the vegetal plate stage after 20 hours of development	Figure 5: Embryo in calcium-free seawater from fertilization after 20 hours of development