de the body, such as heat, light, pressure, and chemicals. exteroreceptor The breakdown of food outside cells. extracellular digestion The substance in which animal tissue cells are embedded consisting of protein and polysaccharides. extracellular matrix (ECM) Four membranes (yolk sac, amnion, chorion, allantois) that support the developing embryo in reptiles, birds, and mammals. extraembryonic me