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  • thin, membranous sac enclosing the embryos of reptiles, birds, and mammals. Source: Noland, George B. 1983. General Biology, 11th Edition. St. Louis, MO. C. V. Mosby
  • Inner, fluid-filled sac composed of a thin double membrane that surrounds the embryo in reptiles, birds, and mammals. Source: Curtis, Helena. 1968. Biology. New York, NY. Worth Publishers