Marieb, Elaine Nicpon. Essentials of human anatomy and physiology. 5th ed. Menlo Park, Calif.: Benjamin/Cummings Pub. Co., 1997. Print.
"Introduction to the Amniota." UCMP - University of California Museum of Paleontology. N.p., n.d. Web. 17 Oct. 2012. <https://www.ucmp.berkeley.edu/vertebra
"Brit_chorion">Entry of 'chorion' in the Encyclopædia Britannica website . Retrieved on 26 Sep 2009. In insects, the chorion is the outer shell of the insectegg "Brit_chorion"