Sherk, Stephanie Dionne. "Prenatal Development". Gale Encyclopedia of Children's Health, 2006. Gale. Archived from the original on 1 December 2013. Retrieved 6 October 2013.
Sherk, Stephanie Dionne. "Prenatal Development". Gale Encyclopedia of Children's Health, 2006. Gale. Archived from the original on 1 December 2013. Retrieved 6 October 2013.