amnion vs hymen

amnion

noun
  • The innermost membrane of the fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended. 

hymen

noun
  • A membrane which completely or partially occludes the vaginal opening in human females. 

  • Marriage. 

How often have the words amnion and hymen occurred in a corpus of books? (source: Google Ngram Viewer )