ovule vs yolk

ovule

noun
  • An immature ovum in mammals. 

  • The structure in a plant that develops into a seed after fertilization; the megasporangium of a seed plant with its enclosing integuments. 

yolk

noun
  • The yellow, spherical part of an egg that is surrounded by the white albumen, and serves as nutriment for the growing young. 

  • The grease in a sheep's fleece; lanolin. 

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