vitellus vs yolk

vitellus

noun
  • The contents or substance of the ovum; egg yolk. 

  • Perisperm in an early condition. 

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 vitellus and yolk occurred in a corpus of books? (source: Google Ngram Viewer )