egg yolk vs vitellus

egg yolk

noun
  • The yellow central part of a chicken’s (or other bird’s) egg. 

vitellus

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

  • Perisperm in an early condition. 

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