What does "yolk" mean?

Yolk: (Zoöl.) An oily secretion which naturally covers the wool of sheep. Yolk cord (Zoöl.), a slender cord or duct which connects the yolk glands with the egg chambers in certain insects, as in the aphids. -- Yolk gland (Zoöl.), a special organ which secretes the yolk of the eggs in many turbellarians, and in some other invertebrates. See Illust. of Hermaphrodite in Appendix. -- Yolk sack (Anat.), the umbilical vesicle. See under Unbilical.