Source : Webster's Revised Unabridged Dictionary (1913)

Yolk \Yolk\ (y[=o]lk or y[=o]k; 277), n. [OE. yolke, yelke,
   [yogh]olke, [yogh]elke, AS. geoloca, geoleca, fr. geolu
   yellow. See {Yellow}.] [Written also {yelk}.]
   1. The yellow part of an egg; the vitellus.

   2. (Zo["o]l.) An oily secretion which naturally covers the
      wool of sheep.

   {Yolk cord} (Zo["o]l.), a slender cord or duct which connects
      the yolk glands with the egg chambers in certain insects,
      as in the aphids.

   {Yolk gland} (Zo["o]l.), 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}.