\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}.