The Collaborative International Dictionary of English v.0.48:
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. (Zool.) An oily secretion which naturally covers the wool
Yolk cord (Zool.), a slender cord or duct which connects
the yolk glands with the egg chambers in certain insects,
as in the aphids.
Yolk gland (Zool.), 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