What is "yolk sack"

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.