Source : Webster's Revised Unabridged Dictionary (1913)

Vitellus \Vi*tel"lus\, n. [L., the yolk of an egg.]
   1. (Biol.) The contents or substance of the ovum; egg yolk.
      See Illust. of {Ovum}.

   2. (Bot.) Perisperm in an early condition.

Source : WordNet®

vitellus
     n : nutritive material of an ovum stored for the nutrition of an
         embryo (especially the yellow mass of a bird or reptile
         egg) [syn: {yolk}]
     [also: {vitelli} (pl)]