The Collaborative International Dictionary of English v.0.48:
Vitellus \Vi*tel"lus\, n. [L., the yolk of an egg.] [1913 Webster] 1. (Biol.) The contents or substance of the ovum; egg yolk. See Illust. of Ovum. [1913 Webster] 2. (Bot.) Perisperm in an early condition. [1913 Webster]WordNet (r) 3.0 (2006):
vitellus n 1: nutritive material of an ovum stored for the nutrition of an embryo (especially the yellow mass of a bird or reptile egg) [syn: yolk, vitellus]