Entry 2 senses Webster, 1913 Vitellus · Vi·tel·lus · IPA 01 n. The contents or substance of the ovum; egg yolk. See Illust. of Ovum. 1. The contents or substance of the ovum; egg yolk. See Illust. of Ovum.(Biol.) See: Ovum 2. Perisperm in an early condition.(Bot.)