Complex sentence: Vitellogenin is an egg yolk precursor protein expressed in females of fish, amphibians, reptiles (including birds), most invertebrates, and the platypus as a precursor for the lipoproteins and phosphoproteins that make up most of the protein content of yolk.
Split into simpler sentences: Vitellogenin is an egg yolk precursor protein expressed in the females of nearly all oviparous species including fish, amphibians, reptiles, birds, most invertebrates, and the platypus. VTG is the precursor of the lipoproteins and phosphoproteins that make up most of the protein content of yolk.