The yolk sac is an important structure for nourishing developing embryos in certain animal species, especially in birds, reptiles, and fish. However, variations in this structure may exist across different groups of organisms. Synonyms for yolk sac may include vitellus, yolker, yolk gland, egg yolk, vitelline sac, and umbilical vesicle. These alternative terms may be used interchangeably in scientific literature and may have different meanings in different contexts. Understanding these synonyms can help scientists communicate more effectively when discussing embryonic development and reproduction in different species and can also aid in broader research efforts to compare biological mechanisms across taxa.