1. Membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional
   - yolk sac, vitelline sac, umbilical vesicle

Type of: sac

• hydrotropic
• hydrotropism
• hydrous
• hydroxide
• hydroxide ion
• hydroxy
• hydroxy acid
• hydroxyacetic acid
• hydroxybenzene
• hydroxybenzoic acid
• hydroxybutyric acid
• hydroxychloroquine
• hydroxyl
• hydroxyl group
• hydroxyl ion
• hydroxyl radical
• hydroxylamine
• hydroxymethyl
• hydroxyproline
• hydroxytetracycline
• hydroxyzine
• hydroxyzine hydrochloride
• Hydrozoa
• hydrozoan
• Hydrus