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

• add to
• add together
• add up
• add-in
• addable
• addax
• Addax nasomaculatus
• addend
• addendum
• adder
• adder's fern
• adder's tongue
• adder's tongue fern
• addible
• addict
• addicted
• addiction
• addictive
• adding machine
• Addis Ababa
• Addison's disease
• Addison's syndrome
• addition
• addition reaction
• additional