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

• recreation facility
• recreation room
• recreational
• recreational drug
• recreational facility
• recreational vehicle
• recreative
• recriminate
• recrimination
• recriminative
• recriminatory
• recross
• recrudesce
• recrudescence
• recrudescent
• recruit
• recruiter
• recruiting-sergeant
• recruitment
• recrystallisation
• recrystallization
• rectal
• rectal artery
• rectal reflex
• rectal vein