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

• resonator
• resorb
• resorcinol
• resorcinolphthalein
• resorption
• resort
• resort area
• resort hotel
• resound
• resounding
• resoundingly
• resource
• resourceful
• resourcefully
• resourcefulness
• resourceless
• resow
• respect
• respectability
• respectable
• respectably
• respected
• respecter
• respectful
• respectfully