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

• resourceful
• resourcefully
• resourcefulness
• resourceless
• resow
• respect
• respectability
• respectable
• respectably
• respected
• respecter
• respectful
• respectfully
• respectfulness
• respecting
• respective
• respectively
• respects
• respell
• Respighi
• respirable
• respiration
• respirator
• respiratory
• respiratory acidosis