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

• imperishability
• imperishable
• imperishableness
• imperishingness
• imperium
• impermanence
• impermanency
• impermanent
• impermeability
• impermeable
• impermeableness
• impermissibility
• impermissible
• impermissibly
• impersonal
• impersonality
• impersonally
• impersonate
• impersonation
• impersonator
• impertinence
• impertinent
• impertinently
• imperturbability
• imperturbable