1. Membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional
   - yolk sac, umbilical vesicle, vesicula umbilicus

Derived forms: vitelline sacs

Type of: sac

• unmelted
• unmemorable
• unmemorably
• unmentionable
• unmercenary
• unmerchantable
• unmerciful
• unmercifully
• unmercifulness
• unmerited
• unmeritorious
• unmet
• unmethodical
• unmilitary
• unmindful
• unmindfully
• unmindfulness
• unmined
• unmingled
• unmissable
• unmistakable
• unmistakably
• unmistakeable
• unmitigable
• unmitigated