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

• nitrogen fixation
• nitrogen mustard
• nitrogen narcosis
• nitrogen oxide
• nitrogen trichloride
• nitrogen-bearing
• nitrogen-fixing
• nitrogenase
• nitrogenise
• nitrogenize
• nitrogenous
• nitroglycerin
• nitroglycerine
• nitrohydrochloric acid
• nitrosobacteria
• Nitrosomonas
• Nitrospan
• Nitrostat
• nitrous
• nitrous acid
• nitrous bacteria
• nitrous bacterium
• nitrous oxide
• nitta tree
• nitty