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

• Silphium
• Silphium laciniatum
• silt
• silt up
• siltstone
• silty
• Silurian
• Silurian period
• silurid
• silurid fish
• Siluridae
• siluriform fish
• Siluriformes
• Silurus
• Silurus glanis
• silva
• silvan
• Silvanus
• silver
• silver age
• silver ash
• silver beech
• silver bell
• silver birch
• silver bromide