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

• polyphonically
• polyphonous
• polyphony
• polyphosphate
• polyphosphoric acid
• polyphyletic
• Polyplacophora
• polyplacophore
• polyploid
• polyploidy
• Polypodiaceae
• Polypodiales
• Polypodium
• Polypodium aureum
• Polypodium glycyrrhiza
• Polypodium polypodioides
• Polypodium scouleri
• Polypodium virgianum
• Polypodium vulgare
• polypody
• polypoid
• Polyporaceae
• polypore
• Polyporus
• Polyporus frondosus