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

• Osteoglossiformes
• osteoid
• osteologer
• osteological
• osteologist
• osteology
• osteolysis
• osteoma
• osteomalacia
• osteomyelitis
• osteopath
• osteopathic
• osteopathist
• osteopathy
• osteopetrosis
• osteophyte
• osteoplasty
• osteoporosis
• osteosarcoma
• osteosclerosis
• osteosclerosis congenita
• osteostracan
• Osteostraci
• osteotomy
• ostiarius