Yolk-sac

an extraembryonic membrane that encloses the yolk of eggs in birds, reptiles, and marsupials and that circulates nourishment from the yolk to the developing embryo.
a similar membrane in placental mammals that encloses a mostly hollow sp-ce and loses its nutritive function entirely as the placenta develops.
historical examples

forest neighbors william davenport hulbert
the anatomy of the human peritoneum and abdominal cavity george. s. huntington
the ethics of medical homicide and mutilation austin o’malley
a guide to the study of fishes, volume 1 (of 2) david starr jordan
the works of francis maitland balfour, volume 1 francis maitland balfour
development of the digestive c-n-l of the american alligator albert m. reese
the works of francis maitland balfour, volume 1 francis maitland balfour
the ethics of medical homicide and mutilation austin o’malley
development of the digestive c-n-l of the american alligator albert m. reese
the anatomy of the human peritoneum and abdominal cavity george. s. huntington

noun (zoology)
the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk
the corresponding part in the embryo of mammals, which contains no yolk
yolk sac
a sac that is attached to the gut of an embryo and encloses the yolk in bony fish, sharks, reptiles, mammals, and birds. in most mammals, the yolk sac functions as part of the embryo’s circulatory system before the placenta develops.