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1. in vertebrates with telolecithal eggs, the highly vascular layer of splanchnopleure surrounding the yolk of an embryo;
2. in humans and other mammals, the sac of extraembryonic membrane that is located ventral to the embryonic disk and, after formation of the gut tube, is connected to the midgut; by the second month of development, this connection has become the narrow yolk stalk; the yolk sac is the first hematopoietic organ of the embryo, and its vitelline circulation plays an important role in the early embryonic circulation; the sac is also the site of origin of the primordial germ cells.
See yolk sac.
sac(sak) [L. saccus, sack, bag]
A baglike part of an organ, a cavity or pouch, sometimes containing fluid. Synonym: saccus See: cyst
In a mammalian lung, an informal term for “pulmonary alveolus.”See: alveolus.
The expanded end of the allantois, well developed in birds and reptiles.
alveolar sacPulmonary alveolus.
The inner fetal membrane that encloses the developing fetus and produces amniotic fluid.See: chorion
The outer fetal membrane that encloses the developing embryo.
The cavity, lined with conjunctiva, that lies between the eyelids and the anterior surface of the eye.
The mesenchymal tissue surrounding a developing tooth.
The expanded distal end of the endolymphatic duct.
In the peritoneum, a saclike protrusion containing a herniated organ.See: hernia
Hilton's sacSee: Hilton, John
The upper dilated portion of the nasolacrimal duct situated in the groove of the lacrimal bone. The upper part is behind the internal tarsal ligament. It is 12 to 15 mm long.
lesser peritoneal sacOmental bursa.
The enclosed, transparent mesothelial sac that is squeezed between the abdominal wall and the abdominal viscera.
vitelline sacYolk sac.
In mammals, the embryonic membrane that is the site of formation of the first red blood cells and the cells that will become oogonia or spermatogonia. Synonym: vitelline sacSee: embryo for illus