sac (sak) a pouch or bag.
yolk sac,
air sacs alveolar s's.
allantoic sac the dilated portion of the allantois, becoming a part of the placenta in many mammals; it becomes the urachus in humans.
alveolar sacs the spaces into which the alveolar ducts open distally, and with which the alveoli communicate. 
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Alveolar sacs (sacculi alveolares).
amniotic sac that formed by the amnion, containing the amniotic fluid.
chorionic sac that formed by the vertebrate chorion, surrounding the embryo, amniotic cavity, and amniotic sac and contributing to the fetal part of the placenta.
conjunctival sac the potential space, lined by conjunctiva, between the eyelids and eyeball.
dental sac the dense fibrous layer of mesenchyme surrounding the enamel organ and dental papilla.
endolymphatic sac the blind, flattened cerebral end of the endolymphatic duct.
gestational sac that comprising the extraembryonic membranes that envelop the embryo or fetus; in humans, that formed by the fused amnion and chorion.
heart sac pericardium.
hernial sac the peritoneal pouch enclosing a hernia.
Hilton's sac laryngeal saccule.
lacrimal sac the dilated upper end of the nasolacrimal duct.
yolk sac the extraembryonic membrane that connects with the midgut; at the end of the fourth week of development it expands into a pear-shaped vesicle (umbilical vesicle) connected to the body of the embryo by a long narrow tube (yolk stalk) . In mammals, it produces a complete vitelline circulation in the early embryo and then undergoes regression.
| yolk sac n. A membranous sac attached to an embryo, enclosing food yolk. Also called umbilical vesicle. |
yolk sac,
a structure that develops in the inner cell mass of the embryo and expands into a vesicle with a thick part that becomes the primitive gut and a thin part that grows into the cavity of the chorion. The cells of the extraembryonic mesoderm differentiate to develop endothelium, primitive blood plasma, and hemoglobin. The yolk sac usually disappears during the seventh week of pregnancy. See also allantois, Meckel's diverticulum.
yolk sac
one of the extraembryonic fetal membranes that balloons out from the fetal midgut. It helps to form a primitive placenta and promotes the development of the vitelline circulation. The vestigial yolk sac can be found about halfway along the small intestine of birds. The yolk sac membrane produces lymphoid stem cells that subsequently colonize the thymus and bursa of Fabricius. The stalk of the yolk sac is sometimes retained as Meckel's diverticulum of the small intestine.
yolk sac infection
see avian omphalitis.
inverted yolk sac
a form of placentation in which fetal splanchnic mesoderm in direct contact with maternal uterine tissue; occurs in laboratory rodents. Called also yolk sac placenta.
yolk sac placenta
see inverted yolk sac (above).
yolk sac tumor
shows varying stages of differentiation into tissues of multiple germ layers. Called also endodermal sinus tumor.
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