Yolk Sac

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Yolk sac

An extraembryonic membrane which extends through the umbilicus in vertebrates. In some elasmobranchs, birds, and reptiles, it is laden with yolk which serves as the nutritive source of embryonic development.

In mammals, as in birds, the yolk sac generally develops from extraembryonic splanchnopleure, and extends beneath the developing embryo. A blood vessel network develops in the mammalian yolk sac lining. Though these blood vessels are empty, they play an important role in absorbing nourishing food and oxygen from the mother. Thus, although the yolk sac in higher mammals may be considered an evolutionary vestige from its yolky-egged ancestors, it still serves important functions in the young embryo. As the embryo ages, the yolk sac shrinks in size, and the allantois takes over the role of nutrition. See Allantois

Yolk Sac


the organ of nutrition and respiration in the embryos of cephalopod mollusks, cartilaginous and bony fish, reptiles, birds, mammals, and man.

The yolk sac arises in the early stages of embryonic development, usually by means of the overgrowing of the yolk with endoderm and with the visceral layer of the lateral plates, and consists of an enlarged outgrowth of the midguts, the cavity of which in the majority of animals (except higher mammals and man) is filled with unbroken yolk. In the wall of the yolk sac blood cells and blood vessels are formed, which provide for transport of nutritive substances to the embryo and for its respiration. As the embryo develops, the size of the yolk sac decreases, its cavity shrinks, and it is either gradually drawn into the body cavity and resorbed or is cast off.

yolk sac

[′yōk ‚sak]
A distended extraembryonic extension, heavy-laden with yolk, through the umbilicus of the midgut of the vertebrate embryo.
References in periodicals archive ?
The yolk sac is a vascularized membrane similar to the placenta in mammals, which involves the yolk during embryonic development and is responsible for the transfer of nutrients to the embryo.
The adhesive organ at the base of the yolk sac allows the attachment of the larvae to some types of substrate (LEGENDRE; TEUGELS, 1991) that may be displaced in currents, playing an important role in larval dispersal.
Whilst, ample evidences in fisheries indicated that in several fish species HPI axis is not efficient before absorption of yolk sac (Veillette et al.
In the second phase, the yolk sac has been completely absorbed and the larvae need external sources for feeding.
10) As reported in prepubertal testis tumor registry, yolk sac tumor is most frequent type of testis tumor in children.
Figure lG shows the anterior and posterior lobes of the inner yolk sac along with the lateral lobe of the inner yolk sac visible through the mantle of an S.
In all cases, ultrasound identified a gestational sac and yolk sac consistent with the gestational age as measured by the last menstrual period," Dr.
The relative weight of yolk sac was expressed as a percentage of body weight, while the relative weight of heart or liver was expressed on the basis of the yolk sac-free chick weight.
The yolk sac was present, measuring 5 cm in diameter, and its grayish-yellow content was sampled for aerobic culture.
The sublethal effects were indicative of stress and may have been causative factors in reduced larval survival; they included precocious hatch, smaller larval size with larger yolk sac at hatch, and reduced growth rate.
The hypoblast grows outward to form a layer of tissue (primitive endoderm) that lines the blastocoele and eventually gives rise to the yolk sac.