fetal membrane

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Fetal membrane

One of the membranous structures which surround the embryo during its developmental period. Since such membranes are external to the embryo proper, they are called extraembryonic membranes. They function in the embryo's protection, nutrition, respiration, and excretion.

There are four fetal membranes—the amnion, chorion, yolk sac, and allantois. In the course of development, the chorion becomes the outermost, and the amnion the innermost, membrane surrounding the developing embryo. As the allantois increases in size, it expands and becomes closely associated, if not fused, with the chorion. The two membranes together are known as the chorioallantoic membrane.

The amniotic cavity within which the embryo is enclosed becomes filled with an aqueous fluid which gives osmotic and physical protection to the embryo during the remainder of its fetal existence. Smooth muscle fibers in the amnion spontaneously contract and gently rock the embryo before it develops the capacity for spontaneous movement.

As the stored nutrients of the yolk are depleted during development, the yolk sac gradually decreases in size and is eventually incorporated into the midgut of the embryo. The yolk sac in the nonyolky eggs of placental mammals is vestigial. It has evolutionary but essentially no functional significance.

At the time of birth or hatching, the embryo becomes completely separated from the amnion and chorion and from the major portion of the allantois. The proximal portion of the latter remains within the embryo, however, as the urinary bladder. See Allantois, Amnion, Chorion, Yolk sac

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

fetal membrane

[′fēd·əl ′mem‚brān]
Any one of the membranous structures which surround the embryo during its development period. Also known as extraembryonic membrane.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
* Stage 5: late embryonic maturation; second embryonic membrane has ruptured, freeing the second antennae; shell spine remains curved along the anterior carapace edge
Introductory Sentences: "The question presents a statement Improved Student Application that birds and mammals are amniotes with (l)-(2)-(3) Guided whose embryos are supported by extra- Reflection embryonic membranes. The purpose of the question is to compare the function and structure of extra- embryonic membranes in typical bird and mammal embryos.
Embryonic viability was estimated by the macroscopical characteristics of the embryonic membranes, degree of vascularization of the allantochorion and embryo development, according to that described by Boshier y Guillomot [3, 8].