from The American Heritage® Dictionary of the English Language, 5th Edition.
- noun A membranous sac that develops from the posterior part of the digestive tract in the embryos of mammals, birds, and reptiles. It is important in the formation of the umbilical cord and placenta in mammals.
from The Century Dictionary.
- noun A fetal appendage of most vertebrates, developing as a sac or diverticulum from the posterior portion of the intestinal cavity.
from the GNU version of the Collaborative International Dictionary of English.
- noun (Anat.) A membranous appendage of the embryos of mammals, birds, and reptiles, -- in mammals serving to connect the fetus with the parent; the urinary vesicle.
from Wiktionary, Creative Commons Attribution/Share-Alike License.
- noun anatomy A
sac, having a number of functions, that develops in the alimentary canalof the embryosof mammals, birdsand reptiles.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- noun the vascular fetal membrane that lies below the chorion and develops from the hindgut in many embryonic higher vertebrates (reptiles, birds and mammals)
from The American Heritage® Dictionary of the English Language, 4th Edition
It is an interesting point that this vascular hood, called the allantois, is represented in the Amphibians by an unimportant bladder growing out from the hind end of the food-canal.
Inside, the amnion encloses the embryo in a protected, moist environment in which nourishment is supplied by the yolk sac, and metabolic waste is stored by the allantois.
A very early condition of Man, with yelk-sac, allantois, and amnion (original).
The very young puppy, with attached ends of the yelk-sac and allantois, and invested in the amnion.
Aristotle's view, arguing that the female seed contrib - utes only nutrition and the allantois, while the male seed forms the other embryonic membranes (chorion, amnion) and bodily parts.
It was known by the 1860's that various nutritive, respiratory, and excretory organs (yolk-sac, allantois) characteristic of embryos are lacking in all adults.
Covering the external face of the amnion and lining the inner face of the chorion is a double membrane, _the allantois_.
In the region of the posterior appendages, _pa_, the section passes through the hindgut, _hg_, and allantois, _al_.
The allantois, _al_, extends cephalad for some distance from the floor of the cloaca.
Thus in many mammals the mesodermal part of the allantois often appears long before the endodermal part, though this is phylogenetically older.