It forms the connecting link between the foetus and the maternal placenta, since it conducts the nutritive vessels from the body of the foetus to the placenta; it comprises, besides, the pedicle of the allantois and the yelk-sac.
Moreover, even the vesicular structure of the allantois disappears at an early stage in the human case; with a deflection of the amnion, it gives rise to the pedicle.
Then the allantois assumes the entire task, and the yolk sack passes away.
The allantois is a membrane which is found in the foetus of the sow, mare, cow, and many other animals.
The fasciculus attached to the embryo is the allantois which becomes the umbilical cord.
The placenta is formed of dilated blood vessels which meet the maternal blood vessels, also dilated, in the uterine wall, allantois later on becomes the umbilical cord.
With the appearance of the allantois on the fourth day, a new feature is introduced.
The allantois first grows out on the dorsal side of the embryo, where it coalesces with the subzonal membrane, over a small discoidal area.
From the ventral wall of the cloacal section, there grows out the bifid allantoic bladder, which is probably homologous with the allantois of the higher Vertebrata.
Ovum in which the mesoblast of the allantois has extended round the inner surface of the subzonal membrane and united with it to form the chorion.
The yolk-sack disappears early, and the allantois (Turner) bulges out into a right and left lobe, which meet above the back of the embryo.
From the first the allantois lies in the pleuroperitoneal cavity.
The allantoisfirst appears as a protuberance of the splanchnopleure just in front of the anus.
We have in tracing the origin of the allantois to deal with a case of what Dohrn would call 'change of function.
As the hindgut becomes folded in the allantois shifts its position, and forms (figs.
The fold at the tail end of the embryo progresses considerably, and during its advance the allantois is formed in the same way as in Birds.
Her blood seeps over into the transformed allantois which is now called a placenta.
The allantois of the chick now gains a new development and an altered function.
The blood vessels of the allantois take in oxygen and give out carbon dioxide through the porous shell.
These two tissues are so closely pressed against each other that the blood vessels of the transformed allantois mesh in with those of the thickened parent wall.
So the allantois of the reptile has become the placenta of the mammal.
One of the reasons why the chicken must turn its eggs in the nest is that, if the allantois remain too long in contact with the upper shell of the egg, it will become attached to it and will not thereafter perform its functions.
There the primitive urine accumulates, and thus the allantois acts as bladder or urinary sac in the embryos of man and the other Amniotes.
The formation of the amnion and the allantois and the complete disappearance of the gills are the chief characteristics that distinguish the Amniotes from the lower Vertebrates we have hitherto considered.
Directly in front of the anus-opening the allantois develops from the hind gut; this is the important embryonic structure that forms into the placenta in the Placentals (including man).
The thin wall of the allantois consists of the same two layers or membranes as the wall of the gut--the gut-gland layer within and the gut-fibre layer without.
With this complete abandonment of the gills is probably connected the formation of another organ, to which we have already referred in embryology--namely, the allantois or primitive urinary sac (cf.
It is only in some of the later Marsupials (Peramelida) and all the Placentals that the allantois develops into the distinctive and remarkable structure that we call the placenta.
In the reptiles and birds the allantois enlarges into a spacious sac, which encloses the embryo with the amnion, and does not combine with the outer foetal membrane (the chorion).
We have already considered the formation and significance of this remarkable embryonic organ when we traced the development of the chorion and the allantois in the human embryo (Chapter 1.
Phylogenetically we must suppose that the allantois originated as a pouch-like growth from the cloaca-wall in consequence of the expansion caused by the urine accumulated in it and excreted by the kidneys.
The allantois is cut near its opening into the hindgut; its walls are thin, the epithelium consisting of but a single layer of more or less flattened cells.
The union of the allantois and the gut accounts for the elongated outline of the enteron in this section.
Figure 4M passes through the region where both the allantoisand the Wolffian ducts open into the hindgut.
Owing to its thin walls and small lumen the allantois was traced only a short distance into the umbilical stalk.
The allantois is probably somewhat larger here than in the other embryos used for this stage, in which it was torn away.
The allantois also contains a fluid which is known as the allantoid liquid.
The allantois is composed of two layers, which form a closed sac in connection with the urachus, or the tube which extends from the fetal bladder through the umbilical cord.
In the uterus the fetus is surrounded by several membranes which are known as the amnion or inner layer, the allantois or central layer, and the chorion or outer layer.
The one layer of the allantois is spread over the outer surface of the amnion and the other over the inner surface of chorion.
The existence of an allantois in the human embryo has been long inferred from the presence of a ligamentous cord extending from the fundus of the bladder to the umbilicus, like the urachus in animals.
Diagram of chick showing relations of amnion, allantois and yolk.
When the company invited had arrived, he took them into a room where he had the allantois of a calf filled with hydrogen gas, and, upon setting it at liberty, it immediately ascended and adhered to the ceiling.
Finally, from the antagonism of the allantois the sexual parts emerge at the very extremity of the body, which is placed opposite to the mouth.
All these animals consequently repeat the Mussels, and this formation admits of being followed out up to Man, where it is left as allantois and primordial kidneys.
Lastly, the allantois appears, and in its prolongation the sexual parts.
In the fowl, however, the blood-vessels of the allantois also probably absorb the albumen of the egg, and may excrete urea into the egg-space.
But the allantois and yolk sac of the rabbit, though they correspond in development, differ entirely in function from the similar organs of the fowl.
Figures to illustrate the relative function and importance of allantois and yolk sac in bird and mammal.
The allantois altogether disappears in the adult fowl; in the adult mammal a portion of its hollow stalk remains as the urinary bladder, and the point where it left the body is marked by the umbilicus or navel.
This becomes at last a great hollow sac, which is applied closely to the porous shell, and the extent of which will be appreciated by looking at Figure 5, Sheet 24, where the allantois is shaded.
We find solid mesoblastic somites, we find primitive streak, allantois and amnion, features we have just been explaining as the consequence of an excess of yolk in the egg.
The embryo hangs by the umbilical cord, which encloses the pedicle of the allantois (al).
We will deal with this remarkable formation and with the allantois later on (Chapter 1.
In this most advanced group of the mammals the allantoisis very small, soon loses its cavity, and then, in common with the amnion, undergoes certain peculiar changes.
When the allantois has attached itself to the inner wall of the chorion and been converted into the placenta, its blood-vessels alone effect the nourishment of the embryo.
This is the allantois or "primitive urinary sac," an important embryonic organ, only found in the three higher classes of vertebrates.
The wall of the allantois and the enveloping serolemma remains smooth and without villi, as in the birds.
In all the amniotes the allantoisquickly appears at the hinder end of the alimentary canal, growing out of the cavity of the pelvic gut (Figure 1.
The arteries of the allantoisare originally the terminations of the primitive aortae.
The allantois is present in Amphibia but is very small.
In those of the higher vertebrates which are developed within an egg (reptiles, birds and monotremes) the sac of the allantois comes to lie beneath the egg-shell and acts as a respiratory organ.
The statement that the umbilical arteries appear as the terminal branches of the embryonal aorta requires to be modified in the following respect: When the allantois develops its arteries are in fact end-branches of the two primitive aortae.
The growth of the mesoderm pushes the intestine against the sacral vertebrae, while the stalk of the allantois with the rudimentary urinary bladder is forced against the ventral abdominal wall.
In the higher mammalia, developed within the uterus, the allantois becomes attached by vascular villi to the uterine wall and establishes a vascular connection between the foetal and maternal blood vessels.
It is impossible to reconcile the conception of mutation with the adaptive relation between this allantois and the expulsion of the egg enclosed in a shell on land.
As the allantois could not be absorbed or retracted again into the abdomen, the umbilicus was evolved--that is to say, the scar formed by the union of the folded edge between the body wall and amnion surrounding the stalk of the allantois.
One of the most obvious examples of this is the allantois in the Amniota.
This is the placenta, developed, in part from the allantois of the embryo, in part from the uterus of the mother.
And even the feeblest attachment of the allantois of the embryo to the wall of the uterus would be of the greatest advantage to the species.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "allantois" in a variety of sentences. We hope that you will now be able to make sentences using this word.
