A crystalline, transparent, colorless substance found in the allantoic liquid of the fetal calf; Ð formerly called allantoic acid and amniotic acid.
The recent discovery of a true allantoic placenta in Perameles removes one reason for regarding {127} the Marsupials as primitive creatures.
It was for a long time held that the Marsupials differed from all other mammals in having no allantoic placenta.
It implies on the whole that the Marsupials have sprung from a stock with an allantoic placenta.
But quite recently this supposed difference has been proved to be not universal by the discovery in Perameles of a true allantoic placenta.
Observe the allantoic bladder, the spleen, gall bladder, portal vein, and pancreas.
Allantoic bloodvessels ramify thickly over its walls, and aeration occurs through the permeable shell.
Deposits of urates now become abundant in the allantoic fluid.
All Mammalia, other than the Monotremata and Marsupialia, have a true allantoic placenta.
They become prolonged backwards, and after receiving the epigastric veins above mentioned from the iliac veins, and also veins from the allantoic bladder, unite behind into a single vessel.
The right allantoic vein disappears before long, and the common trunk formed by the junction of the vitelline and allantoic veins becomes considerably elongated.
In the Carnivora again we have to start from the discoidal placenta, as shewn by the fact that the allantoic region of the placenta is at first discoidal (p.
In the umbilical cord a remnant of the allantoic vesicle was present in the embryos observed by Turner, but in the absence of a large allantoic cavity the Cape Ant-eater differs greatly from the Carnivora.
The allantoic arteries in the cord wind in a spiral manner round the allantoic vein.
The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that in Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion.
According to Rathke the original trunk connecting the allantoic vein directly with the heart through the liver is aborted, and the ductus venosus Arantii is a secondary connection established in the latter part of foetal life.
The left of the two eventually atrophies, so that there is formed an unpaired allantoic vein.
The section following (C) represents, I believe, only the stalk of the allantoic diverticulum.
According to the former the common trunk of the allantoic and vitelline veins in its passage through the liver gives off branches to the liver, and also receives branches from this organ near its anterior exit.
Thus, the secondary, allantoic circulation gradually takes the place of the original vitelline circulation.
In this way the allantoic placenta is formed (Fig.
This foreshadows the final destiny of the proximal intra-abdominal portion of the allantoic sac which persists and is converted into the urinary bladder of the adult.
These animals are therefore distinguished as Aplacentalia from the remaining higher mammals in which the allantoic placenta develops and which are hence called the Placentalia.
A considerable interval also develops between the points of entrance into the cloaca of the intestine proper and of the allantoic stalk (urinary bladder).
The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that of the Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion.
In the Carnivora, again, we have to start from the discoidal placenta, as evinced by the fact that in the growth of the placenta the allantoic region of the placenta is at first discoidal, and only becomes zonary at a later stage.
More remarkable still is the development of a small allantoic placenta.
As a rule there is no allantoic placenta forming the means of communication between the blood of the parent and the foetus, and when such a structure does occur its development is incomplete.
The bladder originates from the hindmost and lowest part of the allantoic pedicle (urachus), which enlarges in spindle shape before the entry into the cloaca.
In the Dipneusts and Amphibia, in which the allantoic sac first makes its appearance, it remains within the body-cavity, and functions entirely as bladder.
The lowest part of the allantoic pedicle (or the "urachus") remains hollow, and forms the bladder.
These urinary organs, acting as reservoir and duct for the urine excreted by the kidneys, originate from the innermost part of the allantoic pedicle.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "allantoic" in a variety of sentences. We hope that you will now be able to make sentences using this word.
