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Updated: May 15, 2025
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. This is the case also with the lowest mammals, the oviparous Monotremes and most of the Marsupials. The latter also is richly permeated with blood-vessels which bring the mother's blood to the embryo.
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.
In the Reptilia the bladder is expanded for the same function, and absorbs oxygen and gives off carbon dioxide through the pores of the shell. 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.
The embryo thus enclosed in the egg finds its protection in the fact that it is encased in a fluid contained in the amnion. It draws its nourishment from the yolk upon which it lives and the nourishment is transmitted to it by blood vessels. It draws its oxygen and throws off its wastes through the instrumentality of the allantois, which covers it over.
Here again we find vesicular growths or appendages of the originally simple gut developing into a variety of organs. Two of these embryonic structures, the yelk-sac and allantois, are already known to us. The two large glands that open into the duodenum, the liver and pancreas, are growths from the middle and most important part of the trunk-gut.
The allantois of the chick now gains a new development and an altered function. In the case of the chick it floats against the shell of the egg and absorbs oxygen through the shell. Inside the body of the mammal this is impossible, because the air is too far away. No shell is formed about the egg because it is not to be laid.
In another class of adaptations the embryonic or larval stage is adapted to new conditions, while the adult condition is either less changed or not changed at all. One of the most obvious examples of this is the allantois in the Amniota. The embryos of Reptiles, Birds, and Mammals all develop two embryonic or foetal membranes, the amnion and the allantois.
That the placenta has resulted from some such process of evolution is proven by its different stages of development in different orders of mammals. 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. This is not the whole explanation; other factors still undiscovered were undoubtedly concerned.
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