Allantois is embryonic membrane found in
A. Mammals
B. Birds
C. Reptiles
D. Birds, mammals and reptiles

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Hint: This is the middle membrane between the amnion and chorion which plays an important role in transporting the oxygen to the embryo.

Complete answer:
Allantois is one of the four membranes of embryos. It is an extraembryonic membrane. In the posterior of the embryo, development begins and there this membrane is found. This layer is characterized by birds, mammals and reptiles. This layer lies between the two membranes which are amnion and chorion. Amnion is the innermost layer which protects the embryo and chorion is the outermost layer which protects the embryonic membranes. So, allantois is the middle layer. The allantois grows to a large size in non-humans and in humans, it is also important as it is a small outgrowth of the developing digestive tract.

> In option A, mammals are given so this membrane is found in mammals therefore this is the correct option.

> The option B is Birds, allantois is found in the birds and therefore this option is also correct.

> The option C is Reptiles and this membrane is characteristic of reptiles too so this is also the correct one.

> The last option is birds, mammals and reptiles. So this option is the correct one as this membrane is found in mammals, birds and reptiles.

Hence, The correct answer is option (D).

Additional information:
The function of allantois includes the storage of urinary waste and it helps in exchange of gases. Its main function is to deliver oxygen to the embryo. It expands between the amnion and chorion in reptiles and birds in order to serve as a temporary respiratory organ while the cavity stores fetal excretions. There is a fourth layer too in humans which is yolk sac.

Note: This membrane is characteristic of all the three mammals, reptiles and birds and it is present in all of them. This layer surrounds the amnion membrane which is the innermost one which supports and surrounds the embryo and protects it from mechanical stress.