What is the Difference Between Amnion and Allantois?

The amnion and allantois are two of the four fetal membranes that protect and nourish the developing embryo. They have distinct functions and structures:

Amnion:

Thin, innermost layer that directly contacts the developing embryo.
Forms a fluid-filled cavity that encloses the embryo.
Does not contain blood vessels and obtains nutrients from nearby chorionic and amniotic fluid, and fetal surface vessels.
Composed of cuboidal and columnar epithelial layers.
Helps control embryonic body temperature and allows the fetus to move freely, aiding in skeletomuscular development.

Allantois:

Extra-embryonic layer located between the amnion and chorion.
Present in birds, reptiles, and mammals (except some marsupials) as a vascular structure.
Expands greatly between the amnion and chorion to serve as a temporary respiratory organ for the fetus.
Stores all the fetal excretions in its cavity and is filled with clear liquid.
Composed of blood vessels that allow exchange of gases from the developing embryo.
Eventually fuses with the chorion and umbilical cord and becomes a part of it.
After birth, the allantois forms a unique structure called the urachus, which connects the bladder to the umbilical cord.

In summary, the amnion is the thin, innermost membrane that directly contacts the developing embryo and provides a protective environment, while the allantois is an extra-embryonic layer located between the amnion and chorion that helps with gas exchange and waste removal for the fetus.