Final answer:
The amnion and chorion are the two layers that fuse as the amniotic cavity enlarges, crucial for protecting the growing embryo.
Step-by-step explanation:
As the amniotic cavity enlarges during embryonic development, the two layers that fuse are the amnion and the chorion. The amnion eventually grows to surround the embryo, filled with amniotic fluid, and is derived from cells of the epiblast extending around the amniotic cavity. The chorion, which includes contributions from the trophoblast and mesoderm, becomes the outermost membrane surrounding the embryo and fetus, playing a key role in the formation of the placenta. These two layers are essential in providing protection and necessary support for the growing embryo within the uterus.