| A second fluid filled cavity, the amnion, develops within the inner cell mass. |
| The allantois forms as an outpocketing of the embryo's hindgut and grows outward into the space between the amnion and chorion. |
| In reptiles and birds, the amnion, chorion, and allantois are further protected by a hard or leathery calcareous shell. |
| The amnion is thin and difficult to visualize and best seen when perpendicular to the ultrasound beam. |
| Vertebrates have three embryonic membranes: the amnion, the chorion, and the allantois. |
| Surrounding and protecting the embryo is the amnion, filled with amniotic fluid, and providing the embryo with a stable fluid environment. |
