When the embryo is developing into that adult organism, there are four extra embryonic membranes that are formed and these four membranes are eventually discarded by that individual following birth. Now, these four extra embryonic membranes include the Coryon, the amnion, the lantoids, as well as the Umbilical vesicle, and an animal such as reptiles and birds. The Umbilical vesicle is actually called a yolksag because it contains a nutritious substance made of liver proteins known as yolk. So in this lecture, we're going to focus primarily on the embryonic development of the human embryo. And to see what we mean by these four extra embryonic membranes. Let's take a look at the following diagram.