In the evolution of vertebrates, one of the turning points in the transition from aquatic to terrestrial life was the evolution of a means of reproduction to provide protection for gamete and embryo without being necessary the presence of free water. The vertebrates became truly terrestrial with evolution, reptiles, from internal fertilization of an egg can be deposited on land. To do this we will see, as a primary element, the cleidoic egg development.

The secret of the cleidoic egg inside is that it has within its shell its own supply of water and all life support systems. When the fertilized egg travels through the reproductive tract, sturdy shell calcium is deposited around it. Then, when the incubation is started, the embryonic development is initiated, and from an unstructured mass of cells that divide rapidly, grow both the fetus and a complex membrane system.

Within the egg, embryo development is completely surrounded by a fluid filled membrane, the amnion. This fluid, amniotic fluid, protects the embryo from desiccation and accidents. The egg also carries a rich food supply, the yolk. The yolk is enclosed within a membrane which is directly connected to the digestive system of the embryo. Because the egg is a closed system (not unlike a spacecraft), it requires some method to get rid of waste. A third membranous sac, the allantois collects liquid wastes of the embryo, completely separated from the rest of the system. The outer surface of the allantois, which has a rich blood supply, is also used for exchanging oxygen and carbon dioxide. These two gases diffuse into and out of the egg. A fourth membrane, the chorion, is located just below the skin, surrounding the embryo and all other membranes. These membranes are formed from embryonic tissue and developed as the embryo grows as discussed below.

In the hen, egg formation, as the reproduction phase, is a continuous process, except the time of moulding, hence egg production has become an important industry as a source of human food, in addition to serving for the propagation of the species.