Birds have a unique reproductive system. For mammals the offspring develop in the uterus of the mother. In birds, however, the offspring develop in an egg the mother laid. The egg provides protection and nutrients to the developing embryo. The egg must contain all the nutrients essential to the development of the embryo it carries. As a result, the egg is a convenient, self-contained package for studying embryology.

Fertile vs Infertile egg

Day 6 embryo

Chick development (University of Kentucky)

Incubation and hatching your own eggs