In hens, internal fertilization take place but they do not give birth to chicks like human beings. After fertilization, the zygote divides repeatedly to form embryo which travels down the oviduct. As it travels down, many protective layer are formed around it.
The outermost protective layer is hard shell which we can see in a hen’s egg. After the hard shell is formed around the developing embryo, the hen finally lays the egg. The hen, then sits on the egg to provide sufficient warmth to the eggs for the development of the embryo into the chicks. The embryo takes about 3 weeks to develop into a complete chick.