This is a uncommon occurrence with hens, when something goes wrong in their reproductive system - an egg wasn't laid and was "absorbed" into another egg as it developed.
the chicken
They don't. Once the egg has developed to a certain stage, the egg is laid in the nest.
An egg gets fertilized in the fallopian tube by a sperm, then it goes inside of the uterus, and inside the uterus it develops.
gods will.
Reptiles are thought to have developed after the evolution of an amniotic egg, which allowed them to reproduce successfully on land without the risk of the egg drying out. This adaptation was a key factor in their ability to transition from an aquatic environment to a terrestrial one.
Egg -> tadpole -> frog (Note, most fail to develop properly.)
Because birds evolved to reproduce in such a manner. For birds, more of their offspring survived if they developed and birthed outside of the mothers body.
They usually lay small and/or deformed eggs at the beginning of their laying cycle. As their egg sack isn't completely developed yet.
Reptiles (I think)
It is called an egg. The egg is developed in a chicken via the Oviduct but only stays there for about 24 hours, the balance of the development is done outside the hens body after the egg is laid by the hen.
many eggs are released one a month once the woman is fully developed