An internal layer is a hen that lays an egg internally---as opposed to externally (outside of her body). It is a reproductive disorder of hens, where her oviduct is not functioning as it should. The disorder occurs as a result of a genetic defect preset since birth, or it can be acquired as a secondary result of an infection, or trauma to her oviduct.
Normally, when a hen with a healthy functioning reproductive system deposits a yolk, it moves through her oviduct, creating a shell coating around it and then ejecting it from her body and into her nest. When a hen is an internal layer, instead of the yolk getting taken into the oviduct, it is released into her abdomen. A single incidence of internal laying isn't necessarily a problem, however if it occurs on a frequently on a continuing basis, all the egg material will start to accumulate inside of the hen's body. The accumulated egg material presents two significant problems:
- Eventually, all the rotting egg material inside of the hen's body will begin to exert pressure on her internal organs, as well as restricting her ability to breath properly. This is why hen's often assume a penguin-like stance, as all the built up internal pressure in her body makes her extremely uncomfortable, and often restricts her ability to breath normally.
- Because egg yolk material is a rich medium for bacteria to grow, it increases the hen's risk of infection, leading to a life threatening condition called Egg yolk peritonitis.