As the embryo develops on the yolk, it causes that part of the yolk to become lighter and float upwards. If, after the
egg is moved, the embryo is downwards it will cause the yolk to rotate in the inner thin white until it is uppermost
again.
The yolk is held in position by a structure called the chalaza and by membranes. It still tends to float upwards,
deforming the inner membranes and eventually the yolk will touch the shell membranes. If this happens with the
embryo uppermost the embryo will stick to the shell and die. For this reason, turning is essential for survival of a
healthy chick.
As the egg is turned the embryo is swept into fresh nutrients, allowing the embryo to develop. This is critical for the
first week when the embryo has no circulation system. After the first week, eggs still need to be turned but not as
often. The turning regime is often different between species and altricial birds tend to need more frequent turning
than precocial birds. Whereas fowl are turned through 80 degrees every hour or so, parrots are often turned through
180 degrees many times an hour in the early stage.