The eggs of the birds acquire the typical oval shape during the laying. In its first phase, the egg, which is spherical and has a malleable shell, advances through the oviduct – the channel through which the gamete or ovule runs – by means of an undulatory movement known as peristaltic, which is directed by a battery of muscles with ring shape surrounding the oviduct.
In order for the egg to advance, the posterior muscles contract and press on the back of the egg , while the front muscles relax. This movement, carried out alternately and synchronously, makes the egg first acquire a conical shape and later an oval shape, when the shell solidifies.
