Observations of hatching movements in the chick, Gallus domesticus.

Made a film analysis of the hatching movements of White Leghorn chicks placed in transparent glass "eggshells." During every hatching burst, each of the limbs first extended, then flexed beyond the original resting point, and finally extended again to the original resting point (as seen from the body). This suggests that all 4 limbs helped to hold the body of the embryo tightly in place inside the egg during a cracking phase, enabling the beak to perform the actual cracking of the shell. Subsequently during a turning phase, the limbs contributed to rotation of the embryo in the eggshell, bringing the animal in the right position for further breaking the shell during the next burst. The effective forces against the shell, leading to rotation of the animal in the egg, were probably performed by (a) the right leg during its flexion, (b) the right wing during its flexion, and (c) the left wing during its re-extension. (17 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)