Previous work shows that folding a posthatching chick into the hatching position results in the re-initiation of hatching. Furthermore, bending the neck to the right or left serves as a selective signal for turning on hatching behavior. The present study addresses the issue of whether sensory rcccptors located in the neck provide this signal. Three groups of chicks were folded into the hatching position and p l d in glass eggs. In the experimental group, sensory input from the neck was eliminated w i t h a local anesthetic, lidocaine. In these chicks, hatching was initiated only after a long latency, correlated with the time at which the anesthetic wore off. In the two control groups, in which saline was iqjected into the neck or lidocaine was iqjected into the thigh, the latency was much shorter. Therefore sensory receptors located in the neck appear to provide input that serves as a selective signal for initiating hatching.
Chick embryos face a problem common to all egg-laying animals: they must escape from the shell at the end of the incubation period (for review, see . To accomplish this, the initiation of hatching is very precisely controlled. Hatching must occur at the end of the 20-21-day incubation period, when the chick is physiologically and behaviorally prepared for survival in the external environment, but has not yet exhausted its yolk supply. Furthermore, hatching must not occur before the yolk sac is withdrawn into the abdomen and the extraembryonic circulation is clamped off. In addition, once the chick has escaped from the egg, the hatching behavior must be turned off.
Hatching behavior has been described extensively