Prehatching motility and hatching behavior in the chick
Abstract In previous communications, the motility of the chick embryo up to 17 days of incubation had been characterized as spontaneous rhythmical uncoordinated random motility. On the other hand, the opening of the shell and hatching are accomplished by a sequence of vigorous back thrusts of the head and beak against the shell. The head movments are part of a coordinated action pattern which includes wriggling movements of the body which have a rotatory component, resulting in the rotation of the body along its axis; and the bracing of the shoulder and tarsal joints against the shell. We have found that similar coordinated movements including a rotatory component are instrumental in several other action patterns which are preparatory to hatching. They include: the lifting of the head out of the yolk sac; the tucking of the head under the right wing; the assumption of the typical hatching position; and pipping. Another preparatory step, namely the penetration of the membrane separating the embryo from the air space does not require coordinated movements. All these movements were observed through windows in the shell; they are described in detail. Coordinated movements appear to originate de novo around day 17. They do not seem to be related to the random movements which are characteristic of earlier embryos. The latter continue until the onset of hatching, but they are suspended temporarily while the coordinated movements are performed during tucking, pipping and hatching.