THE pip, or first break in the eggshell by the bird, is the initial phase of hatching. It furnishes the bird with a weakened point at which to begin cutting the shell and complete the process. The typical pip on a chicken egg is a flatly conical area about one-third of the distance from the large end, near the widest portion of the egg. Approximately 0.2 cm in height and 1 cm in diameter, it is proscribed by cracks in the shell, and has four or five additional cracks radiating from the center. The cracks are parted to varying degrees, depending upon the force from inside the shell that caused the pip. Sometime after pipping the chick begins to rotate within the egg and, starting at the pip, to jab with the egg tooth and beak tip and cut away the shell and shell membrane almost completely around the egg. It then pushes off the loosened cap thus formed at the large end of the egg and tumbles out of the shell, terminating hatching. The Musculus complexus or hatching is a paired structure located on the anterodorsal portion of the neck in birds, originating on several anterior cervical vertebrae and inserting on the occipital crest at the rear of the skull (Fisher, 1958; Bock and Hikida, 1968). In chickens this muscle, as well as the subcutaneous area around it, starts to swell with lymph a few days prior to until it reaches its peak size at the time of pipping. It then loses the fluid in a few days and assumes the appearance of normal muscle rather than being clear or yellowish and bulbous. In the adult bird it functions to raise or elevate the head. The M. complexus is probably present in enlarged form at in most birds, having been described in grebes (Fisher, 1961), ducks (Keibel, 1912; Fisher, 1966), the coot, Fulica americana (J. Fisher, 1962), Franklin's Gull, Larus pipixcan (H. Fisher, 19;62), the Bobwhite, Colinus virginianus (Johnson, 1969), chicken (Fisher, 1958, and many others), and possibly in the House Sparrow, Passer domesticus, and Red-winged Blackbird, Agelaius phoeniceus (Fisher, 1958). As the peak in fluid content of the M. complexus is concurrent with pipping, it is difficult to discount the idea that it must have some role here. Keibel (1912) was apparently the first to attribute the function of breaking the eggshell to the M. complexus, and he felt that the pip was due to a contraction of the muscle, lifting and extending the beak to force the egg tooth through the shell. Pohlman (1919: 103) believed that the muscle could not contract because of its turgid condition and that