The "Hatching Muscle" in the Chick

TH• mechanism of hatching is little understood, and few investigators have concerned themselves with it.It has generally been accepted that the egg-tooth on the dorsal tip of the bill is a part of the apparatus that ruptures the shell.About 130 years ago Yarell (Rosenstadt, 1912) described the tooth and suggested its function.The tooth is usually pressed against the inner surface of the first "pip" or opening in the shell, but what causes the strong thrust that breaks the shell?Since the chick's head and neck are flexed inside the shell, it is logical to presume that extension of the head or any of the cervical vertebrae could force the tooth against the shell.Keibel (1912) believed that M. complexus (Fig. 1) extended the head and thus pro-