In the period from 14 to 150 days, the zygomatic arch of the rat skull possesses a marked positional stability in relation to the parietal bone, the basisphenoid bone and the length axis of the skull. These conclusions have been drawn from a study of 27 male rats, X-rayed at regular intervals between 14 and 150 days. From the X-ray photos, quantitative expressions of angular values between the mentioned structures have been obtained. The significance of the observations is discussed. It is emphasized that the demonstrated stability of these structural components of the rat skull during growth is in marked contrast to the extent of movements which take place between other parts of the skull during orthocephalization.
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