RESUMEN
In the study factors which influence the growth of any tissue, a distinction must be made between determination, the decision as to which of the inherent genetic potentialities a given tissue will realize, and differentiation, the realization of expression of a specific potentiality. In the two tissue systems that are involved in the morphogenesis of the skull namely, synchondroses and syndesmoses, mitosis seems to have an essential place in between the act of determination and the process of differentiation. Mitosis is the first visible sign that a determination has occurred, and it precedes the final differentiation which results in deposition of bone or hypertrophy of cartilage. We have been using tritiated thymidine autoradiography to label pre-mitotic cells in the periodontal ligament (a syndesmosis) and in the spheno-occipital synchondrosis in the rat. In the case of the synchondrosis, growth of the adjacent basal bones can be directly related to mitosis within the cartilage between 40 and 80 days of age by calculations comparable to those of Kember. However, between 1 and 40 days, there is more growth than can be accounted for on the basis of mitoses detected within the cartilage. On the other hand, if one calculates the portion of growth due to endochondral replacement alone, interstitial mitoses in the cartilage are found to be adequate. Perichondral apposition of cartilage cells need not be postulated. The remaining portion of the total growth of the bones is accounted for by apposition of bone from the periosteum (AU)