Experimental investigation of morphogenesis in the growing antler.

D ' A R C Y THOMPSON (1942) described the form of antlers as having developed in a two-dimensional pattern which, during growth, may have become more or less distorted depending on the species. In some (e.g. moose, fallow deer), the antlers may exhibit a palmate configuration; in most deer, however, they are branched structures formed by the repeated two-dimensional bifurcation of the original outgrowth. This process gives rise to a series of tines which vary in number according to age and species. In the sika deer, the first set of antlers, produced in yearlings, are unbranched spikes which may grow as much as 6 inches in length. The following year, these are replaced by branched antlers usually having three points each. Mature bucks ordinarily possess 4 points per antler. The annual growth cycle of antlers has been thoroughly documented by Waldo & Wislocki (1951), and Wislocki (1956) in the Virginia deer. In the sika deer it is briefly as follows. Shedding of the old antlers occurs in the spring, having been preceded by the local swelling of the skin of the pedicle immediately below the base of the antler. There is reason to believe that the incipient growth of the new antler is somehow instrumental in bringing about the loss of the old. Following autotomy, a scab forms on the stump of the pedicle, and wound healing ensues by the migration of epidermis and subjacent connective tissue from the margins. It is from this undifferentiated tissue derived from the dermis of the pedicle skin that the new antler bud appears to develop. Subsequent invasion and proliferation of such cells results in the formation, within a few weeks, of a rounded knob growing on top of the pedicle. This structure branches dichotomously as it grows throughout the spring and summer, attaining its full size by late August. Growth occurs almost exclusively at the apical end of each branch, where there exists a mass of undifferentiated cells resembling the blastema of regenerating structures in lower vertebrates. In older parts of the growing antler, located progressively more proximal, are zones of chondrification