In Sd/ + heterozygotes, tlie bodies of all the vertebrae are reduced. This includes the dens epistrophei -which is absent as a bony element though it is represented by a ligament. In the absence of this pivot for the turning movement of the head, an atypical but functional horseshoe-shaped articulation is established between atlas and epistropheus. Muscles normally inserting in the region of the atypical joint are shifted to a different position. The reduction of the vertebral bodies is due to a degeneration of the notochord which takes place when the vertebrae of the trunk region are still membranous. An hypothesis is put forward according to which the degeneration of the notochord is responsible not only for the reduction of the vertebral bodies, but also for the degeneration which leads to the loss of parts of the tail; this hypothesis requires the assumption that the notochord in these animals is physiologically defective about a day earlier than has so far been established by histological methods. The developmental connexion between urogenital anomalies and the rest of the syndrome remains obscure.
The research reported in this paper was supported by grants from the Medical Research Council and from the Rockefeller Foundation which are gratefully acknowledged. For the drawings of the plate, my thanks are due to Miss Pauhne Tarry.