The development of digital joints of chick embryo paralyzed with dacamethonium bromide and of joints in chick embryo limb buds cultured on chorioallantoic membranes was studied. The digital joints of 45 paralyzed and 40 control embryos and the joints of 37 grafts and 30 control legs were examined histologically. Six preselected grafts were studied by electron microscope. Our results demonstrate that paralysis does not completely inhibit the early stages of joint clefting. In many paralyzed embryos, small clefts appeared in the periphery and occasionally in the central part of the interzone. Unlike in normal embryos, they rapidly disappeared, inducing fusions across the joints. Early, 2-day paralysis during a period of normal clefting inhibited irreversibly the development of the articular cavity that failed to appear after a cessation of the drug. Paralysis initiated in older embryos caused rapid regression and partial fibrous ankylosis of previously fully differentiated joints. Grafts from 4- to 6-day-old embryos usually failed to develop a full range of skeletal segments. Normal development of skeletal segments occurred in grafts from older donors. The articular cavity failed to develop in almost all grafts. Early cavitation accompanied by characteristic cell changes was found in a few joints. These cellular changes are suggestive of an intrisic mechanism of early cavitation, but joint motion is required for the full differentiation and maintenance of the joint cavity.