Genesis and development of the upper extremity of the chick embryo between the second and the fourth day of incubation. The pictures were taken in the opened egg. In an accelerated motion sequence the foldings are shown which occur with the appearance of the head-fold, the formation of the amnion and the folding off of the embryo from the yolk. This illustrates clearly the emergence of the upper extremity as a fold. Then the morphogenetic movements are shown, as they occur in the course of the forming upper arm, fore arm and hand. In addition to these morphogenetic movements, the development of the early spontaneous motoric activities are demonstrated in normal motion.