it is important to note that the past studies of viktor hamburger and rita levi-montalcini in the chick embryo have been further proven by more recent studies which demonstrated that the elimination of neuronal cells by programmed cell death (pcd) is necessary for the correct assembly of the nervous system. overall, spontaneous embryonic activity has been shown to play a role in neuron and muscle development but is probably not involved in the initial formation of connections between spinal neurons.