The embryonic nervous system is refined over the course of development as a result of two main processes: apoptosis (programmed cell death) and selective axon pruning. We simulated a large scale spiking neural network characterized by an initial apoptotic phase, driven by an excessive firing rate, followed by the onset of spike-timing-dependent plastiticity (STDP), driven by spatiotemporal patterns of stimulation. In the apoptotic phase the cell death affected the inhibitory more than the excitatory units. The network activity stabilized such that recurrent preferred firing sequences appeared along the STDP phase, thus suggesting the emergence of cell assemblies from large randomly connected networks.