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The density and proportion of synaptic contacts in the primate motor cortex (Brodmann area 4) were determined in 21 rhesus monkeys ranging in age from embryonic day 41 (E41) to 20 years. Two to 4 vertical electron microscopic probes, each consisting of 150–250 overlapping micrographs traversing the thickness of the cortex, were prepared for each specimen. Synapses were categorized according to their morphology (symmetrical or asymmetrical), cellular location (on spines, shafts or soma), number, and ratio of laminar distribution. The density of synapses was expressed per unit area and volume of neuropil (excluding neuronal and glia cell bodies, myelin sheath, blood vessels and extracellular space). The first synapse in the area of the emerging motor cortex were observed at E53 in the marginal zone (prospective layer I) and in the transient subplate zone situated beneath the developing cortical plate …