The ontogenesis of cortical circuitry is the subject of the investigation reported in this paper!. Strategie elements of cortical circuitry are synaptic contacts: sites of neuronal interaction. If we first determine the location of early formed synap ses, we may then seek the pre-and postsynaptic elements, that is, the structures which meet and interact at each synaptic junction. The dynamic aspects of cortical development cannot be studied directly: neuronal growth cannot be observed in an intact brain. The pattern of develop ment can be approached only by the analysis of static properties at multiple ages. We have selected the newborn dog as a starting point for these studies. Our observations have been restricted to a small portion of primary somesthetic cor tex. This region was selected because, in mature cortex, its input and physiologie properties are relatively weIl known. It is particularly advantageous to study a primary sensory projection area because its morphologie development can be di rectly related to its functional development. (A related investigation of the onto genesis of electrical activity in somesthetic cortex of the dog will be reported separately). A selective review of the literature is necessary in order to place this work in the context of a few relevant phenomena of cortical development.