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.