We have analysed the development of the larval PNS of Drosophila, with the aim of understanding the genetic programme that underlies this development. 2. The achaete-scute gene complex (AS-C), which is required for the development of the adult PNS, is also necessary for the larval PNS. The analysis of different AS-C lesions shows that the larval PNS results from the superimposition of two independent subpatterns, each of which depends on one AS-C gene. 3. The analysis of the two subpatterns reveals hidden homologies between the very different arrangements of sense organs observed on different segments, suggesting that the initial pattern is the same in all segments and is later modified in the different segments. 4. The early arrangement of sensory mother cells can be visualised in a special transgenic line, A37. In this line the initial repetitive pattern inferred above can be directly observed. Furthermore this line makes it possible to decide whether a given mutation acts on the very early steps of the PNS development (determination) or at later stages (differentiation). 5. The line A37 has been used to show that mutations that reduce the PNS such as AS-C- or da- alter the very first steps of the process, while mutations which result in a hypertrophied PNS such as N seem to alter a subsequent step. We end up with an overview of the genetic operations that generate the arrangement of sense organs and sensory neurons.
