One of the major achievements in neurobiology during this century and especially during the last 10 years has been to gain an understanding of the initial strategy that is used to assemble part of the central nervous system of Drosophila. The anterior-posterior and ventral-dorsal axes of the embryo are established and different cell types are generated by the formation of perpendicular concentration gradients of proteins that regulate gene expression, which initiate dynamically changing patterns of gene expression in different parts of the embryo, depending on the concentrations of gene regulators to which each nucleus is exposed. These Cartesian coordinates determine the initial developmental fate of each nucleus. Cells in the nervous system have both internal and external molecular addresses. The internal molecular address, which is the combinatorial set of gene regulators that is expressed by the cell, is determined by the position of the cell in the embryo, whereas external molecular addresses are cell membrane molecules that have positional information and probably are required to form synaptic circuits.
Nirenberg M. The Initial Strategy for Assembly of Part of the Central Nervous System. Arch Neurol. 2000;57(1):51-52. doi:10.1001/archneur.57.1.51