The concept of field homology has recently been reformulated under the modern perspective of evolutionary developmental biology (Puelles and Medina, 2002), based on the existence of mor-phogenetic fields as major units of development and evolution, linking the genotype to the phenotype, which are considered as the natural comparison characters for homology considerations. Morphogenetic fields are self-regulated entities, having specific properties (such as expression of specific combinations of developmental regulatory genes), and specific interactions with other fields, which change dynamically during development. They exist in specific topological locations within the organism and give rise to specific sets or lineages of cells. In the brain, morphogenetic fields are represented by segments, and by divisions and subdivisions within them (different hierarchies). Based on the predominant radial glial-guided migration of neurons during development, the majority of their cellular derivatives stay within single radial histogenetic divisions, which therefore constitute natural comparison units for field homology considerations in the adult brain. However, homology of a specific morphogenetic field in two animals does not necessarily mean complete homology of the cell groups in the derived histogenetic divisions, since some evolutionary divergence may have occurred, involving the appearance of novel, nonhomologous cell groups in one of the animals, or in both (which may be native cells of the same field or immigrant cells from other fields).
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