![]() ![]() The expression of active Dpp/Bmp on this side is prevented by a second key element in the conserved signalling system, the Sog/Chordin protein, which antagonizes the action of the Dpp/Bmp morphogen. ![]() A key element in this signalling system is the Dpp/Bmp protein, a member of the TGFβ gene family, which is thought to exert an antineurogenic effect that represses neural identity and limits the extent of the neuroectoderm to one side of the embryo. In the following, I consider four key aspects of brain development as well as the underlying molecular genetic control elements that appear to be conserved.Īt the earliest stage of brain development, a conserved molecular signalling system polarizes the embryonic ectoderm into a neuroectoderm, from which the central nervous system (CNS) develops, and into a ‘non-neural’ ectoderm ( De Robertis 2008). Indeed, comparable developmental genetic mechanisms appear to operate in invertebrates and vertebrates during all major stages of brain development from the early specification of the neuroectoderm to the formation of neural circuitry (for recent reviews see Lichtneckert & Reichert 2005 Arendt et al. Brusca & Brusca 1990).Ĭontrasting with this notion, and in support of earlier ideas (reviewed by Arendt & Nübler-Jung 1994), are more recent neurogenetic analyses carried out in several vertebrate and invertebrate model systems that reveal striking similarities in the expression and action of regulatory genes that control brain development. Based on these differences, a number of investigators proposed an independent evolutionary origin for the nervous systems of vertebrate and invertebrate animals (gastroneuralia–notoneuralia concept e.g. By contrast, when the nervous systems of vertebrates were compared with those of invertebrates, apparent differences in neuroanatomy were found in both adult structure and embryonic morphogenesis. The subsequent successful application of both theories to the analysis of vertebrate brains uncovered fundamental similarities in the neural organization of different vertebrate brains, arguing for a common evolutionary origin of major brain elements in all vertebrates. These unifying theories were the cell theory of Schleiden and Schwann, which states that all living organisms are composed of and derived from cells, and the evolutionary theory of Darwin, which explains the origin of organismal complexity and diversity through the action of natural selection ( Schwann 1839 Darwin 1859). ![]() Moreover, this edition discusses recent work in molecular systematics, and there is a large new section on Kingdom Protista (replacing Protozoa ) containing new contemporary views of these organisms (arranged in 17 phyla).Despite the extensive history of investigations on the brain, which reaches back over two millenia, fundamental insight into the nature and origin of the brain remained elusive until the nineteenth century when two central theories of the biological sciences were first proposed. Other key changes from the 1st edition (1990) include: the incorporation of all the new developments in phylogenetics, developmental biology and molecular genetics major changes at the highest levels among the invertebrates three phyla that appeared in the original book Pentastomida, Pogonophora and Vestimentifera no longer exist, and an entire new phylum, Cycliophora, has been erected. The text is accompanied by detailed line drawings and - new to this edition - four-colour photographs. Detailed classifications, phylogenetic trees, and references for all phyla are provided. It is organized around the themes of bauplans (body plans) and evolution (phylogenetics). Invertebrates 2nd edition presents a modern survey of the 34 animal phyla (plus the Protista) and serves as both a college course text and a reference on invertebrate biology. ![]()
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