Fig. 1

Phylogeny and ontogeny of the CNS. a Phylogenetic relationships among main deuterostome clades, as cited in the text. b Ontogeny of the CNS, including the brain, in vertebrates. The origin of the different elements of the CSF system has been indicated. Similarities between deuterostome clades and CNS development in vertebrates have also been indicated. 1 Adult starfish possess a neural plate-like nervous system similar to one from early vertebrate embryos, before the formation of the neural tube; 2 Adult Hemichordata possess a hollow dorsal nerve chord similar to one from vertebrate embryos before the closure of the anterior neuropore; 3 Urochordata and cephalochordata larvae possess a hollow dorsal nerve chord anteriorly open similar to one from vertebrate embryos before the closure of the anterior neuropore; 4 Urochordata and cephalochordata adults possess a closed dorsal nerve chord similar to one from vertebrate embryos after the closure of the anterior neuropore; 5 In vertebrates, the neural tube becomes a physiologically sealed system from a very early developmental stage and the site of the embryonic transfer system is depicted