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ABSTRACT
The results of recent electron microscope (EM)-level reconstructions of the anterior end of the larval nerve cord in Branchiostoma floridae are summarized. The anterior cord has three main regions: the anterior and posterior parts of the cerebral vesicle, and the primary motor center. Landmarks include the frontal eye and balance organ in the anterior cerebral vesicle; the lamellar body, ventral commissure, and dorsal tectum in the posterior cerebral vesicle, and the giant cell complex in the primary motor center. These are arranged in a fashion that is sufficiently brain-like to suggest that the anterior cord of Branchiostoma and the vertebrate brain are at least partially homologous. Identifying antecedents of lancelet CNS landmarks in other invertebrates is more difficult, but there is some neuroanatomical evidence to support the idea that the chordate nerve cord is derived from the ciliary band system of an ancestral dipleurula-type larva, and considerable scope for further research, at both the microanatomical and molecular levels, to test this idea in detail.