Fig. 4.

Patterning and evolution of the peripheral nerves in chordates. (A) Peripheral nerves of the amphioxus “head.” As indicated by arrows, rostral peripheral nerves are likely to be patterned segmentally by the presence of somites, as spinal nerves are in vertebrates. This does not mean that amphioxus nerves are identical to vertebrate spinal nerves. Redrawn from Hatschek (1892). (B) Developmental logics responsible for the spatial distribution patterns of crest cells (peripheral nerve primordia) in vertebrate embryos. In all the vertebrate embryos examined, the cephalic crest cell streams adhere to the even-numbered rhombomeres (r2, r4, r6, and posterior) of the hindbrain, whereas in the trunk, crest cell streams are subdivided into a somitomeric pattern by the presence of somites (s). Abbreviations: mb, midbrain; ot, otic vesicle; sc, spinal cord; sp, spinal nerve primordia; V, VII + VIII, IX, and X, cranial nerve primordia. Modified from Kuratani (2004).