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Fig. 1. Elements of cranial organization at the core of the `New Head
Hypothesis'. The New Head Hypothesis (NHH) of Carl Gans and Glenn Nothcutt
(Gans and Northcutt, 1983), as
schematized in mouse (A-D) and human (E) embryos. (A-D) Schematics of
embryonic day 9.5 (E9.5) mouse embryos, highlighting specific major tissue
components of the NHH. The NHH posits that vertebrates exhibit unique
characteristics, such as (A) an expanded alar plate of the forebrain; (B)
formation of neurogenic sensory placodes from the surface cephalic ectoderm;
(C) a migratory mesenchymal population of cranial neural crest cells that
invade the branchial arches (BA) and surround the expanded forebrain and
rostral primary sensory placodes; and (D) an expanded skeletal system to
support the new rostrally expanded forebrain, rostral sensory capsules and
BAs. (E) The hypothesized shift from a passive suction feeding
protovertebrate to a predatory vertebrate with jaws made from modified BA
elements necessitated a high degree of craniogenic developmental and
functional integration. Abbreviations: BA, branchial arches; di, diencephalon;
hb, hindbrain; mb, midbrain; olf, olfactory; op, optic; ot, otic; tel,
telencephalon. (A-D) Modified, with permission, from Depew and Olsson
(Depew and Olsson, 2008).
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