Glossary

branchial arches The third and several additional visceral arches (usually five in all), which in fishes are in the region of the gills.

epimere The dorsal portion of the mesodermal layer in the developing body wall and head; also called paraxial mesoderm. It forms a segmental, rostrocaudal series of mesodermal masses called somites in the body and the caudal part of the developing head. Further rostrally in the head, it forms incompletely divided, segmental masses called somitomeres.

hypomere The ventral portion of the mesodermal layer in the developing body wall; also called lateral plate mesoderm. It gives rise to the smooth muscle of the gut and to the cardiac muscle of the heart.

mesomere The middle portion of the mesodermal layer in the developing body wall; also called the nephric ridge. It gives rise to the kidneys and gonads.

neural crest Ectodermally derived cells initially located at the lateral edge of the invaginating neural tube that subsequently migrate and contribute to numerous parts of the developing nervous system and body, including some of the bipolar neurons that lie in the ganglia of most sensory cranial nerves, all of the bipolar neurons of the sensory spinal nerve ganglia, the postganglionic neurons of the autonomic nervous system, most of the cranium, and the visceral arches.

neurogenic placodes Thickened regions of the epidermis that give rise to many of the bipolar neurons that lie in the ganglia of most of the sensory cranial nerves. Placodes occur only in the developing head region.

somites The segmental, mesodermal masses of the developing body wall and caudal head that are derived from epimere. In the body, somites give rise to the striated skeletal muscle of the body wall and limbs. In the caudal head, the somites give rise to most of the branchial arch muscles of the palate, pharynx, and larynx (innervated by cranial nerve X) and to the hypobranchial muscles of the tongue (innervated by cranial nerve XII).

somitomeres The incompletely divided, mesodermal masses of the developing head that are derived from epimere. They give rise to the striated muscles of the eyes (innervated by cranial nerves III, IV, and VI) and of the first three visceral arches (innervated by cranial nerves V, VII, and IX).

visceral arches A series of skeletal arches that occur in the region of the jaw and throat (pharynx) in mammals; they include the mandibular arch that forms the jaw, the hyoid arch, and the branchial arches. Most of the tissues of these arches are in fact somatic rather than visceral in developmental origin and function.

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