|Schematic of developing fetus with first, second and third arches labeled.|
|Floor of pharynx of human embryo about twenty-six days old.|
|Gray's||subject #13 65|
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In the development of vertebrate animals, the pharyngeal arches (also called branchial arches or gill arches in fish) develop during the fourth and fifth week in utero as a series of mesodermal outpouchings on the left and right sides of the developing pharynx. In fish, the branchial arches give rise to gills.
These grow and join in the ventral midline. The first arch, as the first to form, separates the mouth pit or stomodeum from the pericardium. By differential growth the neck elongates and new arches form, so the pharynx has six arches ultimately.
Each pharyngeal arch has a cartilaginous bar, a muscle component which differentiates from the cartilagenous tissue, an artery, and a cranial nerve. Each of these is surrounded by Mesenchyme. Arches do not develop simultaneously, but instead possess a "staggered" development.
Pharyngeal or branchial pouches form on the endodermal side between the arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches. 
The pouches line up with the clefts, and these thin segments become gills in fish.
There are six pharyngeal arches, but in humans the fifth arch only exists transiently during embryologic growth and development. Since no human structures result from the fifth arch, the arches in humans are I, II, III, IV, and VI. 
Use in staging
The development of the pharyngeal arches provide a useful morphological landmark with which to establish the precise stage of embryonic development. Their formation and development corresponds to Carnegie stages 10 to 16 in mammals, and Hamburger-Hamilton stages 14 to 28 in the chicken.
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