Branchiomerism and the theory of head segmentation

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Abstract
The summary of this paper is as follows: A critical review of the developmental evidence shows that the branchial pouches are formed in cephalocaudal sequence subsequently to the segmentation of the dorsal mesoderm. The pouches interrupt a continuous sheet of mesoderm to form the branchial arches. The arches when formed do not correspond topographically to the dorsal somites. Branchiomerism does not therefore coincide with somitic metamerism. The branchial structures do not support the theory of head segmentation. The nervi trigeminus, facialis, glossopharyngeus, and vagus cannot be regarded as segmental nerves. There is no evidence that branchial pouches or arches have been elided from the series. The problem of meristic homology is briefly discussed.