Branchiomerism and the theory of head segmentation

The summary of this paper is as follows:1 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. 2 The pouches interrupt a continuous sheet of mesoderm to form the branchial arches. 3 The arches when formed do not correspond topographically to the dorsal somites. 4 Branchiomerism does not therefore coincide with somitic metamerism. 3 The branchial structures do not support the theory of head segmentation. 3 The nervi trigeminus, facialis, glossopharyngeus, and vagus cannot be regarded as segmental nerves. 3 There is no evidence that branchial pouches or arches have been elided from the series. 3 The problem of meristic homology is briefly discussed.