Early Development of Oral, Olfactory and Adenohypophyseal Structures of Agnathans and Its Evolutionary Implications

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Abstract

Ideas concerning the earliest evolution of the vertebrate head must be based on paleontological evidence as well as on adult and embryonic morphology of present day agnathans. Unfortunately, paleontological evidence for evolution of certain soft structures of the head is not really available, so that embryonic development of such structures is often the basis for speculation in this field (e.g. Stensiö, 1968). Here, we would like to address one phase of this problem; the formation of the agnathan adenohypophysis and some contiguous structures from the nasopharyngeal canal, and the presumed homology of the nasopharyngeal canal with the adenohypophyseal anlage, or Rathke’s pouch, of higher vertebrates.