In the Vertebrata the great majority of cells producing hormonal peptides belong to the APUD series and share its distinctive cytochemical and ultrastructural characteristics. According to the concept all members of the series are to be regarded as derivatives of neuroectoderm or of specialized (placodal) ectoderm. For most of the APUD cells this criterion is fulfilled in that their origin from neural tube, neural ridges or neural crest can be considered proven. Complete proof is not yet available for the APUD cells of the gastrointestinal tract and pancreas, and indeed much contrary evidence can be cited. Despite the latter, our embryological studies show: (1) that the hypothalamohypophyseal complex is wholly neuroectodermal; (2)that the chronology of neural crest dispersion is such that this tissue could be responsible for observed APUD cell contributions to the foregut; (3) that placodal ectoderm makes important contributions to pharyngeal pouch endocrine derivatives in birds and mammals; and (4) that the amphibian parathyroid gland is derived from the same layer of neural ectoderm as the hypothalamo-hypophyseal axis. Supporting immunocytochemical studies indicate that peptides belonging to the APUD series are more widely distributed than hitherto recognized and it is concluded: (1) that the whole of peptide endocrinology is neuroendocrinology; and (2) that the APUD cells, with a few cells hitherto regarded as being outside the series, form a third (Endocrine) division of the nervous system to add to the existing Somatic and Autonomic divisions.
