The segmental Bauplan of the rostral zone of the head in vertebrates.

Observations in a wide selection of lower vertebrate embryos have confirmed classical descriptions concerning segmentation of the early head mesoderm. The premandibular (PM) segment is seen as the most rostral representative of a continuous rostro-caudal series of condensations in the paraxial mesoderm, luminisations within which are secondary and variable in occurrence and form. The premandibular condensations are typically in continuity across the midline; the confluence, which comes to lie behind Rathke's pouch, marks the site of first mesoderm formation behind the oral membrane. The underlying consistency of pre-otic segmental pattern throughout the vertebrates is frequently obscured by superimposed variation in morphological detail between species, reflecting the dynamic nature of the morphogenetic tissue processes. Luminisation is one such morphogenetic epiphenomenon. Variations in it account for the terminal (Platt's) vesicle and "proboscis pores": such structures cannot therefore be safely used to infer evolutionary homologies. Many previous difficulties facing segmentation theory are explained as the result of failure to take account of the dynamic nature of the responsible morphogenetic events.