The ontogeny of the notochord of Branchiostoma lanceolatum

Anatomical details of the ontogeny of the notochord of the european lancelet (ˆ amphioxus), Branchiostoma lanceolatum, are described, based on complete series of alternating thin and semithin sections for light and transmission electron microscopy of early neurula and larval stages, as well as observation of living specimens. The notochord is formed by the outgrowth of dorsal medial cells of the archenteron. A gradient along the anterioposterior axis is discerned, the more anterior part of the notochord being further developed. The differentiation of the central notochordal cells to muscle plates with a spacious intracellular vacuole is demonstrated. The arrangement of these cells as a `stack of coins' is established early during ontogeny and is shown to be a plesiomorphy within the Chordata. These cells are capable of contraction in one gill slit larvae. The existence of dorsal and ventral MuÈ ller cells is proven. These cells are interconnected by desmosomes, form extracellular spaces between them, and show signs of high transcriptional activity.