Fig. 2

Notochords in phylogeny: invertebrate chordates, annelids, and nemerteans. a Midsagittal section of early neurula of amphioxus with blastopore (bp) and notochord rudiment (nr, horizontal hatching). b Cross section at level indicated by dotted line in a; notochord rudiment (nr) and somites indicated, respectively, by horizontal and diagonal hatching. c Cross section of later neurula of amphioxus showing notochord (no) and dorsal nerve cord (nc); somites indicated by diagonal hatching. d, e Early and late larvae, respectively, of an ascidian tunicate. Cross sections through the tail showing nerve cord (nc); muscles (diagonally hatched) and notochord (no); asterisk indicates endodermal strand (discovered by Seeliger [114]). f, g Inverted annelid scenario (after Semper [48]); following inversion (f looped arrow), fibers (fi) associated with the nerve cord (nc) are precursors of the notochord (no) in g. h, i Variant annelid theory (after Ehlers [50]); annelid after inversion (h, looped arrow), the position of the siphon (si) corresponds to the vertebrate notochord (no) in i. j–n Nemertean scenario (after Hubrecht [54]); gastrula (j) has a first invagination (arrowhead) for gut and a second invagination (arrow) for the proboscis, while mesenchyme cells (mc) ingress into the blastocoel. Subsequently k, a through gut forms from mouth (m) to anus (a), and mesenchyme cells condense around the proboscis (pr). l Schizocoely produces a proboscis coelom (prc) and a proboscis sheath (prs). The arrow in m indicates the proboscis (pr) pulling out of the proboscis coelom (prc), leaving behind a few mesenchyme cells. In n, the mesenchyme cells in the proboscis coelom have extensively proliferated to form the notochord (no); the remains of the proboscis have become the anterior pituitary (ap), while the dorsal nerve cord (nc) has formed by the dorsal migration and fusion of the lateral nerve cords