Figure 3

Histology and fine structure of the embryo of Saccoglossus kowalevskii at 36 h pf. A - C, Electron micrographs. D Semithin sagittal sections. E-F Scanning electron micrographs. (A) The lumen of the archenteron (ar) is still continuous with the anterior primordal protocoel (ppc). (B) The apical cell processes of the primordal protocoelic cells (pcc) are goblet-shaped and their basal portion is resting on ecm (arrowheads) separating the protocoel from the ectoderm (ec). (C) The epithelial endodermal cells (edc) are filled with several vesicles (vs) apically. Inset: Each cilium is connected to the cytoplasm by an anchoring complex. (D) Sagittal sections of two specimens. At the onset of mesoderm formation, the endoderm (ed) shows two shallow constrictions (arrowheads) mirroring the embryo’s future tripartite body organization. The lower embryo is slightly older indicated by the completely separated protocoel (pc). The primordal mesoderm (pmd) starts to establish laterally, at the middle and posterior regions of the endoderm. The extracellular matrix (ecm) is indicated by the dotted line. (E-F) Sagittally dissected embryo showing the internal organization of the posterior end (note the position of the telotroch (tt)). Ectoderm and endoderm have close contact to each other. A third layer of cells, the primordal mesoderm, reaches between ecto- and endoderm. ac accessory centriole, bb basal body, ecc ectodermal cell, ci cilium, cr ciliary rootlet, mi mitochondrion, mv microvilli, nn nerve net, yo yolk, za zonula adherens.