In Amphioxus, Serial Block-Face Scanning Electron Microscopy Reveals the Absence of Hatschek's Right and Left...

AbstractHatschek published the first comprehensive description of amphioxus development in the late nineteenth century. For him, a key event in early embryology was the evagination of the anterior end of the pharynx to form a right diverticulum and a left diverticulum-precursors, respectively, of the rostral coelom and preoral organ. Here we reexamine Hatschek's proposed diverticula with serial block-face scanning electron microscopy, a technique for generating fine-structural models of tissues in three dimensions. We find that no such diverticula ever form in the embryo. Instead, the anterior tip of the gut transforms into a mass of irregularly organized cells, the source of the peritoneal lining of the rostral coelom. Moreover, a cluster of cells associated with the first left segment is the likely source of the preoral organ. The discussion considers how the absence of Hatschek's gut diverticula impacts previously suggested homologies relating deuterostome head cavities.