ABSTRACT
Introduction Although the biology of hagshes overall remains highly controversial, authors past and present unanimously agree on the crucial role of hag-sh in elucidating the origin and early evolution of vertebrates. Still, an ongoing quest since the late nineteenth century has produced surprisingly little information on one of the most anticipated mysteries about these archaic-looking sh: prenatal development. Hagsh embryos are notoriously difcult to obtain. Consequently, hagsh embryology throughout the twentieth century relied on a small sample of specimens
Introduction ...................................................................................................... 95 Hagsh: A vertebrate that wasn’t?............................................................ 96 Vertebrate origins and hagsh ................................................................ 102
Development of hagsh ................................................................................ 104 Fertilization ................................................................................................ 104 Early development: Zygote to neurula .................................................. 105 Late development: Pharyngula to hatchiling ........................................ 107
Evolutionary implications of hagsh development.................................. 109 Neural crest ................................................................................................ 109 Neurogenic placodes and lateral line ..................................................... 112 Vertebrae ......................................................................................................116 Craniofacial patterning and the skull ......................................................118
Concluding remarks .......................................................................................119 Acknowledgments ......................................................................................... 121 References ....................................................................................................... 121
occasionally collected from the eld. As multiple projects converged on the same-and suboptimally prepared-materials, erroneous, anecdotal, or incomplete observations reinforced dubious interpretations, which stood uncontested for a long time (e.g., neural crest origins in an epithelial outpocket: Conel, 1942; endodermal origins for the nasohypophyseal canal: Gorbman, 1983).
