Development of head and trunk mesoderm in the dogfish, Scyliorhinus torazame: I. Embryology and morphology of the head cavities and related structures.

摘要: Vertebrate head segmentation has attracted the attention of comparative and evolutionary morphologists for centuries, given its importance understanding developmental body plan vertebrates origin. In particular, mesoderm is central to problem. The shark embryo provided a canonical morphological scheme head, with epithelialized coelomic cavities (head cavities), which have often been regarded as somites. To understand significance cavities, embryonic development was investigated at histological levels in shark, Scyliorhinus torazame. Unlike somites some enterocoelic mesodermal components other vertebrates, S. torazame appeared irregular cyst(s) originally unsegmented mesenchymal mesoderm, not via an undivided coelom. mandibular cavity first paraxial part followed by hyoid cavity, premandibular last form. prechordal plate recognized rhomboid roof preoral gut, continuous rostral notochord, divided anteroposteriorly into two parts growth hypothalamic primordium. Of those, posterior likely differentiate anterior disappeared later. trunk exhibited significant differences, terms appearance timing differentiation. developed process secondarily; homology reported elasmobranch embryos discussed.