A survey of occurrence of about seventeen circumventricular organs in brains of various vertebrates with special reference to lower groups.

The occurrence of about 17 circumventricular organs is described in 31 species belonging to various groups of vertebrates from cyclostomes to mammals and the phylogeny of each organ is discussed briefly. The neurohypophysis (median eminence and neural lobe) and the subcommissural organ are observed in all species studied. The pineal is also ubiquitous except for the hagfish (Eptatretus) and the caiman (Caiman). These four circumventricular organs are phylogenetically the oldest organs. The subfornical organ and the area postrema may be phylogenetically the youngest organs, although the subfornical organ is found in the lungfish (Lepidosiren) and the area postrema is observed in the dogfish (Mustelus) and the brachiopterygians (Polypterus and Erpetoichthys) among the piscine vertebrates studied. The other significant findings may be summarized as follows. Osteoglossomorphous teleosts (Osteoglossum and Gnathonemus) possess a distinct median eminence. The median eminence of the caiman (Caiman) is divided into the anterior and posterior parts as in birds. The saccus vasculosus is observed in the freshwater ray (Potamotrygon), but not in the freshwater teleost (Osteoglossum). The organum vasculosum laminae terminalis of the dogfish (Etmopterus) contains fuchsinophilic neurosecretory fiber terminals and is considered as the neurohemal organ. The paraventricular organ is not differentiated in the lungfish (Lepidosiren) and the caecilian (Typhlonectes). The paraphysis is found in the dogfish (Mustelus) and the osteoglossomorphous teleosts (Osteoglossum and Gnathonemus). The so-called paraphysis of the lungfish is not a paraphysis, but a dorsal sac. The development of the subfornical organ is variable among amphibians. The area postrema is well developed in the caecilian (Typhlonectes).