Phylogeny of Vertebrates As Seen from Brain Structure

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1. Serial sections of the brains of both adults and embryos of various vertebrates were studied histologically. Special attention was paid to the phylogeny and ontogeny of circumventricular organs. Ubiquitously occurring circumbentricular organs in the vertebrate brains are the subcommissural organ and neurohypophysis. The pineal is also found in most species. These fundamental circumventricular organs appear early in ontogeny in most vertebrates. The phylogenetically new organs such as the subfornical organ and area postrema appear late in ontogeny.2. In the course of study of the brain histology, it was revealed that the brain parenchyma of hynobiid salamanders (Hynobius, Pachypalaminus, and Onychodactulus) is not vascularized at all. This is unusual among vertebrate brains. In these salamanders, the ventricle is wide and the neurons are distributed in the periventricular grey. Considering these features together, it may be concluded that the brain of hynobiid salamanders represents the most primitive type of the vertebrate brain.3. The size of nuclei of the cerebellar granular cells was measured in various vertebrates from cyclostomes to mammals. In piscine vertebrates, the size decreases in the following order; Chondrichthyes, Chondrostei, Holostei, and Teleostei. In tetrapods, the size decreases in the following order; Urodela, Aunra, Reptilia, Mammalia, and Aves. The nuclear size represents the genome size. The small nuclear size of teleosts may be related to the evolution under r selection .The large nuclear size of urodeles my be related to the evolution under K selection. Meanwhile, the small nuclear size of birds may be related to their high metabolic activity.