Introduction

Abstract

AbstractThis book aims to understand vertebrate brains from developmental and evolutionary aspects by focusing on the ontogenesis of the brain of the medaka, as a vertebrate model. In this book we also propose the hourglass model of brain development, a new general principle of the vertebrate brain morphogenesis. This introductory chapter explains the evolution of nervous systems, the medaka, and anatomical terms. The origin of nervous system is evolutionarily old and dates back to the Precambrian Era. There are two general types of the nervous systems in extant animals: the diffuse and centralized types. The diffuse nervous system is found in the Cnidaria/Ctenophora, whereas the centralized type is seen in more evolved animals, bilaterians. The bilaterian nervous system can be divided into two parts, the peripheral (PNS) and central (CNS) nervous systems, the latter of which is composed of the brain and spinal cord in vertebrates. Over one-half of living vertebrate species are fishes, and they exhibit a considerable diversity. Extant bony fishes evolutionarily arose from the jawed fishes, and comprise two subclasses: the lobed-finned and ray-finned fishes. The living ray-finned fishes comprise four groups, and among them the teleost fish is the branch that emerged most recently. The medaka is an acanthopterygian teleost, a modern teleost, and belongs to the family Adrianichthyidae within the order Beloniformes. Many features of the medaka have made this fish a useful vertebrate model. Finally, anatomical terms used in this book are explained.KeywordsAcanthopterygian fishAnatomical termsBony fishCentral nervous system (CNS)EvolutionExperimental animalMedakaNervous systemRay-finned fishTeleost