ABSTRACT

The evolutionary history, or phylogeny, of modern-day vertebrates can be traced back to early, ancestral forms of some 500 million years ago. The time-course of phylogeny is compressed many millionfold during the embryonic development, or ontogeny, of a modern vertebrate. Diverse evolutionary paths from ancestral protochordates have led to the vertebrate classes of fishes, amphibians, reptiles, birds and mammals. In order to achieve a rapid conduction, the axons of vertebrate neurons may be sheathed in myelin, a fatty substance which acts as an electrical insulator. The neurotransmitter at the neuromuscular junctions of vertebrate skeletal muscle is acetylcholine. A vertebrate nervous system of the very simplest type may be seen in the early embryos of advanced vertebrates and in adult, primitive chordates. Since the spinal cord retains many of the features of the simple neural tube, it is structurally rather similar in the different vertebrate classes.