CEPHALIZATION, the tendency exhib ited in many different phyla of animals toward the specialization of the region about the mouth into a distinctive head, and the concentration there of nervous and sensory organs. In the chordates we find a clear development starting from the undifferentiated oral region of Am Phiaxus without any very special differentiation of its central nervous system and sense organs of the most generalized sort. The cyclostomes and fishes form the next stage, in which there is a definite, though small, brain, and the more important of the special senses 6hose of smell, sight, hearing, equilibrium) are centralized in the head, which acquires a special skeleton, the skull. We finally come to the terrestrial ver tebrates, in which all the special senses be come located in the head, the lateral-line sense and dermal chemical sense being lost. The in
creasing size and complexity of the brain is one of the outstanding features of vertebrate evolution. In the arthropods we find a regular graduation from the equal segments of the chaetopod worms, which are probably not far removed from the primitive arthropod, to the well-defined head of the crayfish or insect, with its specialized eyes, antenna, jaws and ganglia. The tendency toward cephalization is associ ated with the fact that the head is often the first part to take shape in the embryo; however, this fact, as exemplified by the trochosphere larva of the annelids, may indicate that the head of certain segmented forms is the true representative of the entire body of their an cestors, and that the formation of the seg mented body may be the reminiscence of an ancestral multiplication by budding.