In certain animals a structural condition occurs in which all or most of the paired parts or structures are repeated at regular intervals, along the antero-posterior axis of the body. The body of such animals is made up of a longitudinal series of elements, in each of which all or most of the systems of the body are represented either by entire paired organs or structures or by a portion of the median unpaired structures. Each such division of the body is called a metamere, somite or segment. In some cases, such as the earthworm, the anterior and posterior boundaries of each segment may be marked externally by a construction of the body wall, therefore, the animal is said to exhibit both external and internal metamerism. In others, however, there is no external metamerism, internal segmentation alone is present. Such animals in which various segments are nearly alike are said to display homonomous segmentation, but in the majority of segmented animals the various segments differ from each other in many respects, such animals are said to possess heteronomous segmentation. Homonomous segmentation is a primitive-condition in which the various segments are more or less independent of each other and each is capable of performing all of the necessary functions of life. In heteronomous condition the segments have become unlike and there exists a division of labour among them, some segments performing some functions and others other functions. It follows therefore that in the evolution of the segmented animals there has been a continuous progression from the homonomous to the extreme heteronomous condition.
The vertebrates display heteronomous segmentation that too is internal. There is no trace of external segmentation. The vertebrates show segmentation, most clearly in the embryonic development, but it is confined to parts of muscular, skeletal and nervous systems. In the embryonic development of vertebrates the change from a somewhat homonomous condition to an extreme heteronomy can be clearly followed.