. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 134 MORFHOLGY OF THE ORGAXS OF VERTEBRATES. increase is impossible. Increase in size is effected here by additions to the exterior, and in the case of the long bones, bodies of the vertebrae, etc., by the appearance of more than one centre of ossification in the cartilage. From these centres ossification in all directions, but for a time there remains a cartilaginous region between the ends (epiphyses) and the main portion in which increase in length is possible. Later these epiph} ses usually become so united or an
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 134 MORFHOLGY OF THE ORGAXS OF VERTEBRATES. increase is impossible. Increase in size is effected here by additions to the exterior, and in the case of the long bones, bodies of the vertebrae, etc., by the appearance of more than one centre of ossification in the cartilage. From these centres ossification in all directions, but for a time there remains a cartilaginous region between the ends (epiphyses) and the main portion in which increase in length is possible. Later these epiph} ses usually become so united or anchylosed to the main portion that the line of division cannot be traced. The skeleton may be divided into internal and dermal por- tions, and the internal in turn is composed of an axial portion, including the vertebral column, skull, ribs, and breast-bone; and an appendicular portion, consisting of the skeleton of the appendages and the girdles supporting them. The vertebral coliinm is developed around the notochord (p. 17). This, as will be remembered, is a rod-like structure of entodermal origin which lies between the alimentarv' tract and the central nervous system, extending from just behind the infundibulum to the posterior end of the body. Its cells gradu- ally become gelatinous, and migrate toward the peripher)', where thev finally become arranged in a man- ner recalling epithelium ; while the mass of the notochord is composed of a reticulum, in the meshes of which is the rather solid jelly. The cellular envelope thus formed and its derivati\es are frequently called the elastica interna. It is clearly of entodermal origin. The notochord has different fates in the various divisions of the verte- brates, as will be detailed later. In the cyclostomes it con- tinues to increase in size throughout life, and constitutes the major portion of the skeletal axis; but in other vertebrates the development of \-ertebrae relegates it to a subordinate position in the adult, wher
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