134 MORPHOLGY OF THE ORGANS 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 extends 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 epiphyses 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 column is developed around the notochord 

 (p. 17). This, as will be remembered, is a rod-like structure 

 of entodermal origin which lies between the alimentary 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 periphery, where they 

 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 derivatives 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 vertebrae relegates it to a very subordinate 

 position in the adult, where it may persist as a very inconspicu- 

 ous remnant. 



Fig. 139. Section through noto- 

 chord of embryonic shark {Acan- 

 thias}. C, centrum of vertebra; 

 E, /, elastica externa and interna ; 

 N y neural process. 



