CHAPTER XIV 

 THE SKELETON 



I. General 



From an embryological point of view, the bones of the bod\% 

 their associated cartilages, the Hgaments that unite them together 

 in various ways, and the joints should be considered together, 

 as they have a common origin from certain aggregations of 

 mesenchyme. The main source of the latter is the series of 

 sclerotomes, but most of the bones of the skull are derived from 

 the unsegmented cephalic mesenchyme. 



Most of the bones of the body pass through three stages in 

 their embryonic development: (1) a membranous or prechondral 

 stage, (2) a cartilaginous stage, (3) the stage of ossification. 

 Such bones are known as cartilage bones, for the reason that 

 they are preformed in cartilage. Many (see p. 433 for list) of 

 the bones of the skull, the clavicles and the uncinate processes of 

 the ribs do not pass through the stage of cartilage, but ossifica- 

 tion takes place directly in the membrane; these are known as 

 membrane or covering bones. The ontogenetic stages of bone 

 formation parallel the phylogenetic stages, membrane preceding 

 cartilage, and the latter preceding bone in the taxonomic series. 

 Thus, in Amphioxus, the skeleton (excluding the notochord) 

 is membranous; in the lamprey eel it is partly membranous and 

 partly cartilaginous; in the selachia it is mainly cartilaginous; in 

 higher forms bone replaces cartilage to a greater or less degree. 

 The comparative study of membrane bones indicates that they 

 were primitively of dermal origin, and only secondarily grafted 

 on to the underlying cartilage to strengthen it. Thus the car- 

 tilage bones belong to an older category than the membrane 

 bones. 



The so-called membranous or prechondi-al stage of the skeleton 

 is characterized simply by condensation of the mesenchyme. 

 Such condensations arise at various ' times and places described 



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