CHAPTER XI 

 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



I. THE SKELETAL SYSTEM 



THE skeleton comprises: (i) the axial skeleton (skull, vertebras, ribs, 

 and sternum) and (2) the appendicular skeleton (pectoral and pelvic gir- 

 dles and the limb bones) . Except for the flat bones of the face and skull, 

 which develop directly in membrane, the bones of the skeleton exhibit first 

 a blastemal, or membranous stage, next a cartilaginous stage, and finally 

 a permanent, osseous stage. 



For a detailed account of the development of the various bones of the 

 skeleton the student is referred to Bardeen, Keibel and Mall, vol. i. 



THE AXIAL SKELETON 



The primitive axial skeleton of all vertebrates is the notochord, or 

 chorda dor sails, the origin of which has been traced on pp. 34 and 36. The 

 notochord constitutes the only skeleton of Amphioxus, whereas in fishes 

 and amphibians it is replaced in part, and in higher animals almost en- 

 tirely, by the permanent axial skeleton. In the development of mammals, 

 this transient elastic rod disappears early, except in the intervertebral 

 discs where it persists as the nuclei pulposi. 



The Vertebrae and Ribs. The mesenchyme derived from the sclero- 

 tomes grows mesad (Figs. 290 and 323) and comes to lie in paired segmen- 

 tal masses on either side of the notochord, separated from similar masses 

 before and behind by the inter segmental arteries. In embryos of about 4 

 mm., each sclerotome soon differentiates into a caudal, compact portion 

 and a cranial, less dense half (Fig. 313 A). From the caudal portions, 

 horizontal tissue masses now grow toward the median line and enclose the 

 notochord, thus establishing the body of each vertebra. Similarly, dorsal 

 extensions form the vertebral arch, and ventro-latera' outgrowths, the 

 costal processes. The looser issue of the cranial halves also grows mesad 

 and fills in 'the intervals between successive denser regions. 



The denser caudal half of each sclerotomic mass presently unites with 

 the less dense cranial half of the sclerotome next caudad to form the an- 



30Q 



