CHAPTER XI 



THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 

 L The Skeletal System 



The skeleton comprises (1) the axial skeleton (skull, vertebrae, ribs, and ster- 

 num), and (2) the appendicular skeleton (pectoral and pelvic girdles 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 skele- 

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



AXIAL SKELETON 

 The primitive axial skeleton of all vertebrates is the notochord or chorda dor- 

 salis, the origin of which has been traced on pp. 33 and 35. The notochord con- 

 stitutes the only skeleton of Amphioxus, whereas in fishes and amphibians it is 

 replaced in part, and in higher animals almost entirely, by the permanent axial 



Myotome- 



Ectoderm— \ 



'■•Sclerotome 



.intersegmental 



artery 

 ... Notochord 



Myotome- 



Ectoderm. 



- Notochord 

 ■Anlage of vertebra 

 ■Intervertebral fissure 

 — Intersegmental artery 



Fig. 313. — Frontal sections through the mesodermal segments of the left side of human embryos. 

 A, at about 4 mm. showing the differentiation of the sclerotomes into less dense and denser regions; B, 

 at about 5 mm. illustrating the union of the halves of successive sclerotomes to form the anlages of the 

 vertebrae. 



skeleton. In the development of mammals, this transient elastic rod disappears 

 early except in the intervertebral discs where it persists as the nuclei pulposi. 



Vertebrae and Ribs. — The mesenchyme derived from the sclerotomes grows 

 mesad (Figs. 290 and 323) and comes to lie in paired segmental masses on either 

 side of the notochord, separated from similar masses before and behind by the 

 intersegmental arteries. In embryos of about 4 mm. each sclerotome soon differ- 

 entiates into a caudal compact portion and a cranial less dense half (Fig. 313 A). 



309 



