312 



THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



Chondrification begins in the future occipital and sphenoidal regions, in the 

 median line, and extends cephalad and to a slight extent dorsad. At the same 

 time, the internal ear becomes invested with a cartilaginous periotic capsule 

 which eventually unites with the occipital and sphenoidal cartilages (Fig. 316). 

 The chondrocranium, as it is termed, is thus confined chiefly to the base of the 

 skull, the bones of the sides, roof, and the face being of membranous origin. 

 Chondrification also occurs more or less extensively in the branchial arches, and, 

 as will appear presently, the first two pairs contribute substantially to the forma- 

 tion of the skull. 



In the period of ossification, which now ensues, it becomes evident that some 

 bones which are separate in adult lower animals fuse to form compound bones in 



• • Interparietal 



Supra^occipital 

 Exoccipital 



Condyle 

 ^asi-occjpital 

 Fig. 317. — Occipital bone of a human fetus 

 of four months (after Sappey). The portions still 

 cartilaginous are shown as a background. 



:f^as 



Fig. 316. — Reconstruction of the chondro- 

 cranium of a human embryo of 14 mm. (Levi in 

 McMurrich). as, Alisphenoid; 60, basi-occipital ; 

 bs, basisphenoid; eo, exoccipital; m, Meckel's car- 

 tilage; OS, orbitosphenoid; p, periotic; ps, pre- 

 sphenoid; so, sella turcica; s, supra-occipital. 



the human skull. The sphenoid and temporal bone; for example, represent five 

 primitive pairs each. As such components may arise either in membrane or 

 cartilage the mixed origin of certain adult bones is explained. 



Ossification of the Chondrocranium. — Occipital 5o«e.— Ossification begins in 

 the occipital region during the third month. Four centers appear at right angles 

 about the foramen magnum (Fig. 3 1 7) . From the ventral center arises the basilar 

 (basi-occipital) part of the future bone; from the lateral centers the lateral (exoc- 

 cipital) parts which bear the condyles, and from the dorsal, originally paired 

 center the squamous (supra-occipital) part below the superior nuchal fine. The 

 squamous (interparietal) part above that hne is an addition of intramembranous 

 origin. These several components do not fuse completely until about the seventh 

 year. 



