312. 



THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



the neural tube. Except in the occipital region, where there are indica- 

 tions of the incorporation into the skull of three or four vertebrae, the skull 

 is from the first devoid of segmentation. 



Chondrification begins in the future occipital and sphenoidal regions, 

 in the median plane and extends cephalad and to a slight extent dorsad. 

 At the same time the internal ear becomes invested with a cartilagin- 

 ous periotic capsule which eventually unites with the occipital and sphen- 

 oidal cartilages (Fig. 316). The chondro cranium 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 formation of the skull. 



Inter parietal 



-l - -Supra-occipital 

 ---/-- -Exoccipital 



-Condyle 

 Basi-occipital 



FIG. 317. Occipital bone of a human fetus 

 of four months (after Sappey). The portions 

 still cartilaginous are shown as a homogeneous 

 background. 



as 



FIG. 316. Reconstruction of the chon- 

 drocranium of a human embryo of 14 mm. (Levi 

 in McMurrich). as, Alisphenoid; bo, basi- 

 occipital; bs, basisphenoid; eo, exoccipital; m, 

 Meckel's cartilage; os, orbitosphenoid ; p, perio- 

 tic; ps, presphenoid; so, sella turcica; s, supra- 

 occipital. 



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

 some bones which are separate in adult lower animals fuse to form com- 

 pound bones in the human skull. The sphenoid and temporal bones, for 

 example, represent five primitive pairs each. As such components may 

 arise either in membrane or cartilage, the compound nature of certain 

 adult bones is explained. 



Ossification of the Chondrocranium. The Occipital Bone. Oss'fica- 

 tion begins in the occipital region during the third month. Four centers 

 appear at right angles about the foramen magnum (Fig. 317). From the 

 ventral center arises the basilar (basi-occipital) part of the future bone; 

 from the lateral centers the lateral (exocipital) parts which bear the 

 condyles; and from the dorsal, originally paired center, the squamous 

 (supra-occipital] part below the superior nuchal line The squamous 



