THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



Squamosum 



transformed into the petrous and mastoid portions of the temporal bone 

 (Fig. 320). The mastoid process is formed after birth by a bulging of the 

 petrous bone, and its internal cavities, the mastoid cells, are formed and 



lined by the evaginated epithelial lining of 

 the middle ear. The squamosal and tympanic 

 portions of the temporal bone are of intra- 

 membranous origin, while the styloid process 

 originates from the proximal end of the 

 second, or hyoid branchial arch. 



Membrane Bones of the Skull. From 

 the preceding account it is evident that, 

 although the bones forming the base of the 

 skull arise chiefly in cartilage, they receive 

 substantial contributions from membrane 

 bones. The remainder of the sides and roof 

 of the skull is wholly of intramembranous 



origin, each of the parietals forming from a single center, the frontal 

 from paired centers. At the incomplete angles between the parietals and 

 their adjacent bones, union is delayed for some time after birth. These 

 membrane-covered spaces constitute the fontanelles. 



Tympanicum 



' Petrosum 



FIG. 320. The left temporal 

 bone at birth. The portion of 

 intracartilaginous origin is repre- 

 sented in stipple. 



Malleus 



Incus 



Stapes 

 Styloid process- 



Tympanic ring 

 Stylo-hyoid lig. 



Temporal squama 



Zygoma 



Mandible 



Cricoid cartilage 



Meckel's cartilage 



Hyoid cartilage (lesser horn) 

 Hyoid cartilage (greater horn) 

 rhyreoid cartilage 



FIG. 321. Lateral dissection of the head of a human fetus, showing the derivatives of. the 

 branchial arches (after Kollmann). 



The vomer forms from two centers in the connective tissue flanking the 

 lower border of the lamina perpendicularis of the ethmoid. The cartilage 

 of the ethmoid thus invested undergoes resorption. 



Single centers of ossification in the mesenchyme of the facial region 

 give rise to the nasal, lacrimal, and zygomatic, all pure membrane bones. 



