162 TEXT-BOOK OF EMBRYOLOGY. 



which ossification later takes place to form the pterygoid hamulus (p. 159). 

 The lateral pterygoid lamina is also of intramembranous origin and fuses with 

 the medial lamina, the two laminae forming the pterygoid process which subse- 

 quently unites with the body of the sphenoid. (See Fig. 138.) 



In the ethmoidal region, only the vomer is of intramembranous origin. An 

 ossification center appears in the embryonic connective tissue on each side of 

 the perpendicular plate (lamina perpendicularis) and these two centers produce 

 two thin plates of bone which unite at their lower borders and invest the lower 

 part of the perpendicular plate. The portion of the latter thus invested 

 undergoes resorption. 



The frontal and parietal bones are purely of intramembranous origin. About 

 the eighth week two centers of ossification, one on each side, appear for the 

 frontal. The bones produced by these centers unite in the medial line to form 

 the single adult bone. In the event of an incomplete union an open suture 

 remains the metopic suture. A single center of ossification appears for each 

 parietal bone at about the same time as those for the frontal. The union of 

 the bones which form the roof and the greater part of the sides of the skull does 

 not occur till after birth. The spaces between them constitute the sutures and 

 fontanelles so obvious in new-born children (Fig. 139). 



A single center of ossification appears in the embryonic connective tissue 

 for each zygomatic, lachrymal and nasal bone, all of which are of intramem- 

 branous origin. 



BONES DERIVED FROM THE BRANCHIAL ARCHES. 



The first branchial arch becomes divided into two portions. One of these, 

 the maxillary process, is destined to give rise to the upper jaw and much of the 

 upper lip and face region. The other, the mandibular process, is destined to 

 give rise to the lower jaw, the lower lip and chin region, and two of the auditory 

 ossicles. The angle between the two processes corresponds to the angle of the 

 mouth, and the cavity enclosed by the processes is the forerunner of the mouth 

 and nasal cavities. (See Fig. 96, also p. 119.) So far as the skeletal elements 

 are concerned, cartilage develops only in the mandibular process where it 

 forms a slender bar or rod known as MeckeVs cartilage. Only a small part of 

 this becomes ossified, the greater portion of the mandible being of intramem- 

 branous origin. No cartilage develops in the maxillary process. This 

 probably indicates a condensation of development in man and the higher 

 animals, for among the lower animals cartilage precedes the bone. In man the 

 maxilla and palate bone also are of intramembranous origin. 



The palate bone develops from a single center of ossification which appears 

 at the side of the nasal cavity in embryos of about 18 mm. This center 



