THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



199 



represents the perpendicular part, the horizontal part appearing in embryos of 

 about 24 mm. as an outgrowth from the perpendicular and not as a separate 

 center of ossification. The orbital and sphenoidal processes also represent out- 

 growths from the primary center and appear much later. 



Opinions regarding the development of the maxilla are at variance. One 

 view is that it arises from five centers of ossification. One of these centers gives 

 rise to that part of the alveolar border which bears the molar and premolar 

 teeth; a second center forms the nasal process and that part of the alveolar bor- 

 der which bears the canine tooth; a third produces the part which bears the 

 incisor teeth; and the two remaining centers give rise to the rest of the bone. 

 All these parts effect a firm union at an early stage, with the exception of the 

 part bearing the incisor teeth which remains more or less distinct as the incisive 

 bone (premaxilla, intermaxilla) . Another view arising from recent work on 



Incisive bone Upper lip 



(intermaxillary) 



Primitive choanae 



Lip groove 



Cut surface Palatine processes 



FIG. 178. Head of human embryo of 7 weeks. His. 

 Ventral aspect of upper jaw region. Lower jaw and tongue have been removed. 



human embryos is that there are primarily only two ossification centers; one of 

 these gives rise to the incisive bone, the other to the rest of the maxilla (Mall). 

 These centers appear at the end of the sixth week (embryos of 18 mm.). 



A very important feature in the development of the maxilla is its agency in 

 separating the nasal cavity from the mouth cavity. The palatine process of the 

 bone grows medially and meets and fuses with its fellow of the opposite side in 

 the medial line, the two processes together thus constituting about the an- 

 terior three-fourths of the bony part of the hard palate. It should be observed, 

 however, that the palatine processes do not meet at their anterior borders, for 

 the incisive bone is insinuated between them (see Figs. 178, 179). 



