UPPEK JAW PROCESS. 24*, 



(Plates I., VI., and VII., k). The first of these gill arches, at 

 present the most interesting to us, which we may call the 

 jaw arch, develops the jaw-skeleton of the mouth (Plate I., u). 

 A small process first grows out from the base of the front 

 gill-arch : this is the upper jaw process. The first gill-arch 

 itself develops a cartilage on its inner side, called after its 

 discoverer, "Meckel's cartilage," on the outer surface of 

 which the lower jaw forms (Figs. 232, u, 236, u). The upper 

 jaw process forms the principal part of the entire framework 

 of the upper jaw, viz., the palate bone and the wing bone. 

 On its outer side the upper jaw bone, in the narrower sense, 

 afterwards arises, while the middle portion of the upper jaw 

 skeleton, the twixt jaw (intermaxillary bone) develops 

 from the anterior portion of the frontal process. (See 

 development of the face in Plate I.) 



In the further characteristic development of the face in 

 the three higher vertebrate classes, the two upper jaw pro- 

 cesses are of the highest importance. From them proceeds 

 the palate roof, the important horizontal partition which 

 grows into the simple primitive mouth-cavity, separating 

 it into two quite distinct cavities. The upper cavity, 

 into which the two nasal cavities open, now develops into 

 the nasal cavity — a respiratory air passage and an olfactory 

 organ. The lower cavity, on the other hand, forms, by itself, 

 the permanent secondary mouth-cavity (Fig. 237, m) — the 

 digestive food passage and the organ of taste. Both the upper 

 smell-cavity and the lower taste-cavity open at the back into 

 the throat (pharynx). The palate roof, separating these two 

 cavities, is formed by the coalescence of two lateral portions 

 — of the horizontal plates of the two upper jaw processes 

 (palate-plates ; Fig. 237, p). When these do not perfectly 



