TUB LOWER VERTB13RATA. 161 



capsules, are also the first-formed structures of the mamma- 

 lian cranium. The cartilaginous skull that results from 

 their union is called the chondro-cranium. 



2. The Jaws. In fig. 80, A and B, there appears 

 beneath the eye a bar of cartilage, the palato-pterygoid 

 cartilage, which is also to be seen from the side in fig. 82. 

 It will be learnt from the latter that this bar is joined 

 in front to the cranium behind the olfactory capsule, and 

 behind to the otic capsule. The cartilaginous bar from the 

 palato-pterygoid to the otic capsule is called the quadrate, 

 and at the point of junction, at the postero-ventral angle 

 of the palato-pterygoid, articulates with the cartilaginous 

 bar which is destined to form the skeleton of the lower 

 jaw Meckel's cartilage. 



3. Cartilage-bones. Fig. 80, c, shows a dorsal view of 

 these structures in a young frog. The parts corresponding 

 to those in A and B will be easily made out, but now 

 ossification (after the manner explained in chap, vi., 11) 

 has set in at various points of this cartilaginous cranium. 

 In front of the otic capsule is the paired pro-otic bone ; 

 behind it at the sides of the parachordal ring is the paired 

 ex-occipital ; in front of the brain-case, and behind the 

 olfactory capsules, is a ring of bone, the sphenethmoid. 

 These are all the cartilage-bones that appear in the cranium 

 and upper jaw of the frog, so that, even in the adult, the 

 skull consists in no small part of cartilage. 



4. Membrane-bones. But another series of bones, 

 developed first chiefly in dermal connective tissue, and 

 coming to plate over the chondro-cranium, are not shown 

 in fig. 80, c. They are, however, in figs. 81 and 83. These! 

 membrane-bones are : on the dorsal side, the parieto-frontals, 

 originally two pairs of bones which fuse in development, 

 and the nasals. . Round the edge of the jaw, and bearing 

 the teeth, are premaxillae and maxillae, and overlying the 

 quadrate cartilage and lateral to the otic capsules are the 

 T-shaped squamosal bones. A quadrato-jugal appears at 

 the angle of articulation with the lower jaw. In the ventral 



ZOOL. 11 



