484 MANDIBULAR AND IIYOID BARS. 



The membranous man(Hbular arch gives off in the embryos of all 

 the Sauropsida an obvious bud to form the superior maxillary pro- 

 cess, and the formation of this bud appears to represent the growth 

 forwards of the pterygoid process in Elasmobranchii, which is indeed 

 accompanied by the formation of a similar bud ; but the skeletal rod, 

 which appears in the axis of this bud, is as a rule independent of 

 that in the true arch (fig. 331, pa, pg). The former is the pterygo- 

 palatine bar ; the latter the Meckelian and quadrate cartilages. 



The pterygo-palatine bar is usually if not always ossified directly, 

 without the intervention of cartilage. 



Born has recently shewn that Parker was mistaken in supposing that 

 the palato-pterygoid bone is cartilaginous in Birds. In the Turtle a short 

 cartilaginous pterygoid process of the quadrate would seem to be present 

 (Parker, No. 458). 



The quadrate and Meckelian cartilages are either from the first 

 separate, or very early become so. 



The quadrate cartilage ossifies as the quadrate bone, and supplies 

 the permanent articulation for the lower jaw. Its upper end exhibits 

 a tendency to divide into two processes, corresponding with the 

 pedicle and otic processes of the Amphibia. The Meckelian cartilage 

 becomes soon covered by investing bones, and its proximal end ossifies 

 as the articulare. The remainder of the cartilage usually disappears. 



Mammalia. The most extraordinary metamorphosis of the hyoid 

 and mandibular arches occurs in the Mammalia, and has been in 

 part known since the publication of the memoir of Keichert (No. 

 461). 



Both the hyoid and mandibular arches develop at first more 

 completely than in any of the other types above Fishes ; and are 

 articulated to each other above, while the pterygo-palatine bar is 

 quite distinct. The main features of the subsequent development are 

 undisputed, with the exception of that of the upper end of the hyoid, 

 which is still controverted. The following is Parker's (No. 452) 

 account for the Pig, which confirms in the main the view originally 

 put forward by Huxley (No. 445 ). 



The mandibular and hyoid arches are at first very similar (fig. 341 

 mn and hy), their dorsal ends being somewhat incurved, and articu- 

 lating together. 



In a somewhat later stage (fig. 342) the upper end of the man- 

 dibular bar (mb), without becoming segmented from the ventral part, 

 becomes distinctly swollen, and clearly corresponds to the quadrate 

 region of other types. The ventral part of the bar constitutes the 

 Meckelian cai'tilage (mk). 



The hyoid arch has in the meantime become segmented into two 

 parts, an upper part (i), which eventually becomes one of the small 

 bones of the ear — tiie incus — and a lower fart which remains per- 

 manently as the anterior cornu of the hyoid (si.h). The two parts 

 continue to be comiected by a ligament. 



