216 OUTLINES OF CHORDATE DEVELOPMENT 



B. THE VISCERAL ARCHES 



The elements of the visceral skeleton are formed in the 

 pharyngeal visceral arches, which are established by the fusion 

 of the serial gill pouches with the ectoderm. AVe have described 

 the formation of the mandibular, hyoid and four branchial 

 visceral arches. In each of these save the last branchial, 

 skeletal elements appear. The mandibular and hyoid skeletal 

 arches appear about the time the full number of visceral 

 pouches is established, as condensations in the mesenchyme of 

 the visceral arch regions, soon becoming cartilaginous. The 

 mandibular arch appears first as a short rod, lying transversely 

 to the axis of the embryo, in the floor and sides of the mouth 

 cavity (Fig. 57). It is very early divided into two parts, the 

 separation between them marking the jaw articulation. The 

 dorsal section, or upper jaw rudiment, known as the palato- 

 quadrate, has already been described; the ventral section, or 

 lower jaw rudiment, becomes subdivided into Meckel's cartilage, 

 or lower jaw proper, and the infrarostral cartilage. The last 

 two elements remain ventral to the olfactory region as small 

 relatively undifferentiated elements during early development, 

 but the palato-quadrate rapidly enlarges and grows backward, 

 becoming roughly parallel with the trabecuke and fusing with 

 them at two points as described above. Later (about 21 mm.) 

 the posterior or quadrate portion of this cartilage forms a con- 

 nection with the auditory capsule. During metamorphosis, 

 as the mouth enlarges and extends far posteriorly, the arrange- 

 ment of the jaw elements is very considerably modified. The 

 upper end of Meckel's cartilage, which has been in the olfactory 

 region, rapidly extends posteriorly and reaches to the quadrate 

 cartilage, below and in front of the auditory capsule. The 

 quadrate, meanwhile, elongates ventro-laterally. That part of 

 the palato-quadrate lying in the orbital region, softens and 

 largely disappears, and the anterior connection with the tra- 

 becula is drawn back in its place and remains as the seat of the 

 future pterygoid and palatine regions. The jaw articulation 

 is thus carried rapidly from the anterior to the posterior region 



