176 



CHORDATE ANATOMY 



Development of the Visceral Skeleton. In the human embryo a series 

 of visceral arches separated by pharyngeal pouches appear in relations 

 corresponding to those of aquatic vertebrates. In the first of those arches 

 the maxilla of the upper jaw and the mandible of the lower jaw develop 



Fig. 165. — Early chondrocranium of Squalus. (The brain in outline.) als, 

 alisphenoid cartilage; ch, anterior end of notochord; h, hyoid arch; ?na, mandibular arch, 

 not yet divided into pterygoquadrate and Meckelian; oc, otic capsule; t, trabecula; 

 1-5, branchial arches; cartilages dotted. (From Kingsley's " Comparative Anatomy of 

 Vertebrates," after Sewertzoff.) 



as membrane bones. The mandible, however, surrounds a cartilage, 

 Meckel's cartilage, which corresponds to the mandibular cartilage of the 

 lower jaw of elasmobranchs. While most of Meckel's cartilage disappears 

 during ontogenesis, that portion which extends into the cavity of the middle 

 ear ossifies in two centers, one of which forms the malleus and the other 





Fig. 166. — Diagram of early elasmobranch skull, bp, basal plate; c, trabecular 

 cornu;^, foramen lacerum; ga^-\ gill arches; gc, gill cleft; /;, hyale; lim, hyomandibular; 

 ii, i^, upper labials; II, lower labials; m, Meckel's cartilage; nc. nasal capsule; oc, otic 

 capsule; of, orbital foramen; ov, occipital vertebrae; pq, pterygoquadrate; s, suspensor 

 ligament; sp. spiracle; si, sphenolateral; t, trabecula; v, vertebrae; I-VII, visceral arches; 

 i-io, cranial nerves. (From Kingsley's "Comparative Anatomy of 'Vertebrates.") 



the incus. The so-called Meckel's cartilage of the mammalian embryo 

 appears therefore to correspond not only to the mandibular or Meckel's 

 cartilage of lower vertebrates, but with their cjuadrate element also. The 

 quadrate element develops into the incus while the articular portion of the 

 mandibular ossifies as the malleus. 



