THE SKELETAL SYSTEM 



235 



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



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 



Fig. 191. — Early chondrocranium of Acanthias. (The brain in outline.) als, 

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

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

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

 Vertebrates," after Sewertzoff.) 



of lower vertebrates, but with their quadrate element also. The quadrate 

 element develops into the incus while the articular portion or articulare 

 element of the mandibula ossifies as the malleus. All this agrees com- 

 pletely with the evidence from comparative anatomy, which makes the 



Fig. 192. — Diagram of early elasmobranch skull, bp, basal plate; c, trabecular 

 cornu; Jl, foramen lacerum; ga'-^ gill arches; gc, gill cleft; h, hyale; hm, hyomandibula; 

 i>, 1 2, 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.") 



malleus homologous with the articulare bone of the lower jaw, and 

 the incus the modified quadrate of the upper jaw of reptiles, so that the 

 articulations of the reptilian jaw become in man and mammals a part of a 

 sound-conducting apparatus. See Fig. 190. 



The cartilages of the remaining visceral arches in the human embryo 

 have a diversified fate. The dorsal part of the second, the hyoid, ossifies 



