IX.] 



DEVELOPMENT. 



M7 



which gives rise to the pterygoid, palatine, 

 maxillary and malar regions (Fig. 47, II.). 



The latter, MeckeVs cartilage (Fig. 52, ?nk; Fig. 47, III.), a 

 slender rod, is proximally in relation with the periotic region 

 of the skull. In the Mammalia, the upper extremity of this 

 rod becomes converted into the malleus, one of the small 

 bones in the tympanic cavity, while in connection with the 

 lower or distal part, the ramus of the mandible or lower jaw 

 is developed, partly by conversion of the cartilage itself, but 

 principally by the ossification of fibrous or cartilaginous 

 tissue deposited around it. The upper end of the ramus 



FIG. 52. Side view of mandibular and hyoid arches of an embryo Pig, an inch and 

 a third long. The main arch is seen as displaced backwards after segmentation 

 from the incus, tg tongue ; ink Meckelian cartilage ; ml body of malleus ; nth 

 manubrium or handle of the malleus; t.ty tegmen tympani ; /incus: st stapes ; 

 ihy interhyal ligament ; st.li stylohyal cartilage ; h.h hypohyal ; bh basi-branchial ; 

 th.h rudiment of first branchial arch ; ja. facial nerve. 1 



afterwards acquires a secondary articular connection with 

 the squamosal bone, its primitive connection with the 

 malleus entirely disappearing. 



The rod of cartilage forming the second visceral arch 

 (Fig. 47, IV.) becomes the anterior hyoid arch, its proximal 

 extremity being modified into the incus. 



From the third arch, which corresponds to the first 

 branchial of branchiate vertebrates, is formed the posterior 

 hyoid arch, or thyrohyal (th.li) (Fig. 47, V.). 



1 Figs. 51 and 52, reduced from the original figures of Parker, are 

 taken with permission from Balfour's Comparative Embryology. 



L 2 



