146 



STRUCTURE OF THE VERTEBRATES 



Those relationships can be ascertained by staining the car- 

 tilages of the early embryo and clearing the tissues. It can be 

 then demonstrated that the IMeckel's cartilage passes from the 

 lower jaw into the skull, and its posterior end is in contact with 

 the quadrate cartilage. As growth continues the jaw part of the 

 cartilage degenerates, and the posterior end ossifies as an ear 

 ossicle. The relationships of the three cartilages is unchanged, 



Hyomandihular- 



Columella 

 (Stapes) 



■Squamosal 

 uadrate 

 Articular 



A. Elasmobranch 



B. Reptile 



'Squamosal 

 uadrate 

 rticular. 



Angular 

 Surangular 



tympanic 

 ring 



r ossicles 

 Squamosal 



Dentary 



C. Mammal-like Reptile 



D. Mammal 



Fig. 83. Jaw Articulation and the Development of the Ear Ossicles. The 

 Dogfish (A) has the three jaw cartilages. The typical reptile (B) has the 

 skull encased in dermal bone, cartilage bones forming the jaw articulation, 

 the hyomandibular being an ear ossicle; the mammal-like reptile (C) 

 shows the back growth of the dentary; and the mammal (D) has the 

 articulation between dermal bones, the quadrate and articular having 

 passed into the middle ear as the incus and malleus. 



