STRUCTURE OF THE VERTEBRATES 



147 



but their position has shifted. The homology is confirmed not 

 only by the embryology of the cartilages, but by the muscles 

 attached to the stapes and their innervation. The latter proofs 

 are too complex for this discussion. 



Pte')-y go-quadrate 



T ympanic ting 



Fig. 84. Mammal Embryo with Unossified Jaw Cartilages. The proximal 



ends of the pterygo-quadrate and Meckel's cartilages lie within the 



tympanic ring and ossify as ear bones. 



The posterior five arches, the branchial arches of the fish, 

 eventually become transformed into the cartilages of the larynx. 

 The primitive amphibia which retain gills naturally retain the 

 branchial arches in reduced number, the others forming discon- 

 nected throat cartilages. The larynx is completed in the mam- 

 mals and will be discussed under the respiratory system. The 

 following chart shows the homologies in tabular form. 



