egg STRUCTURE OF VERTEBRATES 



the stapes, we have already identified as the columella auris. The 

 identification of these bones was made by a comparison of the 

 condition in the embryo with that in the extinct reptiles which 

 are considered to be near the ancestry of mammals (Fig. 457). 

 In these therapsids there was a progressive reduction of all the 

 bones of the lower jaw except the dentary, until finally all the 

 others became a group of loose bones near the angle of the jaw. 

 Their similarit}^ to the embryonic mammalian condition is 

 obvious, and the detailed homologies may be made as follows. The 

 malleus, which is preceded by cartilage, is the articular ; splints 



mall. 



Meek. 



Fig. 457. — Diagrams of the arrangement of jaw and auditory ossicles. ^, in a 

 reptile; B, in a mammal. From Young, The Life of Vertebrates, 1950. Clarendon 

 Press, Oxford. After Gaupp and Ihle. 



ang., Angular : art., articular ; cor., coronoid ; d., dentary ; ex.coL, extra columella ; hy., hyoid ; inc., incus = 

 quadrate; jon., gonial; mall., malleus = articular ; Meek., Meckel's cartilage; pr. fol., prearticular ; 

 q., quadrate ; St., stapes = columella ; suran., surangular ; tymp., tympanic = angular. 



which may be present on the malleus are the surangular (not 

 present in Perameles) and prearticular ; the incus, since it arti- 

 culates with the malleus-articular, must be the quadrate ; the 

 stapes bears the same relation to nerves and blood vessels as do 

 the columella and hyomandibula, and so must be identified 

 with those ; and finally the tympanic, from its general shape 

 and topography, must be the angular. This development of 

 the ear ossicles could only have gone on as the old jaw- joint 

 became freed and as the new squamosal-dentary articulation 

 took its place. 



RESPIRATORY SYSTEM 



We have already covered incidentally most of the more im- 

 portant points in this. The primitive place for gas exchange in 

 animals is undoubtedlv the skin, and in all species where this 



