130 Bulletin American Museum of Natural History. \\o\. XX"\'II, 



The Stapes. 



Whereas comparative anatunii.sts differ as to the homology and origin 

 of the more distal elements of the ossicular chain of mammals, practically 

 all agree that the stapes of mammals is homologous with the stapes, or 

 proximal portion of the columella auris, of reptiles and amphibians. In all 

 these forms the stapes, both in ontogeny and in the adult has essentially 

 similar relations to the internal ear: namely, it is a primitively cylindrical 

 plug, fitting into the fenestra ovalis (cf. Figs. 3.1, St, IC). 



The maminaJian incus and hodij of the malleus. 



Perhaps the majority of present day comparative anatomists and embry- 

 ologists, after the most elaborate and thorough researches (r/. Kingsley, 

 1900; Weber, 1904, ])p. 74-76; Gaupp, 1898, 1908) accept Reichert's 

 view of 1837 that the quadrate of reptiles became free from the squamosal, 

 and, by reduction in size and change of function was transformed into the 

 incus of mammals. 



The emljryology of Alonotremes (cf. Gaupp, 1908) ^larsupials (cf. 

 Parker, 1880, p. 272) and Placentals {cf Parker, 1886; Kingsley, 1900) 

 reveals a remarkable uniformity of conditions, and strong evidence in favor 

 of the view that the mammalian incus and malleus represent transformed 

 jaw elements (cf. Figs. 1, 3, 4, 7). The embryonic incus and malleus in 

 all mammals are relatively very large, and this is in harmony with the view 

 that in their ancestral history they were still larger, and discharged some 

 function other than that of a sound conducting a})j)aratus (Gegenbaur, 

 Kingsley). However the fact that the stapes and the whole auditory capsule 

 are also very large in embryonic mammals de])rives the foregoing argument 

 of much weight, and indeed suggests cjenogenesis (Broom). The malleus 

 and incus form the suspensorium for the Meckelian cartilage (Fig. 2C, 4B, 7) 

 and are held to be derivatives of the pterygo-quadrate-Meckelian, or first, 

 gill arch. 



Perhaps the most weighty item of evidence for regarding the malleus as a 

 transformed articular is that in the embryos of all mammals the malleus 

 is entirely continuous with the Meckelian cartilage and seems to form its 

 posterior portion, just as does the cartilaginous "Anlage" of the articular 

 of reptiles. If the malleus is not homologous with the reptilian articular 

 then this malleo-meckelian connection of all mammals must be entirely sec- 

 ondary, as maintained by Broom (1907.3, pp. 8-10). 



The incudo-malleolar joint suggests the joint between the quadrate and 

 articulare; while the body of the malleus forms the posterior expansion of 



