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though the demonstration of the justice of its adumbrations can 

 only be obtained by the application of the criterion of development. 

 It is twenty-one years since this criterion was applied by Reichert. 

 Since his results were published, they have been, in their main fea- 

 tures, verified and adopted by Rathke, the first embryologist of his 

 age ; and yet they are ignored, and the quadratum of the bird is 

 assumed to be the tympanic of the mammal, in some of the most 

 recent, if not the newest discussions of the subject. Reichert and 

 Rathke have proved, that in the course of the development of either 

 a mammal or a bird, a slender cartilaginous rod makes its appearance 

 in the first visceral arch, and eventually unites with its fellow, at a point 

 corresponding with the future symphysis of the lower jaw. Supe- 

 riorly, this rod is connected with the outer surface of the cartilage, 

 in which the petrosal bone subsequently makes its appearance. 

 Near its proximal end, the rod-like " mandibular cartilage " sends 

 off another slender cartilaginous process, which extends forwards 

 parallel with the base of the skull. "With the progress of develop- 

 ment, ossification takes place in the last-named cartilage, and con- 

 verts it, anteriorly, into the palatine, and posteriorly, into the ptery- 

 goid bone. The mandibular cartilage itself becomes divided into 

 two portions, a short, proximal, and a long, distal, by an articulation 

 which makes its appearance just below the junction of the ptery go- 

 palatine cartilage. The long distal division is termed, from the name 

 of its original discoverer, Meckel's cartilage. It lengthens, and an 

 ossific deposit takes place around, but, at first, not in it. The 

 proximal division in the mammal ossifies, but usually loses its con- 

 nexion with the pterygoid, remains very small and becomes the incus. 

 In the bird the corresponding part enlarges, ossifies, and becomes 

 the os quadratum, retaining its primitive connexion with the ptery- 

 goid. In the mammal, the proximal end of Meckel's cartilage ossi- 

 fies and becomes the malleus, while the rest ultimately disappears. 

 The ossific mass which is formed around Meckel's cartilage remains 

 quite distinct from the proximal end of that cartilage, or the malleus, 

 gradually acquires the form of the ramus of the lower jaw, and 

 eventually developes a condyle which comes into contact and arti- 

 culates with, the squamosal. In the bird, on the contrary, the 

 ramus of the jaw unites with the ossified proximal end of Meckel's 

 cartilage ; which becomes anchylosed with the ramus, but retaining 



