Mr. T. H. Huxley on the Theory of the Vertebrate SkuU. 421 



pterygoid internally and with the quadratojugal externally, which 

 gives attachment to a part of the tympanic memhrane posteriorly, 

 and which is very generally termed the tympanic bone, from its 

 supposed homology with the boue so named in the Mammalia. The 

 resemblance to the tympanic bone, however, hardly extends beyond 

 its relation to the tympanic membrane ; for in no other of the par- 

 ticulars mentioned above do the connexions of the two bones cor- 

 respond. The tympanic of the mammal does not articulate with the 

 lower jaw, nor with the pterygoid*, nor with the jiigal or quadrato- 

 jugal. On the other hand, if the connexions of the tympanic mem- 

 brane were sufficient to determine the point, not only the quadratura, 

 but the articular element of the lower jaw, and even some cranial 

 bones, must be regarded as tympanic. 



Again, if we trace the modifications which the tympanic bone 

 undergoes in the mammalian series, we find that in those mammals, 

 such as Echidna and Ornithorhjnchus, which approach nearest to 

 the Ocipara, and which should therefore furnish us with some hint 

 of the modifications to which the tympanic bone is destined in that 

 group, the bone, so far from increasing in size and importance, and 

 taking on some of the connexions which it exhibits in the oviparous 

 Vertebrata, absolutely diminishes and becomes rudimentary, so that 

 the vast bony capsule of the placental mammal is reduced, in the 

 Monotreme, to a mere bony ring. 



But it is no less worthy of remark, that in these very same animals 

 the malleus and incus have attained dimensions out of all proportion 

 to those which they exhibit in other mammals, and that they even 

 contribute to the support of the tympanic membrane. 



So far, therefore, from being prepared by the study of those 

 Mammalia which most nearly approach the Ovipara, to find, in the 

 most highly organized of the latter, an immense os tympanicum, 

 with a vanishing malleus and incus, we are, on the contrary, led to 

 anticipate the disappearance of the tympanicum, and the further 

 enlargement of the ossicula auditus. Thus far the cautious appli- 

 cation of the method of gradations leads us, and leads us rightly — 

 though the demonstration of the justice of its adumbrations can 

 only be obtained by the application of the criterion of develop- 

 ment. 



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. Sujje- 

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



* Though the pterygoid conies close to it in Monolremata. 



