ON THE THEORY OF THE VERTEBRATE SKULL 551 



ratum, 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 Ornithorhynchus, which approach nearest to 

 the Ovipara, 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 organised 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 application 

 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 development. 



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

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

 features, 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. 

 Superiorly, this rod is connected with the outer surface of the carti- 

 lage, 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 de- 

 velopment, ossification takes place in the last-named cartilage, and 



1 See Note HL 



