DEVELOPMENT OF THE EAR, IN THE COMMON FROG. 177 



homology, on the one hand, with the hyomandibular cartilage of 

 fishes, and on the other with the auditory ossicles of higher Verte- 

 brates. Beginning with the annular cartilage, I may preface my 

 remarks by stating that I have obtained traces of it long before the 

 stage is reached which Professor Parker* takes as his earliest. At 

 its first origin it is far in front of the position in which Professor 

 Parker describes it at its first appearance, and it is therefore removed 

 from the columella, and quite distinct from it. 



Before the fore legs have appeared, and the tadpole has begun to 

 resemble the frog, the Eustachian tube is directed forwards, and its 

 expanded end, which becomes the tympanic cavity, lies close to the 

 palatopterygoid bar, in advance of the eye (see Figs. 12 and 13). At 

 this time the tympanic cavity is surrounded by a mass of cells, which 

 appear closely packed in contrast with the surrounding loose meso- 

 blastic tissue, and this is especially marked towards the ventral sur- 

 face of the tube. This mass is closely applied to the palatopterygoid 

 cartilage ; but when, during the metamorphosis of the tadpole, the 

 tympanic cavity moves backwards, it carries with it the surrounding 

 mass, which becomes denser ; and some time after the metamorphosis 

 has been completed a small cartilaginous bar is formed in it below 

 the ventral edge of the tympanic membrane, and gradually extends 

 until a complete ring grows around the tympanic cavity. This ring 

 is the annular tympanic cartilage. During its growth I can trace no 

 connection between it and the columella close enough to warrant the 

 statement that the two have a common origin. As the columella 

 grows it does come in contact with the annular cartilage ; but at 

 first the separation of the two is complete, and even when contact 

 takes place the relation is only one of close juxtaposition. 



From the above account, it is clear that the constituent cells of the 

 annular cartilage appear in connection with the anterior part of the 

 mandibular arch, and are perhaps derived from it; although it is 

 possible that they may be merely concentrated from the scattered 

 cells surrounding the distal end of the Eustachian tube. The point 

 at which the cartilage originates is in front of and below the eye, 

 and the part of the mandibular arch to which it is applied is the 

 palatopterygoid bar. 



'Structure and Development of Skull in Common Frog," " Phil. Trans.," 1871. 



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