DEVELOPMENT OF EAR AND ACOESSORY ORGANS IN FROG. 543 
and such evidence should have great weight—tends to show that 
the two organs have no connection whatever with each other. 
The Skeleton of the Ear.—In this section is included 
the development of the annular cartilage, stapes, and columella. 
Of these the columella is the only one that is of great general 
interest, from its supposed homology, on the one hand, with 
the hyomandibular cartilage of fishes, and on the other with 
the auditory ossicles of higher Vertebrates. Beginning with 
the annular cartilage, 1 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 de- 
scribes it at its first appearance, and it is therefore removed 
from the columella, and quite distinct from it. 
Before the forelegs 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 mesoblastic tissue, and 
this is especially marked towards the ventral surface of the 
tube. This mass is closely applied to the palatopterygoid car- 
tilage, but when, during the metamorphosis of the tadpole, 
the tympanic cavity moves backwards, it carries with it the sur- 
rounding 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 mem- 
brane, and gradually extends until a complete ring grows 
around the tympanic cavity. This ring is the annular tym- 
panic 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 colu- 
mella grows it does come in contact with the annular catilage ; 
but at first the separation of the two is complete, and even 
1 © Structure and Development of Skull in Common Frog,” ‘ Phil. Trans.,’ 
1871. 
