DEVELOPMENT OF BAR AND ACCESSORY ORGANS IN FROG. 549 
not yet met. Part of the beginning of the septum between utriculus and 
sacculus is represented, as is the small pouch representing the lagena. (X 25.) 
Fie. 9.—Transverse section through the centre of the ear in a tadpole of 
25mm. ‘The full extent of the recessus at this stage is shown. (xX 15.) 
Fie. 10.—Oblique transverse section through the head of a large tadpole, 
just before the anterior legs are protruded. On the left the first branchial 
cleft is shown, as well as the opening of the Eustachian tube into the cesopha- 
gus. On the right hand the centre of the eye is cut, and a section of the 
Eustachian tube is shown considerably in front of its point of origin from the 
throat. (x 10.) 
Fre, 11.—Section through the tympanic cavity and annular cartilage in the 
same specimen. On the left it passes entirely in front of the tympanic cavity, 
cutting the point of union of the palatopterygoid bar with the trabecule. 
The section is completely in front of the eye. (x 12.) 
Fie. 12.—Diagrammatic sagittal section of a tailed frog on the left side, 
showing the course of the Eustachian tube and the position of the annular 
cartilage. It passes through the ear and eye and the angle of the mouth. 
The greater part of the length of the quadrate and palatopterygoid cartilage 
is shown with the Meckelian cartilage below it, as well as the outer portions 
of the hyoid and first three branchial arches. (x 15.) 
Fre. 13.—Oblique section through the ear of the tadpole from which Figs. 
10 and 1] are taken. ‘The columella is shown at an early stage cut at two 
different levels. On the right the interstapedial, and on the left the meso- 
stapedial is shown. The thymus lies a short distance to the outside of the 
columella. (x 10.) 
Fie. 14.—Diagrammatic horizontal section of a tailed frog, to show the 
course of the columella. The eye is cut some distance from its centre, and 
the anterior nares are included in the section. The relations of the columella 
to the auditory capsule, stapes, and thymus are shown. (x 15.) 
Fie. 15.—Oblique transverse section of a young frog’s head, just after 
metamorphosis. The left side is more anterior than the right. On the right 
the whole course of the columella is shown; it is as yet chondrified at its 
base alone. Observe the fragmentary and asymmetrical condition of the 
Eustachian tube. (x 15.) 
Fie. 16,—Section similar to the last, but through an older frog. The 
columella is now chondrified throughout. Observe that the Eustachian tube 
is complete on the left, but fragmentary on the right. (x 15.) 
Fic. 17.—Diagrammatic transverse section of an adult frog’s head. The 
whole course of the recessus is shown, with the connection between the two 
sides, both above and below the brain. On the right as much of the columella 
is shown as possible, and the whole course of the Eustachian tube is repre- 
sented. The anterior parts of the sternum and thymus are cut, and the 
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