144 



THE FROG. 



end of the Eustachian tube at a time when this lies below the 

 eye. During the metamorphosis, this ring of cells preserves its 

 relation with the outer end of the Eustachian tube, or tympanic 



AS 



FIG. 68. A transverse section across the posterior part of the head of an 

 adult Frog, showing the position and relations of the auditor}' organs,. 

 Eustachian tube, and hyoid apparatus. On the right side the section 

 passes through the tympanic cavity and the columella ; on the left side 

 through the anterior cornu of the hyoid. The cartilage is dotted, and 

 the bones, except the columella, represented black. 



A, parasphenoid. AS, angulosplenial. B, bnccal cavity. C, columella. D. tym- 

 panic membrane. E, Eustachian tube. F, anterior cornu of the hyoid. FP, fronto- 

 parietal. Gr, glottis. H, arytenoid cartilage. I, posterior cornu of the hyoid. K, 

 auditory nerve. L, vestibule of the ear. M, anterior vertical semicircular canal. 

 N, horizontal semicircular canal. O. pro-otic. P, pterygoid. Q, quadrate cartilage. 

 R, quadratojugal. S, squamosal. T, tympanic cartilage. V, vocal cord. X, mid-brain. 



cavity, and gradually shifts back with this latter to its adult 

 position. A bar of cartilage appears in its ventral portion, 

 which gradually extends at its ends until it forms the complete 

 annular tympanic cartilage. 



The development of the auditory ossicle, or columella (Fig. 

 68, c), will be described in the section dealing with the develop- 

 ment of the skull (p. 209). 



4. The Cutaneous Sense Organs. 



During the tadpole stage, while the animal is leading an 

 aquatic life, special sense organs in the form of small epidermal 

 papillas are present, arranged in rows along the body, round the 

 eyes, and on other parts of the head. They are supplied by the 

 lateral line series of branches of the trigeminal and pneumo- 

 gastric nerves, which have already been described (pp. 130, 132) ; 

 they are lost completely at, or shortly after, the time of the 

 metamorphosis. 



The mouth of the tadpole is also provided with special 



