THE SKULL. 



200 



branchial bars, the backwardly directed thyrohyals (Fig. 94, T) 

 are formed ; while the rest of the fourth branchial bar, and the 

 whole of the first three bars disappear. 



The fenestra ovalis has been described above as a large 

 aperture, left in the outer and ventral wall of the auditory 

 capsule (Fig. 90, SA). This aperture is closed by membrane, 

 and in this membrane, shortly before the metamorphosis, the 

 stapes is formed as a disc-like plug of cartilage (Fig. 93, SA). 



The columella appears, in tadpoles of about 40 mm. length 

 (cf. Fig 93, CL), as a minute bar of cartilage immediately in 

 front of the stapes, and with its outer end directed forwards. 

 At the time of the metamorphosis the columella grows rapidly, 



FIG-, i'4. The skull of an adult Frog, from the right bide, x 2. 



A, parasphenoiil. AS, angulosplenial. B, anterior cornu of hyoid. C, coluiueHa 

 ~D, deutary. E. exoccipital. F, nostril. FP, fronto-parietal. BE, body of hyoiil. L 

 aperture for exit of optic nerve. M, maxilla. MM, mento-meekelian. M'. upertui c 

 for exit of fifth ami seventh nerves. M", nasal. O, pro-otic. P, pterygoM. PM, prc- 

 maxilla. Q,, quadratojugal. B, a]>erture for exit of ninth and tenth nerves. S. 

 squamosal. SE, sphen-ethmoiil. T, posterior cornu of hyoid, or thyrohyal. 



and becomes rotated outwards, so that its outer end conies into 

 close relation with the tympanic membrane, while its inner end 

 fuses with the stapes. At first, the columella lies in the dorsal 

 wall of the tympanic cavity, but this latter gradually extends 

 upwards around it, until the columella acquires its adult relations, 

 and appears to cross the tympanic cavity (cf. Fig. 68, C, p. 144). 

 In its actual development, the columella of the frog shows no sign 

 of any connection with either the hyoid or mandibular bars, but 

 comparison with other vertebrates renders it very possible that 

 the frog is in this, as in so many other respects, in a modified 

 rather than a primitive condition. 



The bones of the skull are of two kinds : (i) cartilage bonec, 

 which are formed in direct connection with the primary or 



