In view of our imperfect knowledge of the development of most Piscine skulls I would refrain from expressing a decided opinion in favour of either of these alternatives.
Fig. 339. Tadpole with tail beginning to shrink; side view of skull without the branchial arches. (From Parker.)
n.c. notochord; au. auditory capsule; between it and eth. the low cranial side wall is seen; eth. ethmoidal region; st. stapes; 5. trigeminal foramen; 2. optic foramen; ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu trabeculæ; u.l. upper labial, in outline; su. suspensorium (quadrate); pd. its pedicle; ot.pr. its otic process; or.p. its orbitar process; t.m. temporal muscle, indicated by dotted lines passing beneath the orbitar process; pa.pg. palatopterygoid bar; mk. Meckelian cartilage; l.l. lower labial, in outline; c.h. ceratohyal; b.h. basihyal. The upper outline of the head is shewn by dotted lines.
As the tail of the Tadpole gradually disappears, and the metamorphosis into the Frog becomes accomplished, the mandibular arch undergoes important changes ([fig. 339]): the palatopterygoid attachment (pa.pg) of the quadrate subocular bar becomes gradually elongated; and, as it is so, the front end of the subocular bar (su) rotates outwards and backwards, and soon forms a very considerable angle with the trabeculæ. The Meckelian cartilage (mk) at its free end becomes at the same time considerably elongated. These processes of growth continue till ([fig. 330]) the palatopterygoid bar (Pt) forms a subocular bar, and is considerably longer than the original subocular region of the quadrate; while the Meckelian cartilage (Mck) has assumed its permanent position on the hinder border of the no longer suctorial mouth, and has grown forwards so as nearly to meet its fellow in the median line.
Fig. 340. Young Frog, near end of first summer; upper view of skull, with left mandible removed, and the right extended outwards. (From Parker.)
b.o. basioccipital tract; s.o. supraoccipital tract; fo. frontal fontanelle; e.n. external nostril; internal to it, internasal plate; a.t. tympanic annulus.
Bones: e.o. exoccipital; pr.o. prootic, partly overlapped by p. parietal; f. frontal; eth. rudiment of sphenethmoid; na. nasal; pmx. premaxillary; mx. maxillary; pg. pterygoid, partly ensheathing the reduced cartilage; q.j. quadratojugal; sq. squamosal; ar. articular; d. dentary; m.mk. mento-Meckelian.
The metapterygoid region of the quadrate gives rise to a posterior and dorsal process ([fig. 339], ot.pr), the end of which is constricted off as the tympanic annulus ([fig. 340], a.t); while the proximal part of the process remains as the otic (metapterygoid) process, articulating with the auditory cartilage.
The pedicle (pd) retains its original attachment to the skull.