In the earliest stage it forms a simple bar in the membranous mandibular arch, parallel to and very similar to the hyoid bar behind ([fig. 337], Mn). In the next stage observed, that is to say in Tadpoles of four, five, to six lines long, an astonishing transformation has taken place. The mandibular arch ([fig. 338]) is turned directly forwards parallel to the trabecula, to which it is attached in front (p.pg) and behind (pd). The proximal part of the arch thus forms a subocular bar, and the space between it and the trabecula a subocular fenestra. In front of the anterior attachment it is continued forwards for a short distance, and to the free end of this projecting part is articulated a small Meckelian cartilage directed upwards (mk). The Meckelian cartilage is at this stage placed in front of the nasal sacks, in the lower lip of the suctorial mouth. The greater part of the arch, parallel with the trabeculæ, is equivalent to what has been called in the Axolotl the quadrate, while its anterior attachment to the trabeculæ is the rudiment of the palatopterygoid cartilage. The posterior attachment is known as the pedicle.
Fig. 338. Tadpole of Common Toad, one-third of an inch long; cranial and mandibular cartilages seen from above; the parachordal cartilages are not yet definite. (From Parker.)
nc. notochord; ms. muscular segments; au. auditory capsule; py. region of pituitary body; tr. trabecula; c.tr. cornu trabeculæ; p.pg. palatopterygoid bar; pd. pedicle; q. quadrate condyle; mk. Meckelian piece of mandibular arch; so.f. subocular fenestra; u.l. upper labial cartilage. The dotted circle within the quadrate region indicates the position of the internal nostril.
The condition of the mandibular arch during this and the next stage ([fig. 339]) is very perplexing. Its structure appears adapted in some way to support the suctorial mouth of the Tadpole.
Reasons have been offered in a previous part of this volume for supposing that the suctorial mouth of the Tadpole is probably not simply a structure secondarily acquired by this larva, but is an organ inherited from an ancestor provided through life with a suctorial mouth.
The question thus arises, is the peculiar modification of the mandibular arch of the Tadpole an inherited or an acquired feature?
If the first alternative is accepted we should have to admit that the mandibular arch became first of all modified in connection with the suctorial mouth, before it was converted into the jaws of the Gnathostomata; and that the peculiar history of this arch in the Tadpole is a more or less true record of its phylogenetic development. In favour of this view is the striking similarity which Huxley has pointed out between the oral skeleton of the Lamprey and that of the Tadpole; and certain peculiarities of the mandibular arch of Chimæra and the Dipnoi can perhaps best be explained on the supposition that the oral skeleton of these forms has arisen in a manner somewhat similar to that in the Frog; though with reference to this point further developmental data are much required.
On the other hand the above suppositions would necessitate our admitting that a great abbreviation has occurred in the development of the mandibular arch of the otherwise more primitive Urodela; and that the simple mode of growth of the jaws in Elasmobranchii, from the primitive mandibular arch, is phylogenetically a much abbreviated and modified process, instead of being, as usually supposed, a true record of ancestral history.
If the view is accepted that the characters of the mandibular arch of the Tadpole are secondary, it will be necessary to admit that the adaptation of the mandibular arch to the suctorial mouth took place after the suctorial mouth had come to be merely a larval organ.