Two types worked out by Parker, viz. the Axolotl and the common Frog, may be selected to illustrate the development of the mandibular arch.
Fig. 336. Young Axolotl, 2¼ inches long; under view of skull, dissected, the lower jaw and gill arches having been removed. (From Parker.)
nc. notochord; oc.c. occipital condyle; f.o. fenestra ovalis; st. stapes; tr. trabecular cartilage; i.n. internal nares; c.tr. cornu trabeculæ; pd. pedicle of quadrate; q. quadrate; pg. outline of pterygoid cartilage; 5´. orbito-nasal nerve; 7. facial nerve.
Bones: pa.s. parasphenoid; e.o. exoccipital; v. vomer; px. premaxillary; mx. maxillary; pa. palatine; pg. pterygoid.
In the Axolotl, which may be taken as the type for the Urodela, the mandibular arch is constituted at a very early stage of (1) an enlarged dorsal element, corresponding with the pterygo-quadrate of the lower types, but usually known as the quadrate; and (2) a ventral or Meckelian element. The Meckelian bar very early acquires its investing bones, while the dorsal part of the quadrate becomes divided into two characteristic processes, viz. an anterior dorsal process which grows towards and soon permanently fuses with the trabecular crest, and a posterior process known as the otic process, which applies itself to the outer side of the auditory region. The anterior of these processes, as pointed out by Huxley, is probably homologous with the anterior process of the pterygo-quadrate bar in Notidanus, which articulates with the trabecular region of the cranium, while the otic process is homologous with the metapterygoid process. Hardly any trace is present of an anterior process to form a pterygoid bar, but dentigerous plates forming a dermal palatopterygoid bar have already appeared.
At a somewhat later stage a fresh process, called by Huxley the pedicle, grows out from the quadrate, and articulates with the ventral side of the auditory region ([fig. 336], pd). Shortly afterwards a rod of cartilage grows forward from the quadrate under the membranous pterygoid (pg), which corresponds with the cartilaginous pterygoid bar of other types ([fig. 336]), and an independent palatine bar, arising even before the pterygoid process, is formed immediately dorsal to the dentigerous palatine plate (pa), and is attached to the trabecula. These two bars eventually meet, but never become firmly united to the more important membrane bones placed superficially to them.
The mandibular arch in the Frog stands, so far as development is concerned, in striking contrast to the mandibular arch of the Axolotl, in spite of the obvious similarity in the arrangement of the adult parts in the two types.
Fig. 337. Embryo Frog, just before hatching; side view of head, with skin removed. (From Parker.)
Na. olfactory sack; E. involution for eyeball; Au. auditory sack; Tr. trabecula; Mn. mandibular; Hy. hyoid; Br.I. first branchial arch; the gill-buds are seen on the first two branchial arches; l. labial cartilages.