Fig. 341. Embryo Pig, two-thirds of an inch long; elements of the skull seen somewhat diagrammatically from below. (From Parker.)
pa. ch. parachordal cartilage; nc. notochord; au. auditory capsule; py. pituitary body; tr. trabeculæ; c.tr. trabecular cornu; pn. prenasal cartilage; e.n. external nasal opening; ol. nasal capsule; p.pg. palatopterygoid tract enclosed in the maxillopalatine process; mn. mandibular arch; hy. hyoid arch; th.h. first branchial arch; 7a. facial nerve; 8a. glossopharyngeal; 8b. vagus; 9. hypoglossal.
Both the hyoid and mandibular arches develop at first more completely than in any of the other types above Fishes; and are articulated to each other above, while the pterygo-palatine bar is quite distinct. The main features of the subsequent development are undisputed, with the exception of that of the upper end of the hyoid, which is still controverted. The following is Parker’s (No. [452]) account for the Pig, which confirms in the main the view originally put forward by Huxley (No. [445]).
The mandibular and hyoid arches are at first very similar ([fig. 341] mn and hy), their dorsal ends being somewhat incurved, and articulating together.
In a somewhat later stage ([fig. 342]) the upper end of the mandibular bar (mb), without becoming segmented from the ventral part, becomes distinctly swollen, and clearly corresponds to the quadrate region of other types. The ventral part of the bar constitutes the Meckelian cartilage (mk).
Fig. 342. Embryo Pig, an inch and a third long; side view of mandibular and hyoid arches. The main hyoid arch is seen as displaced backwards after segmentation from the incus. (From Parker.)
tg. tongue; mk. Meckelian cartilage; ml. body of malleus; mb. manubrium or handle of the malleus; t.ty. tegmen tympani; i. incus; st. stapes; i.hy. interhyal ligament; st.h. stylohyal cartilage; h.h. hypohyal; b.h. basibranchial; th.h. rudiment of first branchial arch; 7a. facial nerve.
The hyoid arch has in the meantime become segmented into two parts, an upper part (i), which eventually becomes one of the small bones of the ear—the incus—and a lower part which remains permanently as the anterior cornu of the hyoid (st.h). The two parts continue to be connected by a ligament.