5QO MANDIBULAR AND HYOID BARS. 



(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). 



The hyoid arch has in the meantime become segmented into 

 two parts, an upper part (z), which eventually becomes one of 



FIG. 342. EMBRYO PIG, AN INCH AND A THIRD LONG; SIDE VIEW OF 

 MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID ARCH IS SEEN AS DIS- 

 PLACED BACKWARDS AFTER SEGMENTATION FROM THE INCUS. (From Parker.) 



tg. tongue; ink. 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 ; ^.//.basibranchial; th.h. rudiment 

 of first branchial arch; -ja. facial nerve. 



the small bones of the ear the incus and a lower part which 

 remains permanently as the anterior cornu of the hyoid (st./i). 

 The two parts continue to be connected by a ligament. 



The incus is articulated with the quadrate end of the mandi- 

 bular arch, and its rounded head comes in contact with the 

 stapes (fig. 342, st) which is segmented from the fenestra ovalis. 

 The main arch of the hyoid becomes divided into a hypohyal 

 (h.h) below and a stylohyal (st. h] above, and also becomes articu- 

 lated with the basal element of the arch behind (b/i). 



In the course of further development the Meckelian part of 

 the mandibular arch becomes enveloped in a superficial ossifica- 

 tion forming the dentary. Its upper end, adjoining the quadrate 

 region, becomes calcified and then absorbed, and its lower, with 

 the exception of the extreme point, is ossified and subsequently 

 incorporated in the dentary. 



The quadrate region remains relatively stationary in growth 



