404 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



from the yentral part, becomes distinctly swollen, and 

 clearly corresponds to the quadrate region of other types. 

 The ventral part of the bar constitutes Meckel's carti- 

 lage {mk). 



The hyoid arch has in the meantime become seg- 

 mented 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 as the anterior cornu 

 of the hyoid (st.h). The two parts continue to be con- 

 nected by a ligament. 



The iQcus is articulated with the quadrate end of 

 the mandibular arch, and its rounded head comes in 

 contact with the stapes (Fig. 136, st) which is segmented 

 from the fenestra ovaHs. 



According to some authors the stapes is independently formed 

 from mesoblast cells surrounding a branch of the internal carotid 

 artery. 



The main arch of the hyoid becomes divided into 

 a hypohyal (h.h) below and a stylohyal (sth) above, and 

 also becomes articulated with the basal element of the 

 arch behind (6^). 



In the course of further development the Meckelian 

 part of the mandibular arch becomes enveloped in a 

 superficial ossification 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 incorpo- 

 rated in the dentary. 



The quadrate region remains relatively stationary in 

 growth as compared with the adjacent parts of the skull, 

 and finally ossifies to form the malleus. The processus 



