■VI.] THE VISCERAL AUCHES. 165 



branch, staxting from near the dorsal beginning of the 

 fold, runs ventralwards and forwards, tending to meet the 

 corresponding branch from the fold on the other side, at 

 a point in the middle line nearer the front of the head 

 than the junction of the main folds. The two branches 

 do not quite meet, being separated by a median process, 

 which at the same time grows down from the extreme 

 front of the head, and against which they abut. Between 

 the main folds, which are directed somewhat backwards 

 and the branches which slant forwards, a somewhat 

 lozenge-shaped space is developed which, as the folds 

 become more and more prominent, grows deeper and 

 deeper. In the main folds are developed the man- 

 dibles, and in the branches the supenor maxilloe : the 

 lozenge-shaped cavity between them is the cavity of the 

 mouth, and the descending process which helps to 

 complete the upper margin of this cavity is called, from 

 the parts which will be formed out of it, the fronto- 

 nasal process. 



Part of the mesoblast of the two succeeding pairs of 

 visceral folds is transformed into the hyoid bone, which 

 wiU be best considered in connection with the develop- 

 ment of the skull. The two last arches disappear with- 

 out giving rise to any permanent structures. 



With the exception of the first the visceral clefts 

 become obliterated at an early stage of embryonic life ; 

 but the first persists, although it loses all trace of its 

 original branchial function and becomes intimately con- 

 nected with the organ of hearing, of which in fact it 

 forms a most essential part, becoming converted into 

 the Eustachian tube and tympanic cavity. The outer 

 opening and the outer part also of the cleft become 



