DEVELOPMENT OF THIRD DAY 499 



The first one is called the hyomandibular cleft, while the remaining 

 ones are known respectively as the II, III, and IV gill clefts. 



Between these clefts, as well as immediately anterior and posterior 

 to them, there is a pair of thickened regions, each pair of w r hich meets 

 ventrally in the midline and merges with its mate from the opposite side 

 of the body. These thicknesses are called visceral arches, gill arches, 

 or branchial arches, also numbered from the anterior end, caudad. The 

 first is called the mandibular, the second the hyoid, and from here caudad 

 the III, IV, and V. 



The hyomandibular cleft lies between the mandibular and hyoid 

 folds or arches. 



It is well at this point to anticipate a little as to what is to become 

 of these structures later (Fig. 296). 



All the clefts close, with the exception of the hyomandibular. This, 

 too, begins closing at the end farthest from the pharyngeal opening, but 

 retains the opening into the pharynx. The unclosed end itself becomes 

 the tympanic cavity, while the remaining portion of the cleft becomes 

 the Eustachian tube. 



The external auditory meatus is formed by a depression in the sur- 



Fig. 296. 



Head of a 5 l /2 day chick embryo, a.n.f., lateral nasal process ; 

 au, eye ; bulb.ao.~ bulbus aortae ; i,n.f., inner nasal process ; fc.6. 1 

 and fc.fo, 2 mandibular and hyoid arches ; max, upper process of the 

 mandibular arch ; nas.gr., nasal groove ; st.f., frontal process ; 

 tr.nas., tear-duct running to nasal cavity ; ventr., ventricle ; v.h., 

 forebrain. (After Duval.) 



face ectoderm opposite the position of the tympanic cavity. The outer 

 end of the closed hyomandibular cleft thus lies between the tympanic 

 cavity and the external auditory meatus, the tissue formed by the closure 

 of the cleft forming the tympanic membrane. 



The most posterior two gill arches or folds entirely disappear in 

 adult stages of the chick. 



The pair of mandibular arches grow toward each other on the ven- 

 tral side and fuse to form the basis of the mandible, or lower jaw. From 



