OF ARTS AND SCIENCES. 119 



arch. This elevation is marked off from the rest of the arch by two 

 erooves, both of which run into the first cleft at their lower ends ; but 

 in the upper part of their course they differ. The posterior one — 

 sulcus tubo-tympanicus — gradually becomes shallower till, at its upper 

 end, it is no longer distinguishable ; the anterior, — sulcus lingualis, — 

 however, bends forward at its upper end, and opens to the outside at 

 the angle between the maxillary and mandibular processes. The latter 

 subsequently bounds the outer margin of the tongue ; the former is the 

 beginning of the Eustachian tube and the tympanic cavity. 



At its upper or basal portion the margins of the colliculus are thick- 

 ened into crura ; the posterior — crus superius — retains its perpen- 

 dicular direction ; the anterior — crus inferins — bends forward nearly 

 parallel with the maxillary process, and is the first trace of the palatine 

 arch. The edges of the superior crura of the opposite sides of the em- 

 bryo subsequently fuse, thus forming the common sinus, characteristic 

 of birds, into which the Eustachian tubes directly open. 



Cross sections of a two-day embryo chick show that none of the 

 clefts are formed, althougli the entoderm is in contact with the ecto- 

 derm in the region of the first cleft, preparatory to the formation of an 

 opening. Similar sections of 3-4-day chicks — in which all four clefts 

 are open — exhibit the sulci as narrow curved hornlike fissures. The 

 dorsal horn, — sulcus tubo-tympanicus, — faintly indicated in more ante- 

 rior sections, becomes deeper in the more posterior, and finally opens 

 out through the first cleft. Its extent is shown by the position of the 

 sections of the branchial artery ; owing to its arched course the latter 

 is twice cut in cross sections ;• the sulcus begins near the inner section 

 of this vessel, where its presence causes the dorsal wall of the pharynx 

 to protrude, and it extends outward not farther than the outer section 

 of this artery. 



On the sixth day the clefts are all closed except a small posterior 

 portion of the first. The sulcus tubo-tympanicus has a bend which gives 

 its outer end a dorsal direction. In a section through the branchial 

 furrow, which cuts through the territory of the mandibular arch rather 

 than the hyoid, there is in a line with the sulcus a slight depression of 

 the ectoderm ; immediately in front of this there is a prominent eleva- 

 tion (a section of the colliculus posterior superior), while in front of 

 this elevation there is a second more deeply depressed region, — the 

 oblique valley seen in surface views, — which terminates abruptly in 

 front at a second elevation, the section of the anterior superior colli- 

 culus. This deeply depressed region lies opposite the bend in the 

 sulcus, and corresponds to the thinnest part of the wall ; it marks the 

 place of the tympanic membrane. 



