118 PROCEEDINGS OP THE AMERICAN ACADEMY 



for the chick. Ilis account embraces a description of the changes 

 as observed both from the outside and from the inner side of the bran- 

 chial region, and is made as definite as possible through the aid of what 

 is really indispensable, — series of sections both transveree and frontal. 



In the chick of the fourth day the first cleft is already closed except 

 in its posterior * third. The position of the fissure is, however, still 

 recognizable by the presence of a furrow which is least conspicuous in 

 the region of the middle third of the fissure, which latter, moreover, is 

 the place in which fusion first occurs and whicli marks the position of 

 the future tympanic membrane. The total length of the branchial fur- 

 row is .53 mm., of which the posterior open part occupies .17 mm. 



By the sixth day the furrow has increased to .83 mm., but the pos- 

 terior open portion remains unchanged in length, nor is it really open 

 at this time, since deep down a welding has closed the passage. Two 

 pairs of elevations — coUiculi branckiales externi — arise from the 

 edges of the first and second arches, and surround all of the furrow ex- 

 cept a small anterior portion. Of the superior pair the posterior eleva- 

 tion corresponds in position to the root of the first arch, the anterior 

 to the posterior end of the mandibular process. These elevations are 

 so placed that the furrow is broken into a zigzag, the most imj)ortant 

 part of which, since it corresjionds to the region of first closure, is a 

 valley, the sides of which are bounded by the upper anterior and the 

 lower posterior elevations, and the ends of which abut against the two 

 remaining colliculi. 



On the seventh and following days the colliculi become more promi- 

 nent and the corresponding valley deeper ; the lower posterior elevation 

 soon begins, however, to become flattened, while the three remaining 

 colliculi become confluent, and thus form a sharp lip, which slightly 

 arches over the depression from the upper and anterior sides, and 

 thus converts the latter into an oblique passage, the entrance to which 

 is on the side of the flattened and finally obsolete colliculus. 



The inner surface of the first arch in the chick of the fourth day is 

 raised into an elongated elevation, — colliculus palato-pharyngeus, — 

 which, beginning near the origin of the maxillary process, extends 

 downward across the root of the first arch perpendicularly toward the 

 first cleft. It gradually increases in prominence from above down- 

 ward, and ends with a rounded margin at the lower edge of the first 



* Moldenliauer's method of designating directions is borrowed from anthro- 

 potomy, so that liis "anterior" refers to the ventral face of tlie embryo, and 

 his " posterior" to tlie dorsum. For tlie same reason the cepiiahc end is called 

 " upper " or " superior " ; the caudal, " lower " or " inferior." 



