STRUCTURE OF FIFTY-HOUR CHICKS 103 



participation in the formation of the embryonic arterial system, 

 of some of the ductless glands, of the eustachian tube, and of 

 the face and jaws. 



The visceral clefts are formed by the meeting of ectodermal 

 depressions, the visceral furrows, with diverticula from the 

 lateral walls of the pharynx, the pharyngeal pouches. During 

 most of the time the visceral furrows are conspicuous features 

 in entire embryos, they may be seen in sections to be closed by 

 a thin double layer of tissue composed of the ectoderm of the 

 floor of the visceral furrow and the entoderm at the distal ex- 

 tremity of the pharyngeal pouch (Fig. 36, A). The breaking 

 through of this thin double layer of tissue brings the pharyngeal 

 pouches into communication with the visceral furrows thereby 

 establishing open visceral clefts. In birds an open condition of 

 the clefts is transitory. In the chick the most posterior of the 

 series of clefts never becomes open. Although some of the 

 clefts never become open and others open for but a short time 

 the term cleft is usually used to designate these structures which 

 are potentially clefts, whether open or not. 



The position of the visceral clefts is best seen in entire em- 

 bryos. They are commonly designated by number beginning 

 with the first cleft posterior to the mouth and proceeding 

 caudad. The first post-oral cleft appears earliest in develop- 

 ment and is discernible at about 46 hours of incubation. Vis- 

 ceral cleft II appears soon after, and by 50 to 55 hours three 

 clefts have been formed (Fig. 34). 



Between adjacent visceral clefts, the lateral body walls about 

 the pharynx are thickened. Each of these lateral thickenings 

 in the mid- ventral line meets and merges with the corresponding 

 thickening of the opposite side of the body. Thus the pharynx 

 is encompassed laterally and ventrally by a series of arch-like 

 thickenings, the visceral or gill arches. The visceral arches like 

 the visceral clefts are designated by number, beginning at the 

 anterior end of the s$r,ies. Visceral arch I lies cephalic to the 

 first post-oral cleft, between it and the mouth region. Because 

 of the part it plays in the formation of the mandible it is also 

 designated as the mandibular arch. Visceral arch II is fre- 

 quently termed the hyoid arch, and visceral cleft I, because of 

 its position between the mandibular and hyoid arches, is known 

 as the hyomandibular cleft. Posterior to the hyoid arch the 



