Caudal movement of tissues ventral to the cervical somites and the development of the shoulder in the chick embryo

Abstract It has been suggested that formation of the shoulder in the embryonic chick is caused by caudal movement of the pleural coelom (Yander and Searls, '80a,b; Searls, '83). The cells of the somatopleure over the pleural coelom move into the base of the wing causing a thickening at the cranial base of the wing (the shoulder). In the previous work cited what caused the pleural coelom to move in a caudal direction could not be determined. It has now been observed that part of this movement is due to increase in the cranial‐caudal width of the hyoid arch. The mandibular arch does not move relative to the base of the wing or the somites. The caudal margin of the hyoid arch on the surface of the neck moves caudally at a rate of four to six somites per day, overgrowing the cervical sinus in the process. The aortic arches move in the same direction at a rate of 2 to 3 somites per day. It is suggested that the more medial elements of the hyoid arch are increasing in size, displacing the cervical sinus in a caudal direction. Part of the caudal movement of the cranial margin of the pleural coelom is also due to increase in length of the trachea. The trachea branches from the larynx ventral to about the eighth somite from 4 to 7 days of development. The distance from the cranial end of the trachea to the bifurcation that produces the left and right bronchi increases so that the bronchi become medial to the shoulder at 5.5 days, and the distance from the cranial end of the trachea to the pleural coelom increases so that the pleural coelom becomes medial to the shoulder at 4.5 days.