Studies on the Development of the Foregut in the Chick Blastoderm: 2. The Morphogenetic Movements

Abstract

The morphogenetic movements which take place in the endoderm of the chick during foregut formation have been traced by means of carbon marking on blastoderms explanted in vitro. Two types of movement are distinguished, called ‘two-dimensional’ and ‘three-dimensional’, the former occurring in the original plane of the endoderm, the latter folding certain regions medio-ventrally to form the floor of the foregut. The most anterior tip of the foregut is believed to be formed as a pocket between two sets of opposing movements in the endoderm: (a) a forward, twodimensional movement beneath the head process, and (b) an obliquely backward, three-dimensional movement on either side. During the formation of the foregut the oblique movements result in a U-shaped ridge in the endoderm bordering the anterior intestinal portal. These movements spread progressively backwards and result in the two limbs of the ridge being brought together in the midline, where they fuse and thus gradually close off the cavity of the foregut ventrally from that of the yolk sac. Some forward movement takes place at the anterior end of the developing foregut. In the posterior half of the area pellucida there are extensive displacements of endoderm cells in a posterior direction.