GASTRULATION AND HOMEOBOX GENES IN CHICK-EMBRYOS

We review the early stages of chick embryogenesis, in particular the f ormation of the hypoblast, and the ingression of endoderm and mesoderm through the primitive streak. The formation of a trilaminar embryo du ring gastrulation is accompanied by the specification of body axes. Th e first axis is already present in the unfertilized egg and runs from the cytoplasmatic animal to the yolk rich vegetal pole. Already within the uterus a second axis conveys bilateral symmetry to the embryo. It extends from a dorsal/anterior to a ventral/posterior position. These axial poles segregate during gastrulation to form the classical coord inates, a dorsal-ventral and an anterior-posterior axis. The establish ment of axes is accompanied by the expression of specific combinations of homeobox genes during gastrulation in the chick, as in other metaz oa. We review the avian specific information and compare it with findi ngs in other species. A combinatorial homeobox code for the specificat ion of identities during development is discussed. (C) 1997 Elsevier S cience Ireland Ltd.