Gastrulation and homeobox genes in chick embryos.

We review the early stages of chick embryogenesis, in particular the formation of the hypoblast, and the ingression of endoderm and mesoderm through the primitive streak. The formation of a trilaminar embryo during gastrulation is accompanied by the specification of body axes. The 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 coordinates, a dorsal-ventral and an anterior-posterior axis. The establishment of axes is accompanied by the expression of specific combinations of homeobox genes during gastrulation in the chick, as in other metazoa. We review the avian specific information and compare it with findings in other species. A combinatorial homeobox code for the specification of identities during development is discussed.