Depolarized chick blastoderms fail to self-generate bilateral symmetry

We have previously shown that when an initial cell suspension of chick epiblastic cells cultured under well- defined conditions to form a flat disk is covered with a normal hypoblast, it can differentiate and generate axial embryonic structures. Almost invariably, the epiblastic cells differentiate into ectoderm neural tissue and endoderm, and in some cases, signs of axial mesoderm are observed as well as differentiation of fairly symmetrical neural tubes. We demonstrate that, when both layers are reaggregated so that a reconstituted epiblastic disk is covered with a disk of aggregated hypoblastic cells, the epiblastic cells differentiate into disorganized neural tissue and unorganized mesoderm but fail to generate an axial system. The problem of self-generating polarity is thus examined, and it is concluded that the polarity which is earlier imprinted upon the chick blastoderm through the action of gravity is indeed necessary for axial development.