The ability to initiate an axis in the avian blastula is concentrated mainly at a posterior site.

Cell interactions during early vertebrate development are crucial for embryonic mesoderm induction and axis initiation. In the avian embryo two unique layers of cells, the epiblast and the hypoblast, constitute the blastoderm before the primitive streak develops (stage XIII). It was suggested that cells of the hypoblast have the ability to induce competent cells in the epiblast to form the mesoderm and to initiate the embryonic axis. Recent results suggest, however, that at stage XIII the hypoblast does not act by inducing the epiblast to form a primitive streak. Since the hypoblast at stage XIII does not induce the epiblast, experiments were designed in this work to examine whether other subregions of the avian blastula have the ability to initiate the embryonic axis. To distinguish the contribution of a particular fragment to the formation of the embryonic axis, cell-marking examinations with lysinated rhodamine dextran (LRD) were designed. The results of the experimental series discussed in this report suggest that at stage XIII it is mainly the posterior side of the marginal zone and/or of the posterior region of the epiblast layer which has the abilities to initiate the embryonic axis. However, the posteriolateral part of the marginal zone region also has such abilities, which are inhibited during normal development. LRD examinations have demonstrated that a graft of a particular posterior blastoderm region, or posteriolateral marginal zone, can initiate an ectopic streak, and is able to recruit other neighboring cells to the developing ectopic streak. No evidence was found that Koller's sickle itself can initiate an ectopic axis in the epiblast at stage XIII. It is proposed that the cells which are important to initiate the avian embryonic axis are concentrated mainly at the region of the posterior marginal zone including Koller's sickle and in the posterior region of the epiblast layer. The cells in this region, which also express the goosecoid and cVg1 genes, may have organizer properties which induce the mesoderm and determine the initiation site of gastrulation in the chick embryo.