The formation of the spot-like Spemann-type organizer (dark green/yellow) responsible for the dorso-ventral pattern is restricted to the blastopore. Thus, since the blastopore became a large ring , the dorsal organizer has automatically a pronounced off-axis position as required. The Spemann-type organizer with its spot-like extension is by itself insufficient to generate a dorsoventral pattern along the long-extended anteroposterior axis. An important step is, therefore, the generation of a midline organizer
It is the marginal ring and not the Spemann-Organizer that provide the positional information for the AP patterning. Evidence exists that WNT signals, generated in the ring, are involved in the posterior transformation of the more anteriorely located cells that form the future brain [1,2]. Also the activation of more posterior HOX genes takes place in the ring except of the organizing region . For the zebra fish it has been shown that AP-markers appear in the correct order even in the complete absence of a dorsal organizing region . The details of the AP specification support the view that that the AP axis is generated in two parts. The ancestral anterior part (brain and heart) occurs by by posterior transformation
under the influence of a morphogen signal. In contrast, the more posterior part is patterned sequentially. An oscillation generates the periodic somites and the sequential pattern of HOX-gene activation
The enlargement of the posterior organizer to a large ring is a specific feature of vertebrates. In spiders, for instance, the blastopore remains a small spot while the anterior pole enlarged to a ring (below left) . Likewise in Drosophila, at the critical stage the oocyte has the shape of a cone. The tip is posterior, the circular antipodal side anterior. The movement of the nucleus underneath the membrane is necessarily connected with a symmetry breaking .