i ntermed iate
mesoderm
Fig. lA,B. Four-signal model summanzmg inductive processes during dorsoventral pattern formation of the amphibian embryo.A Maternally supplied
signals emanating from vegetal cells induce mesoderm in the marginal zone of
the early blastula embryo. Signals from ventral vegetal cells (VV) induce ventral mesoderm (VM) in a broad marginal region, while signals from dorsal
vegetal cells (DV) induce dorsal mesodenn, also called the Spemann organizer
(0), in a comparably small dorsal marginal domain. B The initial dorsoventral
pattern is refined under the control of zygotic signals generated by the embryo
itself. Dorsalizing signals from the Spemann organizer (0) convert the initially
ventrally specified mesoderm of dorsolateral marginal positions into more intermediate fates (Ml-M3). In addition, they induce neural specification in animal dorsal regions of the embryo which would otherwise give rise to epidermal fates. Both inductions of Spemann organizer signals are antagonized by
ventralizing signals generated in the rest of the embryo. VM will form blood,
M3 muscle of the anterior somites. and 0 notochord