The general vegetal signal apparentlyremains operational in ventralized embryos produced by UV irradiation offertilized eggs these radially symmetrical embryos have the normal amountof mesoderm, but all of it is ventral in character.

The latter region is the future dorsal lip of the blastopore,which is also known as Spemann's organizer after Hans Spemann, who demonstratedthat transplantation of this region of an amphibian embryo will organizea second embryonic axis through induction. These observations haveled to the proposal that there are two mesoderm-inducing signals: (1) ageneral vegetal signal that operates around the circumference and inducesventral mesoderm and (2) a dorsal vegetal signal that induces the axialmesoderm (Slack et al. The dorsal vegetal cells(the Nieuwkoop Center) induce axial mesoderm of the dorsal midline (notochordand segmented muscle), whereas the remaining vegetal cells induce ventrolateralmesoderm (mesothelium, mesenchyme, blood cells). The vegetalcells are not equal in their inductive capacities. This implies that cellsabove this boundary require contact with cells below the boundary (the vegetalcells) to produce mesoderm, and that is what is observed when animal andvegetal explants are combined (Browder et al., Fig. 12.1), whereas no cells from above the pigmentboundary will form muscle or notochord in isolation. The fate map shows that abouthalf of the mesoderm arises from cells above the equatorial pigment boundary(Browder et al., Fig. Mesoderm induction occurs over an extended period of time in the equatorialregion of the embryo from about the 32-cell stage to the beginning of gastrulation.The requirement of induction for production of mesoderm is evident by comparingthe embryonic fate map (which shows the fates of regions of the embryo duringnormal development) to the specification map (which shows what tissue explantscan do in isolation) during cleavage stages. Most contemporary investigations have utilized Xenopus.The two major inductive events during early Xenopus development are(1) mesoderm induction and (2) neural induction (Slack, 1994, Fig. Amphibiansare the most extensively studied vertebrates for investigations into embryonicinduction. This implies both the capacity to produce a signal(ligand) by the inducing cells and the competence of the responding cellsto receive and interpret the signal via a signal transduction pathway. Induction is the processby which one group of cells produces a signal that determines the fate ofa second group of cells.

Vertebrate embryos rely extensively upon inductive interactions to diversifythe number of different kinds of cells in the embryo.

When does the blastopore become evident