Fig. 1. Organizer grafts result in induction of a secondary axis. (A) Schematic of the organizer graft created by Spemann and Mangold, using a light-gray newt donor (Triturus cristatus) grafted into a dark-gray host (Triturus taeniatus). The gastrulae are shown in hemisection for illustrative purposes only (dorsal is towards the right, and the dimensions of these embryos are more Xenopus-like than Triturus-like). (B) The famous result of an optimal grafting experiment (Spemann and Mangold, 1924), showing a section through the trunk of a twinned embryo. The light-gray graft has contributed to the notochord, medial somite and floor plate of the secondary axis. The graft has an induced neural tube, somites, a pronephros and a secondary archenteron cavity. (C, D) Contemporary organizer grafts from Andrea E. Wills (UC Berkeley, CA, USA). (C) The section shows a rafted organizer labeled with lacZ mRNA and stained with Red-Gal; the section is taken through the trunk of a stage 28 Xenopus laevis embryo, where the axial tissues are also stained with Tor70 antibody. (D) Twinned Xenopus embryo, resulting from an organizer graft carried out at stage 10.