247function prior to the molecular era, owing to ease of identifying induced neural
plates and nervous system tissue in sectioned material. Beginning with the obser-
vations that dissociation and delayed reaggregation of amphibian animal cap ecto-
derm could cause neural differentiation (Grunz and Tacke 1989 , 1990 ; Saint-Jeannet
et al. 1990 ; Godsave and Slack 1991 ), and the cloning of organizer-specific tran-
scription factors (Cho et al. 1991 ; Dirksen and Jamrich 1992 ; Taira et al. 1992 ), a
series of experiments from many labs identified antagonism of Bone Morphogenetic
Protein (BMP) signaling activity as a key activity of the organizer in vertebrate
axial development (Smith and Harland 1992 ; Sasai et al. 1994 , 1995 ; Wilson and
Hemmati- Brivanlou 1995 ; Zimmerman et al. 1996 ). The early history of the
molecular characterization of the organizer and its impact on the fields of develop-
mental biology and the evolution of development has been extensively reviewed
1 ̊ axis2 ̊ axisdorsal lip2 ̊ axis1 ̊ axisdonor-derived tissue
(notochord, floor plate, somite)vdvdabcFig. 6.10 The organizer experiment of Spemann and Mangold. (a) Diagrammatic model of
Spemann and Mangold’s dorsal lip transplantation from a lightly pigmented species (light gray) to
the ventral region of a darker species (dark gray). (b) Image of a Xenopus tadpole following the
successful grafting of an early gastrula dorsal lip, showing the endogenous axis (1° axis) and the
induced partial axis (2° axis). The dark pigment in the head of the 2° axis is related to abnormal
head development in the induced axis. (c) Diagram of a cross section through an embryo resulting
from a dorsal lip transplant as in (a). The typical lineage contribution of the donor lip is lightly
shaded reflecting the species origin, indicating contributions to the notochord, floor plate and
medial somite (c is after Spemann and Mangold 1924 ). v ventral, d dorsal
6 Vertebrate Axial Patterning: From Egg to Asymmetry