The fate map of the prospective neural tissue and of the Spemann Organizer, which is thought to be the major neural-inducing tissue, are shown as they appear in the very early gastrula stage of X. laevis (Fig. 1, stage 10-) (10). The prospective nervous system forms a crescent on the dorsal side, lying mostly above the equator at this stage. It consists of prospective fore- and midbrain (F, M), hindbrain or rhombencephalon (RH), and spinal cord (SC). The prospective RH and SC are very wide in the mediolateral direction and very short in the anterior-posterior direction, extending about five to seven cells along the dorsal midline (10) (Fig. 1). These regions converge (narrow) mediolaterally, and extend (lengthen) anterior-posteriorly greatly during gastrulation and neurulation, described in detail below. This equatorial-subequatorial annulus of neural tissue undergoing convergent extension closes around the blastopore (Fig. 1, stage 12) and is often called the "noninvoluting marginal zone" (NIMZ) (11).
The "involuting marginal zone" (IMZ) lies vegetal to the prospective neural tissue, and its dorsal sector (about 30°) contains the "Spemann Organizer" (12). The Organizer contains much of the neural-inducing activity, based on the experiments of Spemann and Mangold in urodeles (13), as well as others since on Xenopus (14), in which this tissue was grafted to the ventral side of another embryo where it induced the formation of a second axis, including a nervous system. For simplicity, only the tissues of the Organizer lying in the dorsal midline are shown in Fig. 1. These tissues are prospective prechordal mesoderm (PM) or "head mesoderm," and prospective notochord (N), both in the deep, nonepithelial region (Fig. 1, stage 10-). The superficial epithelial layer of the Organizer consists of prospective endoderm (E) (Fig. 1, stage 10-), a fact that is often overlooked. This "suprablastoporal" endoderm lies above the site of blastopore formation and involutes to form the archenteron roof (Fig. 1, stages 10- to 17). It is distinct from the "subblastoporal" or vegetal prospective endoderm (VE), which lies below the site of blastopore formation. During gastrulation, the VE is covered over by the IMZ, including the archenteron roof, and becomes the archenteron floor (Fig. 1, stage 17). The vegetal end of the suprablastoporal epithelium consists of specialized endodermal cells, prospective bottle cells (BC), which play a role in initiating gastrulation (see Subheading 1.3.1.). At its lateral regions, the organizer includes some prospective anterior somitic mesoderm (not shown in Fig. 1).
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