Fig. 2. Prospective caudal neural tissue is located on the ventral side of the gastrula ectoderm. Lateral views with dorsal towards the right for gastrula stage embryos and to the bottom for 1-day-old embryos. The vegetal limit of the germ ring is indicated by white lines in A, D and G. Gastrula (A,D,G) and 1-day-old (B,C,E,F,H) embryos in which fluorescein was uncaged in ventral vegetal (A) and lateral vegetal (D) ectoderm, or widely throughout dorsal and lateral ectoderm (G), then fates of fluorescent cells were examined the next day. Ventrovegetal cells (A, green fluorescent cells) give rise to tail neural tube (B, bracket; and C in high magnification), whereas laterovegetal ectodermal cells (D, green fluorescent cells) contribute to trunk neural tube (E, bracket; and F in high magnification). Besides neural tissue, some ventrally labelled cells ended up in somite muscle (B, white arrowhead) and in the pronephric duct (B, black arrowhead). In embryos with widespread dorsolateral uncaging of fluorescein (G), labelled cells in the caudal tail are restricted to floor plate, notochord and hypocord, and are absent from more dorsal neural tube (H). (I) Summary of fate-mapping data. Positions at which fluorescein was uncaged in vegetal ectodermal cells are categorised as: V, ventral; VL, ventrolateral, L, lateral; DL, dorsolateral; D, dorsal. The relative contributions of cells to the trunk (rostral to the end of the yolk extension) and tail (caudal to the end of yolk extension) neural tube are indicated by the different colours in each column. The numbers of embryos examined (n) for each position are indicated beneath the columns. (H) Summary of ectodermal expression and fate-mapping data. Ventral ectoderm towards the animal pole generates epidermis/non neural fates, whereas remaining ectoderm generates neural tissue with cells fated to give rise to the most caudal CNS located in ventrovegetal ectoderm and those contributing to the most rostral CNS located in dorsal animal ectoderm (see above) (Kimmel et al., 1990; Woo and Fraser, 1995). The cartoon does not indicate precursors of ventral CNS midline (such as floor plate and hypothalamus) that are positioned close to the organiser (Mathieu et al., 2002; Shih and Fraser, 1996) and distribute widely along the AP axis of the CNS. Indeed, other ventral spinal cord cell types also originate closer to the gastrula organiser than do dorsal spinal cord neurones (Kimmel et al., 1990). Some of the genes selectively expressed in different regions of the ectoderm (see Fig. 7) are indicated. The dorsal (D) and ventral (V) position of cells at gastrula stage does not necessarily correspond with the eventual DV position of the structures to which the cells contribute. For example, spinal cord of the tail (a dorsal structure in 24 hpf embryos) originates from ventrally positioned ectoderm cells of the gastrula [see also recent fate-mapping studies of mesodermal precursors in Xenopus (Kumano and Smith, 2002; Lane and Sheets, 2002)]. fp, floor plate; hp, hypocord; nc, notocord; nt, neural tube.