Fate maps of the late blastula stage of the Xenopus laevis embryo indicate that the cells of the vegetal pole area are destined to become part of the endoderm germ layer (Keller, 1975; Heasman, Wylie, Hausen & Smith, 1984). By labelling single cells from this region and transplanting them into the blastocoel cavity of host embryos, we have shown that the determinative process that restricts blastomeres to this their normal fate occurs between the early blastula and early gastrula stages (Heasman et al. 1984). To progress towards an understanding of this process, we need to establish some fundamental points. In particular, the following issues are discussed here. (1) Is cell interaction required for determination to proceed? (2) What is the cellular nature of determination?

We have used the labelling and transplantation technique described previously (Heasman, Snape, Smith & Wylie, 1985; Heasman, Snape, Smith, Holwill & Wylie, 1985) to study these questions in relation to the mechanism of determination of vegetal pole cells in Xenopus laevis.

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