Fig. 2. Gastrulation movements induced by activin treatment of animal pole cells. Control explants on the top form spheres; those treated with activin, on the bottom, elongate.

When animal pole-vegetal pole conjugates have been made, observation with a fluorescent microscope should reveal that the cells undergoing gastrulation movements are those derived from the animal cap.

2. Observation of later morphology: After 3 d of culture, when sibling embryos are at tadpole stages, animal caps treated with mesoderm-inducing factors form characteristic structures. In the case of activin, these have been termed "embryoids," because they resemble miniature embryos (20). Treatment with FGF causes the formation of translucent balloon-like structures with a layer of smooth muscle beneath epidermis. Uninduced animal caps remain as opaque darkly pigmented spheres.

3. Numerous molecular markers are available that allow the identification not only of mesoderm, but also of which region of the mesoderm has been induced. Suitable markers are listed on the Xenopus molecular marker resource: URL http:// Expression of these marker genes can be studied by RNase protection or by reverse-transcription PCR (see Chapter 39).

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