A major issue in neural development is determining to what degree detailed regional patterning of the nervous system is owing to self-organizing processes within the neural plate, elicited by general signals from the mesoderm, and what is owing to direct induction by pre-existing regionalized pattern within the mesoderm. This issue can be addressed by recombining parts of the prospective neural plate and Organizer mesoderm in vertical apposition (see ref. 36). In this type of preparation, as in the neural plate isolates described above, care should be taken to separate mesodermal and neural areas cleanly before recombining, so that one does not have crosscontamination of regional tissues. One must also carefully define the regions involved, which is not a trivial task, considering the dramatic movements occurring in both neural and mesodermal tissues. Although the published fate maps can serve as a guide, it is best to check the prospective fates with control marking of tissues. Similar concerns apply to recombination of mediolateral components of the neural and mesodermal tissues.
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