Specification and Establishment of Dorsal‐Ventral Polarity in Eggs and Embryos of Xenopus laevis

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Abstract
Dorsal-ventral (D-V) polarization in Xenopus eggs and embryos is achieved by passing through a series of complicated phenomena such as initial specification of the polarity before first cleavage, establishment of polarity during cleavage stages resulting in an acquisition of a unique developmental capacity by each blastomere, regional differentiation of mesoderm, and finally neural induction by Spemann''s organizer. In order to gain an insight into basic mechanisms which govern D-V polarization, experimental modifications or perturbations of the body axis of embryos, including physical or chemical treatments of eggs, altered orientation of eggs under the normal gravity, centrifugation, manipulation of blastomeres, cytoplasmic withdrawal, and bisection or partial ligation of fertilized eggs are reviewed: all data are consistent with the concept that a cytoplasmic activity which becomes localized in the dorsal side of the egg is responsible or indispensable for the establishment of the D-V axis. The cytoplasmic activity is tentatively called "anterodorsal structure-forming activity". A model which explains the specification, establishment, and realization of D-V polarity in Xenopus laevis is proposed.