ISOLATED DORSAL ANIMAL BLASTOMERES OF XENOPUS-LAEVIS ARE CAPABLE TO FORM MESODERMAL DERIVATIVES, WHILE THE VENTRAL ANIMAL BLASTOMERES DIFFERENTIATE INTO CILIATED EPIDERMIS ONLY

Isolated two dorsal animal blastomeres of 8-cell-stage Xenopus embryos differentiated in about 24% of the cases into mesodermal structures, while the two Ventral animal ones formed exclusively atypical epidermi s. Of special interest is the fact that most of the dorsal animal blas tomeres without mesodermal structures, differentiated into atypical ep idermis with large parts of cement gland. Cement glands could not be d etected in the derivatives of the ventral animal blastomeres. In earli er concepts it has been suggested that the animal hemisphere of the am phibian egg is an uncommitted area, which receives its instructions fo r further determination and differentiation from the vegetal part of t he embryo. However, the results, shown in this paper, support the view that the developmental determinants are distributed in distinct gradi ents already in the early cleavage stages. Although the highest concen trations of these putative determinants are located in the dorsal vege tal area (Melton, 1995) and the dorsal marginal zone, lower concentrat ions of these substances are thought to be present in the animal part, especially in the dorsal animal hemisphere of the egg and the early e mbryo. Our results indicate that dorsal animal blastomeres of early cl eavage stage embryos possess the capacity to form mesodermal derivativ es after their separation from the vegetal hemisphere.