Xenopus laevis cement gland as an experimental model for embryonic differentiation. II. The competence of embryonic cells.

Explants of the blastocoelic roof of Xenopus laevis embryos were incubated for 6 h in Holtfreter solution containing 10 mM ammonium chloride and 10 mM sodium bicarbonate. After this incubation they were transferred for 5 days to Barth's physiological salt solution. Under these conditions the explants underwent differentiation into cement gland tissue. The ability to produce cement gland was highest when explants were dissected from young gastrulae (stage 10). Explants from younger and older embryos displayed much smaller cement glands. There is no difference in the response of ventral or dorsal portions of the blastocoelic roof of stage-10 embryos under these in vitro conditions involving ammonium chloride stimulation. Although the treatment changed the fate of some ectoblastic tissues, it was unable to force endodermal or mesodermal tissues of the blastoporal lip to differentiate into cement gland. The amount of cement gland in the ectoblastic explants varied according to the egg-batch and on the average accounted for 55 percent of the total tissue. The remaining tissue was undifferentiated. Most of this undifferentiated tissue originates from the adjacent layer of the ectoblast. Therefore it may be estimated that the cement gland accounts for 80 to 100 percent of the volume of the competent superficial layer.