Permeability Alterations in Amphibian Embryos Caused by Salt Solution and Measured by Tritiated Thymidine Uptake.

IN attempts to label the nuclei of intact amphibian embryos with tritiated thymidine we found, in agreement with other authors (Tencer, 1961; Quertier, 1962; Chibon, 1962), that when the nucleoside is dissolved in tap water or media of low salt concentration it fails to enter the embryo. However, when tritiated thymidine was either injected into the embryo or applied to isolated tissues immersed in salt solution, the marker diffused readily through the tissues. Evidently the outer surface of the amphibian embryo is furnished with a special permeability barrier which prevents or reduces the entrance of these molecules. This barrier corresponds most likely to what Holtfreter (1943a, 1943Z>) has described as the * surface coat'. In order that the results of our present experiments may be better understood some of the pertinent properties of the coat will be reviewed.