Changes in the cell coat at the onset of gastrulation in Xenopus laevis embryos.

Four developmental stages of Xenopus laevis embryos were fixed in standard electron microscopic fixatives with 1% lanthanum nitrate added. This treatment reveals a lanthanum staining material (LSM) on the cell surface and in extracellular spaces. Morula and blastula stage cells have a small amount of LSM; in blastulae, 3 +/- 3 SD % of the cell surface is coated with LSM. In early gastrulae, 29 +/- 6 SD % of the cell periphery is covered with LSM. In late gastrulae, 82 +/- 13 SD % of the cell periphery is either coated with LSM or faces a space with LSM in it. There is also an appreciable accumulation of LSM within extracellular spaces during gastrulation.