AN ELECTRON MICROSCOPIC STUDY OF CHORDAMESODERM-NEURECTODERM ASSOCIATION IN GASTRULAE OF A TOAD, XENOPUS-LAEVIS

Gastrulae from South African Clawed Toads, Xenopus laevis, were fixed in phosphate buffered glutaraldehyde, postfixed with Os O4 and prepared for electron microscopy. Mesodermal cells did not contact cells of the ectodermal layer. Furthermore, unexpected quantities of extracellular material filled the space between layers during early stages of gastrulation. Extracellular material consisted of glycogen, ribonucleoprotein particles, and fibrillar material, similar to material present in lobes of mesodermal cells during the first half of gastrulation. Mesodermal lobes and extracellular materials were also present in ectodermal sandwiches incubated with mesoderm for three hours, but were absent in explants cultured less than three hours. Extracellular materials were also absent in ectodermal sandwiches cultured with presumptive epidermis (non‐inducing cells) for three hours. Ectodermal cells appear to incorporate fibrillar material from the interspace by pinocytosis during late stages of gastrulation. Autoradiographs suggest that labeled RNA is not transferred to ectoderm during gastrulation, but is involved in protein synthesis within mesodermal cells. The product of this synthesis might be visualized as extracellular fibrillar material. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company