The distribution of mesoderm, the structure of mesoderm cells and relationship between mesoderm and ectoderm were examined by SEM in embryos at stages 3 to 5. The mesoderm was displayed by removal of the endoderm and by fracturing the embryos through mesoderm containing regions. Within the mesoderm layer four zones could be distinguished by their cell shape and arrangement--the primitive streak, a multilayered compact area around the margins of the area pellucida, multilayered and loosely arranged cells near the primitive streak and a flattened monolayer of cells around the advancing lateral and anterior edges of the mesoderm sheet. Secretion of basement membrane by the ectoderm was seen to precede the arrival of mesoderm cells. This suggests that ectoderm alone can synthesize basement membrane without mesodermal contribution.