Homozygous tw18 embryos die as the result of abnormalities in mesoderm formation. Analyses of growth rate and mitotic characteristics in the epiblast show that the mutants are small but with high mitotic activity and abnormalities in the orientation of dividing cells. The high mitotic index may be the result of cells arresting in division. These lesions can be identified in pre-primitive streak embryos. The proliferative zone of mutant embryos does not show the mitotic disturbances characteristic of the remaining epiblast and may therefore be the site at which ectodermal precursors are located.