In early embryonic epithelia most cell divisions are oriented with their cleavage furrows perpendicular to the free surface of the tissue. This results in the generation of two daughter cells, which are equivalent with regard to their apical surface. Mitotic figures, in which the long axis through the centers of the chromosome plates (chromatin masses) is perpendicular to the surface, are seen only occasionally. The lens placode and early lens cup of mouse embryos homozygous for the gene aphakia show significantly greater numbers of maloriented mitoses than normal control lenses. The axis connecting the centers of the chromosomal plates is frequently perpendicular or oblique to the surface. This may result in the production of daughter cells, which are unequal as far as their position at the tissue surface is concerned. Later, the lumen of the mutant lens vesicle fills up with cells, which are apparently released from the lens epithelium. Further development of the lens and the eye is grossly disturbed. The release of these cells may be a consequence of the malorientation of their divisions, or both may be the result of a defect in the spatial organization of the lens tissue, possibly due to abnormal cellular adhesion.