In all vertebrates the ocular lens is a cellar structure that develops initially as a hollow spherical invagination of the surface ectoderm. ' The hollow sphere becomes filled by elongating posterior cells, the primary lens fibers. Subsequent growth takes place at the periphery through the anterior-posterior growth of new cells formed at the equator, the secondary lens fibers. The continued peripheral growth of the lens, resulting in the compression of the older tissue toward the centre, has the important optical consequence of producing a lens of variable refractive index, the index being highest at the centre. This variation in index is an important means by which lens spherical aberration is controlled.