Fig. 4. A section through the lens vesicle of a 10-mm embryo. Complete closure of the vesicle has occurred, and separation from the surface ectoderm is commencing. The posteriorly situated cells of the vesicle are already beginning to elongate into the primary fibers. Note the presence of several epitrichial cells trapped in the lumen of the vesicle. Carnegie embryo 6517. (Smelser GK: Embryology and morphology of the lens. Invest Ophthalmol 4:398, 1965)