AbstractPolycrylamide gel electrophoresis of chicken lens proteins showed 17 crystallins, divided over three groups. Within each group physicochemical heterogeneity was combined with (partial) immunological homogeneity. It is assumed that more than one gene is involved in the synthesis of any crystallin species. During development of the chicken embryo, α‐crystallin was first demonstrated by immunofluorescence in centrally located lens fibers at 3 days. At 8 days the epithelium became positive and the fibers lost some fluorescence. This continued until in 5‐week‐old chickens the lens core was negative. Lens placode cells showed immunofluorescence for δ‐crystallin at 52 hours, mainly in their basal parts. The reaction gradually spread and at 3 days the entire lens was positive. From 8 days on the epithelium reacted progressively weaker, but the fibers remained positive. Five weeks after hatching, epithelium and cortex were negative, while the center still showed strong fluorescence. The behavior of β‐crystallin was intermediate between that of the other two. Immunoelectrophoresis suggested a differential production onset for the components of each single crystallin type. Under normal conditions no crystallins were found outside the lens. Therefore, crystallin synthesis occurs after placode formation has taken place and must be restricted to the lens itself. Autoradiography after3H‐thymidine treatment indicated that all placode cells still replicate, though some already produce crystallins. A generation time of 8 to 10 hours was determined with an M phase of 30 minutes, an S phase of 6 hours, and a G2of 2 ½ hours. During DNA synthesis the nuclei were located in the basal parts of the cells, and for mitosis they migrated to the lumen. Autoradiography after3H‐glucosamine application suggested that the placode cells take active part in the synthesis of the basement membrane interposed between lens rudiment and optic cup. This membrane later becomes the lens capsule, and in mice with the “shrivelled” gene, abnormal masses of anterior epithelial cells also clearly produce extra capsule material. This results in anterior polar cataracts. Several of the above findings are in disagreement with some of the current theories on the regulation of lens differentiation. No substitutes are prese