Fig. 7. Transverse section showing the close apposition of the optic stalk (o.s.) and lateral margin of the optic vesicle (o.v.) to the ventral and lateral surface ectoderm. Arrows indicate appositions that prevent movement of crest cells beneath the eye until later stages. The thickening of the lateral surface ectoderm is the lens placode. (Johnston M, Noden D, Hazelton R et al: Origins of avian ocular and periocular tissues. Exp Eye Res 29:27, 1979)