The scleral ossicles, a ring of membrane bones surrounding the cornea of the domestic fowl are foreshadowed in early development by transient thickenings in the overlying conjuctiva, the conjunctival papillae. Each ossicle is induced by its corresponding papilla and develops in a collagen-bearing bed deposited beneath the papilla. L-azetidine-2-carboxylic acid (LACA) an analog of proline, was injected into the chorioallantoic veins of chick embryos to transiently disrupt collagen synthesis. Embryos treated at 5 or 6 days of incubation (but not those treated before or after this interval) developed retarded papillae and later developed ossiclular rings lacking one or more ossicles. Control embryos, injected with D-azetidine-2-carboxylic acid or with water, developed normally. Thus, the action of LACA in aborting the induction of scleral ossicles is age-restricted and stereoisomerically-specific.