Observations on the role of ectodermal spreading in the early stages of lens placode invagination in the chick embryo.

The importance of differential epithelial spreading in the early stages of chick lens placode invagination in the chick has been demonstrated experimentally in two ways. Generalized swelling of the head caused by immersion in 50% ethanol and restricting the freedom of ectoderm in the eye region to spread by means of ligatures produce both precocious and exaggerated invagination. Inequality of spreading between the placode and surrounding ectoderm in normal development is caused by the adhesion of the former to the optic vesicle, while the ectoderm is free to spread and move over underlying tissues. Folds, i.e. invaginations, form at the boundary between the placode and its surroundings.