The Differentiation of the Crystalline Lens

The first sign of lens development in the vertebrate embryo is the appearance of a thickening of the head ectoderm in the area of contact between the ectoderm and the eye vesicle (lens placode). The cytoplasm of the placode cells in the chicken embryo loses its vacuolization, the cells acquire a cylindrical form, the nuclei arrange themselves perpendicularly to the contact surface and move to the base of the cells (McKeehan, 1951). At about the same time chemical changes take place in the placode cells. Ten Cate and Van Doorenmaalen (1950), using serological methods, were able to demonstrate the presence of specific lens proteins in the placodal stage of the lens in axolotl and chicken embryos. They could not entirely exclude the possibility of the presence of these proteins in the ectoderm at earlier stages, but with their sensitive method they got no indication of them before the placodal stage.