The Differentiation of the Crystalline Lens

T H E 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. At present it is, therefore, impossible to decide whether the appearance of this early protein specificity must be regarded as a pre-morphological cell differentiation or whether it occurs synchronously with or after the morphological changes described by McKeehan. One is inclined, however, to assume that processes of chemodifferentiation by which the typical building-blocks of the cells are produced will precede visible morphological changes. It is generally accepted that the changes in the head ectoderm are due to an influence which the eye vesicle exercises upon that ectoderm at the contact surface (embryonic induction). One might consequently assume that the production of specific lens proteins takes place under the influence of an inductive action of the eye vesicle. To investigate this assumption the following experiments were done. Saline extracts of presumptive lens ectoderm of axolotl neurulae (before the appearance of a lens placode) were mixed with rabbit anti-serum against lens proteins. No precipitin reaction could be demonstrated. Neither could a precipitin reaction be found when saline extracts of young eye vesicles (freed from head