ABSTRACT
At stage 23 the ventral and lateral walls of the primary eye vesicle become thicker in comparison with its dorso-medial wall. The primary eye vesicles have been fully developed at stage 25, but do not show any sign of invagination before stage 26, at which local protrusions develop on the inner surface of the lateral wall. The eye cup deepens during the following stages and the margins of the optic cup forming the choroid fissure have made contact with each other at stage 32. With the beginning of invagination of the retinal layer of the eye cup, the overlying sensorial layer of the ectoderm, which is in intimate contact with the retinal layer, begins to thicken as a first sign of lens formation. The first pigment granules become visible in the thin outer layer of the optic cup at stage 32.
