But how does the eye develops?

         In the embryo the eye develops as an outpocketing of the brain. The primary optic vesicles form which later become the retina. Those primary optic vesicles grow outside till they join the ectoderm. Next they become flattened and form the first strage of the eye, which becomes attached to the brain by the streak of cells. This primary eye consist of two layers. The outer mesodermal layer is made up of the choroid and the sclera, the inner is made up of the retina and the pigmental layer. The outer layer will later form the outgrowths and the rod cells of retina. The inner layer will form the nervous cells. The nervous fibres (neuroaxous) of these cells alongate and go through the streak of cells which connects the eye to the brain. They join the synapses and the axons of the visual area of the brain.

         The place in which the primery optic vesicles join the ectoderm thickens and forms a circular mass of cells. This mass of cells becomes later the lens of the eye. The cells of the ectoderm above the forming lens make up the transparent layer of the cornea.

EVOLUTION OF THE EYE

         The skull bones form the eye cavity and serve to protect more than half of the back part of the eye-ball. The brows stop the sweat from getting into the eye, the lids and eyelashes aid in protecting the eye from the front. Tears wash away small foreign objects that enter the eye. A sac lined with an epithelial membrane separates the front of the eye from the eyeball proper and aids in the destruction of the bacteria that may enter from the outside.

         The eyeball is spherical in shape and its diameter is about 25 mm.