B, Diagram showing the nature of this infolding.
By a process of infolding, the thicker end is partially invaginated, the middle portion or " hind-body " and the organism may now present a superficial likeness to a cercaria.
More usually, by an infolding of the layer of cells in development, we get three layers under the lens; the front layer is the corneagen layer, and is separated by a membrane from the other two which, more or less, fuse and contain the nerve-end-cells (retinal layer).
As a general rule, the depression caused by the infolding of the surface of the incisor (the " mark ") is obliterated in the first or central incisor at six years, in the second at seven years, and in the third at eight years.