Let us take one particular example, a very remarkable one, which has been cited by both writers--Wolff's experiment on the lens of the eye. The lens is just behind the pupil or central aperture in the iris or coloured ring at the front of the eye, and behind the cornea which is to the eye what a watch-glass is to a watch. If the lens of the eye be removed from a newt, as it is from human beings in the operation for cataract, the animal will grow another one. How does it do it? In certain cases a tiny fragment of the lens has been left behind after the operation, and the new one grows from that. This is sufficiently wonderful, but by no means so wonderful as what happens in other cases in which the entire lens has been removed and the new lens grows from the outer pigmented layer of the margin of the iris. To the unbiological reader one source of origin will not seem more wonderful than the other, but there is really a vast distinction between them. At an early stage in the development of the embryo, the cells composing it become divisible into three layers. It is even possible, as Loeb maintains, that this differentiation is present in the unsegmented ovum, in which case the facts to be detailed become still more remarkable and significant. These layers are known as epi-, meso-, and hypo-blast; and from each one of them arise certain portions of the body, and certain portions only. It would be as remarkable to a biologist to find these layers not breeding true as it would to a fowl-fancier to discover that the eggs of his Buff Orpingtons were producing young turkeys or ducks. Now the lens is an epiblastic structure, and the iris is mesoblastic. Hence the wonder with which we are filled when we find the iris growing a lens. Loeb attempts to explain this in the first instance by telling us that the cells of the iris cannot grow and develop as long as they are pigmented; that the operation wounds the iris, allows pigment to escape, and thus permits of proliferation. We may accept this, and yet ask why it takes on a form of growth familiar to us only in connection with epiblast? The reply is: "Young cells when put into the optic cup always become transparent, no matter what their origin; it looks as if this were due to a chemical influence, exercised by the optic cup or by the liquid it contains.