ens 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.

"Lewis has shown that when the optic cup is transplanted into any other
place under the epithelium of a larva of a frog the epithelium will
always grow into the cup where the latter comes in contact with the
epithelium; and that the ingrowing part will always become transparent."
A most remarkable and interesting experiment; it has this very important
limitation--that it is always _epithelium_ with which it has to do,
whereas in Wolff's experiment the regeneration takes place from
mesoblastic tissue. The cause of the transparency may be a chemical
reaction--it depends a good deal upon our definition of that phrase. Is
protoplasm a chemical compound? Some have considered it so, and spoken
of its marvellously complicated molecule. Of course it is made up of
carbon, hydrogen, and other substances within the domain of chemistry.
But is it, therefore, merely a chemical compound? The reply involves the
whole riddle of Vitalism. The author would say that it, as well as all
the living things to which it belongs, is purely and solely a chemical
compound; and he must take the consequences of his belief. One of these
consequences, from which doubtless he would not shrink, would be that a
super-chemist (so to speak) could write him and his experiments and his
book down in a series of chemical formulae--a consequence which takes a
good deal of believing. But it also involves him in a belief in the
rigidity of chemical reactions; and we are entitled to ask for an
explanation of the identical behaviour of the chemical reaction in
connection with epiblastic and mesoblastic cells--both pure chemical
compounds _ex hypothesi_ and, as far as we can tell from their normal
behaviour, widely differing from one another. The optic cup,