"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, or its contained fluid, is one chemical compound; epithelium is another; mesoblast is a third. We want an explanation of the identical behaviour of the first with _either_ of the two latter; and this should be borne in mind--that the reaction is not a mere matter of "clearing" of a tissue as the histologist would clear his section by oil-of-cloves or other reagent, but of the construction of a different type of cell--epithelial, not connective tissue.