has, therefore, been reversed and the eggs have gone back into the resting stage.[177] Wasteneys has found that the rate of oxidation which was raised considerably by the artificial membrane formation goes back to the value characteristic for the resting eggs after the reversal of their developmental tendency.[178] Similar results were obtained in eggs activated with NH₄OH. It appears from this as though the change in the cortical layer which leads to the development of the egg and the increase in the rate of oxidations were reversible in the egg of _Arbacia_.[179] [177] Loeb, _Arch. f. Entwcklngsmech._, 1914, xxxviii., 277. [178] Wasteneys, H., _Jour. Biol. Chem._, 1916, xxiv., 281. [179] F. Lillie thinks that the KCN in this experiment merely inhibits the change of the cortical layer necessary for development. This is contradicted by two facts: first, the writer has shown in 1906 that KCN does not inhibit the membrane formation, and, second, the eggs will not return to the resting stage when put back into sea water too soon; in that case they will disintegrate. This shows that in the KCN something more happens than the mere block to disintegration. The writer had previously noticed that eggs of _Strongylocentrotus purpuratus_, which had been treated for two hours with hypertonic sea water, not infrequently began to divide into two, four, or eight cells (and sometimes more) and then went back into the resting state (except that they possessed the second factor required for development as stated in Chapter V). It may be remarked incidentally that such eggs at the time of cell division contained the centrosomes and astrospheres, and yet went back into a resting state, thus showing that the centrosomes are only transitory organs or organs which are only active under certain conditions. It is quite possible that in these phenomena of reversal not the whole of the cortical layer has undergone alteration. The writer must leave it undecided whether the changes from the resting to the active state in body cells can also be explained in analogy with these experiments. 9. In the formation of the lens we have already noticed an instance where the adjacent organ influences growth inasmuch as the optic cup controlled the formation of the lens. Such influences are quite commonly observed. A piece of _Tubularia_ when cut out from a stem and suspended in water will regenerate at the aboral pole not a stolon but a polyp, so that we have an animal terminating at both ends of its body in a head. The writer called such cases in which an organ is replaced by an organ of a different kind heteromorphosis.