2. Lyon[118] tried to ascertain whether by centrifuging the sea-urchin egg it was possible to modify its structure and thereby affect the later embryo. He and subsequent experimenters found that it only is possible to change the position of the nucleus and the distribution of the pigment in the egg. It follows from this that the nucleus and the pigment are suspended in rather fluid material, the former in the centre, the pigment at or near the surface. The position of the nucleus determines the first plane of segmentation, since the nuclear division precedes the division of the cytoplasm of the egg and the plane of nuclear division becomes also the plane of the division of the whole egg--a point which need not be discussed here. It was found, however, by Lyon and the subsequent investigators that the place where the micromeres are formed and where the intestine of the embryo later originates is little influenced by the centrifuging of the egg. The localization of this spot must therefore be determined by a structure sufficiently solid not to be shifted by the centrifugal force. The intestinal stratum in the egg contains the forerunners of the tissues which secrete hydrolyzing enzymes, _e. g._, trypsin into the digestive tract. [118] Lyon, E. P., _Arch. f. Entwcklngsmech._, 1907, xxiii., 151; Morgan, T. H., and Spooner, G. B., _ibid._, 1909, xxviii., 104; Morgan, _Jour. Exper. Zoöl._, 1910, ix., 594; Conklin, E. G., _ibid._, 1910, ix., 417; Lillie, F. R., _Biol. Bull._, 1909, xvi., 54. When the surrounding solution is altered in constitution or when the temperature is too high, the intestine instead of growing into the hollow sphere grows outside, we get an evagination instead of an invagination of the intestine. Such larvæ may live for a few days but they cannot grow into a living organism. The forces which make the intestine grow into the hollow sphere are unknown; it may possibly be only the difference between the tension on the external and internal surfaces of the hollow sphere; under normal conditions, the resistance on the inner surface being smaller, the intestine grows into the hollow sphere. The intestine is one of the organs required for the self-preservation of a more complicated organism, in fact a higher organism without a digestive tract is not capable of living for any length of time. In the gastrula--_i. e._, the blastula with an intestine--we have an organism which is durable, but the processes leading up to the formation of the intestine are so simple that it is difficult to understand why the assumption of a “supergene” should be required in this case.