The discovery that pieces below a certain minimum size are incapable of producing a whole organism is of capital importance. It has been pointed out that pieces of the egg of the sea-urchin less than one-sixteenth of the whole do not produce even the gastrula stage. In amphioxus the one-eighth blastomere seems to be near the lower limit of development. It has also been found that there is a lower limit for pieces of adult organisms below which they do not regenerate. This has been shown for hydra, tubularia, planarians, and stentor, and is probably true for all forms. This result is especially interesting in those cases in which the parts contain all the elements necessary to produce a new organism, and come from parts of the body that are totipotent in these respects. It seems certain that the lack of power of development in these cases is due entirely to the smallness of the piece. We can express the idea in another way by stating that a certain volume is necessary in order that a piece may produce the typical organization. This conclusion is important as showing that the organization is something enormously large as compared with the size of the chemical or physical molecules, and even of the crystal molecule. The size of a piece that is at the lower limit of organization is also very much larger than the smallest cells of which the embryo is made up, and this relation is a point in favor of the view that the organization is not simply the resultant of the interaction of the cells, but is something much larger than these cells; and we may even go further, I think, and add that it dominates the cells rather than is controlled by them. In the light of the questions discussed in the preceding pages, we may now attempt to follow out in a more connected way some of the modern views and hypotheses dealing with the problem of development. Hertwig, as we have seen, has opposed the Roux-Weismann hypothesis, and has formulated a view of his own. According to Hertwig, the cleavage divides the egg into equivalent parts,--an idea very similar to that of Pflüger. The cells he regards as units, and the development as the result of the interaction of the cells,--a process that in a way Roux had also assumed to take place between the different parts of the later embryo. Thus, while Hertwig’s hypothesis contains little that is really new, it has selected portions from several already existing hypotheses, and united them into a consistent whole. It has been objected to Hertwig’s view that the interaction of equivalent cells could never account for the introduction of new processes in the development; but if we grant that the cells are never entirely equivalent, whatever their potence may be, this objection can, I think, be met. Hertwig’s chief service has been his destructive criticism of the Roux-Weismann idea of qualitative nuclear division.