It might be objected to this that it would be simpler not to assume a segregation of the germ-plasm into determinant-complexes at all in order to explain the process of development, but rather to credit each cell with a complete equipment of germ-plasm from the beginning to the end of the ontogeny, and to attribute the differences in the cells, which condition the structure of the plant and its differentiation, solely to the different influences, external and internal, to which the cell is exposed, and which rouse some determinants to activity at one part and others at another. Perhaps the botanists would be more readily reconciled to this idea, but it seems to me that there are two points which tell against the possibility of its being correct. In the first place, it is far from being established that every cell in the higher plants is capable of giving rise, under favourable conditions, to a whole new plant; every tree and every higher plant has a multitude of cells in its leaves, its flowers, and so on, which cannot do this, which are in fact differentiated in one particular direction, that is, they contain only one kind of determinants, like the histologically differentiated cells of the tissues of the human body. Secondly, there are other organisms besides plants, and a theory of development cannot be based on the phenomena to be observed among plants alone, any more than a theory of heredity can. There are obvious differences in the processes of life among plants as contrasted with those among animals, but it is improbable that there is any thoroughly fundamental difference. It is, however, indubitable that the cells forming the tissues of higher animals, the nerve, muscle, and glandular cells, are really differentiated in one direction, and are quite incapable, under any circumstances whatever, of growing into an entire organism, and even from this alone we might conclude that they contain only one primordium or determinant. Are we then to assume that the vascular cells, epidermis-cells, wood-cells, and so on, of the higher plants, which are also differentiated in one direction, do nevertheless contain the complete germ-plasm? I do not see any ground for such an assumption. To conclude what can be said on the subject of regeneration we must return to the question of an ultimate explanation of this marvellous phenomenon. I have declined to attempt any explanation at all, because I do not consider it possible to give a sufficient one as yet, but I should like at least to give an indication as to the direction in which we must look for it.