Let us try to construct a notion of what this organisation in the developing ovum must be. In the 16-blastomere stage of the sea-urchin egg we have a “system” of parts. In the case of normal development each of these parts has a certain actual fate--it will form a part of the larva into which the embryo is going to develop: It has, as Driesch says, a _prospective value_. But let the normal process be interfered with, and then each of these parts does something else. In the extreme case of interference, when the blastomeres are separated from each other, each blastomere, instead of forming only a part of a larva, forms a whole larva. The _prospective potency_ of the part, that is its possible fate, is greater than its prospective value. Normally it has a limited, definite function in development, but if necessary it may greatly exceed this function. What any one blastomere in the system will become depends upon its position with regard to the other blastomeres. When the egg of the frog is floating freely in water it lies in a certain position with the lighter part uppermost, and then development is normal, each of the two first blastomeres giving rise to a particular part of the body of the larva; that is, each of them is affected by the contact of the other and develops into whatever part of the normal embryo the other does not. But let the egg in the 2-cell stage be turned over and held so that the heavy part is uppermost: the protoplasm then begins to rotate so as to bring the lighter part uppermost; but the two blastomeres do not, as a rule, adjust themselves to the same extent, and at the same rate, and corresponding parts may fail to come into contact with each other. Lacking, then, the normal stimulus of the other part, each blastomere begins to develop by itself, and a double embryo is produced. It is clear, then, both from this case and the last one, that the actual fate of any one part of the system of blastomeres _is a function of its position_. What it will become depends precisely on where it is situated with respect to the other parts. Driesch, then, calls the system of parts in such cases as the 2-cell frog embryo, or the 16-cell sea-urchin embryo, an _equipotential system_, since each part is potentially able to do what any other part may do, and what the whole system may do. But in normal development each part has a definite fate and its activity is co-ordinated with that of all the other parts. It is, therefore, an _harmonious equipotential system_, each part acting in harmony, and towards a definite result, with all the others; although if necessary it can take the place of _any_ or _all_ of the others.