The hypothesis that Pflüger, Hertwig, and Driesch have adopted, namely, that the cleavage divides the egg into potentially equal parts, stands in sharp contrast to the Roux-Weismann conception of development. There are two ideas in the former view which should be kept, I think, clearly apart: the first is, that the blastomeres are potentially equal (isotropous), because they are exactly alike; the second is, that despite the differences that may exist amongst them they are still potentially able to do the same thing, _i.e._ they are totipotent. The former alternative is that adopted by Pflüger, Hertwig, and Driesch; the latter view, to which Driesch seems more inclined in his later writings, is the one that I should prefer.[126] The first four blastomeres of the sea-urchin’s egg appear to be exactly alike, and we find that each can make a whole embryo. If we assume, however, that despite their likeness and their totipotence they are different in so far as there is present in the protoplasm a bilateral structure, we are nearer, in my opinion, to the truth; for, unless we assume the bilateral structure to be determined later by some external factor, of which there is no evidence, we must suppose that after fertilization, at least, there must be a bilateral structure to the protoplasm, and this view is borne out in one sense by the subsequent mode of cleavage of the blastomeres if they are separated. Whether this bilaterality of the fertilized egg leads to the bilaterality of the cleavage is, however, a different question. In some cases this _appears_ to be the case, in others it is clearly not the case, and we must suppose that some other condition determines the bilaterality of the later stages than that which influences the cleavage. Many facts of experimental embryology and of regeneration show, moreover, that a new bilateral structure may be readily assumed by pieces that have lost their connection with the rest of the organism.