At present, however, I see no other probable inference from the facts. If we suppose a bilateral structure is present in the fertilized egg, and that it corresponds to the first plane of cleavage, a change of the sort that we have just sketched above may be supposed to take place when the blastomeres are separated. The stimulus is found in the new spherical form assumed by the isolated blastomere, and we may imagine the change to take place, in the way indicated, by virtue of the old bilaterality that is present, the change beginning at the side originally in contact with the other half. There are several facts which seem to indicate that a change in the axial relations of the egg is very easily brought about before any definite organs have appeared. The fact that the point of entrance of the spermatozoon in the egg of the frog[127] and of the sea-urchin[128] may determine the first plane of cleavage points to this conclusion. The fact that, in the frog, and also in the triton, the median plane of the embryo corresponds sometimes to the first, sometimes to the second plane of cleavage, and sometimes to neither one, shows that the bilaterality of the embryo-structure may or may not coincide with the plane of cleavage. In the fish also there seems to be no correspondence between the planes of cleavage and those of the embryo, so that different factors may determine the two. We should not be justified in concluding from this evidence that a bilateral structure is absent, but rather that it is of such a sort as to be independent of the cleavage, and that it can be also easily changed. It is probable that the kind of organization that we must suppose to exist in the egg is of a very simple sort, and capable of easy readjustment. There is certainly no evidence in favor of the view that the organization of the egg need be anything like the organization of the embryo that comes from the egg, although the organization of the egg may be perfectly definite in its character. Until we know more of the nature of this organization, it is useless to speculate further as to how it can be altered.