Wilson (’94) has also rejected Roux’s hypothesis of qualitative nuclear division, and adopts the view of the totipotence of the early blastomeres. He has also advanced the view that there is during development a progressive differentiation of the cells. In a later contribution (’96), he accepts “the view of Hertwig and of Driesch that the various degrees of partial development beginning with the echinoderm egg and culminating in the gasteropod may be due to varying conditions of the egg cytoplasm in the different forms.” Wilson points out that the series of forms represented at one end by amphioxus and at the other end by the ctenophore and the gasteropod may be brought under a common point of view, “for it is certain that development must be fundamentally of the same nature throughout the series, and the differences must be of secondary moment.” If we reject, as several students of experimental embryology and of regeneration have done, the Roux-Weismann idea of the existence of pre-formed germs in the nucleus, and also the idea of Hertwig of the equivalency of the first-formed blastomeres, and Driesch’s vitalistic principle, what position can we take in regard to the problem of development? We may at least attempt to formulate our present position. There must be assumed to exist in the egg an organization of such a kind that it can be divided and subdivided during the cleavage without thereby losing its primary character. The refusion of the cells after each division by means of protoplasmic connections indicates how this may be possible. The organization must be thought to be of such a kind that the factors determining the cleavage may be different from those that determine the median plane of the body. This is demonstrated by Pflüger’s experiment in which the position of the cleavage planes is changed, but the embryo appears in relation to the primary meridians. The first-formed blastomeres, that result from the division of the egg, do not seem to be strictly equivalent, but they appear to be in most cases, at least, totipotent. The characteristics of each part of the protoplasm may be a factor in determining what sort of structure may come from that part of the egg, but back of this lies the fundamental character of the protoplasm itself, that determines what each part, in its relation to the whole, can do. The division of the nucleus appears to be in all cases an exact quantitative division, and there is some evidence to show that the early nuclei are all equivalent,--or at least totipotent. The division of the protoplasm is often into unlike parts, and the kind of cytoplasm contained in a part may or may not limit the potencies of each part.