99_a_. The patient labors of many observers (how patient only those can conceive who have made such observations!) have detected something of this wondrous history, and enabled the mind to picture some of the incessant separations and reunions, chemical and morphological. Each stage of evolution presents itself as the consequence of a preceding stage, at once an emergence and a continuance; so that no transposition of stages is possible; each has its appointed place in the series (PROBLEM I. § 107). For in truth each stage is a _process_--the sum of a variety of co-operant conditions. We, looking forward, can foresee in each what it will become, as we foresee the man in the lineaments of the infant; but in this prevision we always presuppose that the regular course of development will proceed unchecked through the regular succession of special conditions: the infant becomes a man only when this succession is uninterrupted. Obvious as this seems, it is often disregarded; and the old metaphysical conception of _potential_ powers obscures the real significance of Epigenesis. The potentiality of the cells of the germinal membrane is simply their capability of reaching successive stages of development under a definite series of co-operant conditions. We foresee the result, and personify our prevision. But that result will not take place unless all the precise changes that are needful serially precede it. A slight pressure in one direction, insufficient to alter the chemical composition of the tissue, may so alter its structure as to disturb the regular succession of forms necessary to the perfect evolution. 100. The egg is at first a microscopic cell, the nucleus of which divides and subdivides as it grows. The egg becomes a hollow sphere, the boundary wall of which is a single layer of cells, all so similar that to any means of appreciation we now possess they are indistinguishable. They are all the progeny of the original nucleus and yolk, or cell contents. Very soon, however, they begin to show distinguishable differences, not perhaps in _kind_, but in _degree_. The wall of this hollow sphere is rapidly converted into the _germinal membrane_, out of which the embryo is formed. Kowalewsky (confirmed by Balfour) has pointed out how in the Amphioxus the hollow sphere first assumes an oval shape, and then, by an indentation of the under side, with corresponding curvature of the upper side, presents somewhat the shape of a bowl. The curvature increases, and the curved ends approaching each other, the original cavity is reduced to a thin line separating the upper from the under surface. The cavity of the body is formed by the curving downwards of this double layer of the germinal membrane.