The facts of Polychæte embryology inform us that the change of the trochonphore into the perfect worm consists, first of all, in a growing out of the posterior section of its body and a gradual reduction of the anterior part, segmentation appearing at the same time. Thïs phenomenon depends upon a marked change of the mesodermal bands, situated on each side of the intestine. Each of them is separated into two cell-layers, which spread out toward the mid-ventral and mid-dorsal lines. Then a segmentation makes its appearance in them, proceeding from in front backwards, and almost simultaneously the two layers of the bands separate from each other, by the formation of a cavity in each section or segment. That new segments are gradually formed one after the other, and from before backwards, in the hinder part of the growing body, holds true universally, but, at the extreme end a segment has been individualised from the first, or at a very early phase of the segmentation, and this segment does not divide again even to the end of development. In this way the pygidium is formed, and it represents the posterior individual portion of the trochophore. The growth of Polychæte-larvæ is therefore anteanal and entirely in the penultimate segment.