Its earliest appearance is involved in some obscurity, though it probably arises as a simple cavity in the midst of the lower layer cells (Pl. 7, fig. 1). In its second phase the floor ceases to be formed of lower layer cells, and the place of these is taken by the yolk, on which however a few scattered cells still remain (Pl. 7, figs. 2, 3, 4). During the third period of its history, a distinct cellular floor is again formed for it, so that it comes a second time into the same relations with the blastoderm as at its earliest appearance. The floor of cells which it receives is in part due to a growth inwards from the periphery of the blastoderm, and in part to the formation of fresh cells from the yolk. Coincidently with the commencing differentiation of hypoblast and mesoblast the segmentation cavity grows smaller and vanishes. One of the most important features of the segmentation cavity in the Elasmobranchii which I have studied, is the fact that throughout its whole existence its roof is formed of _lower layer cells_. There is not the smallest question that the segmentation cavity of these fishes is the homologue of that of Amphioxus, Batrachians, etc., yet in the case of all of these animals, the roof of the segmentation cavity is formed of epiblast only. How comes it then to be formed of lower layer cells in Elasmobranchii? To this question an answer was attempted in my paper, "Upon the Early Stages of the Development of Vertebrates[133]." It was there pointed out, that as the food material in the ovum increases, the bulk of the lower layer cells necessarily also increases; since these, as far as the blastoderm is concerned, are the chief recipients of food material. This causes the lower layer cells to encroach upon the segmentation cavity, and to close it in not only on the sides, but also above; from the same cause it results that the lower layer cells assume, from the first, a position around the spot where the future alimentary cavity will be formed, and that this cavity becomes formed by a simple split in the midst of the lower layer cells, and not by an involution. Footnote 133: _Quart. Journ. of Microscop. Science_, July, 1875. [This Edition, No. VI.] All the most recent observations[134] on Osseous Fishes tend to shew that in them, the roof of the segmentation cavity is formed alone of epiblast; but on account of the great difficulty which is experienced in distinguishing the layers in the blastoderms of these animals, I still hesitate to accept as conclusive the testimony on this point. Footnote 134: Oellacher, _Zeit. f. Wiss. Zoologie_, Bd. XXIII. Götte, _Archiv f. Mikr. Anat._ Vol. IX. Haeckel, _loc. cit._