This very early appearance of distinction in the blastoderm between the end at which the embryo will appear, and the non-embryonic end is important, especially as it shews the affinity of the modes of development of Osseous fishes and the Elasmobranchii. Oellacher (_Zeitschrift für Wiss. Zoologie_, Vol. XXXIII. 1873) has shewn, and, though differing from him on many other points, on this point Götte (_Arch. für Micr. Anat._ Vol. IX. 1873) agrees with him, that a similar absence of symmetry by which the embryonic end of the germ is marked off, occurs almost immediately after the end of segmentation in Osseous fishes. In the early stages of development there are a number of remarkable points of agreement between the Osseous fish and the Dog-fish, combined with a number of equally remarkable points of difference. Some of these I shall point out as I proceed with my description. The embryonic end of the germ is always the one which points towards the pole of the yolk farthest removed from the egg-capsule. The germ grows, but not very rapidly, and without otherwise undergoing any very appreciable change, for some time. The growth at these early periods appears to be particularly slow, especially when compared with the rapid manner in which some of the later stages of the development are passed through. The next important change which occurs is the formation of the so-called "segmentation cavity." This forms a very marked feature throughout the early stages. It appears, however, to have somewhat different relations to the blastoderm than the homologous structure in other vertebrates. In its earliest stage which I have observed, it appears as a small cavity in the centre of the lower layer cells. This grows rapidly, and its roof becomes composed of epiblast and only a thin lining of "_lower layer_" cells, while its floor is formed by the yolk (Pl. 3, fig. 3, _sg_). In the next and third stage (Pl. 3, fig. 4, _sg_) its floor is formed by a thin layer of cells, its roof remaining as before. It has, however, become a less conspicuous formation than it was; and in the last (fourth) stage in which it can be distinguished it is very inconspicuous, and almost filled up by cells. What I have called the second stage corresponds to a period in which no trace of the embryo is to be seen. In the third stage the embryonic end of the blastoderm projects outwards to form a structure which I shall speak of as the "embryonic rim," and in the fourth and last stage a distinct medullary groove is formed. For a considerable period during the second stage the segmentation cavity remains of about the same size; during the third stage it begins to be encroached upon, and becomes smaller both absolutely, and relatively to the increased size of the germ. The segmentation cavity of the Dog-fish most nearly agrees with that of Osseous fishes in its mode of formation and relation to the embryo.