During this stage the distribution of these nuclei in the yolk becomes somewhat altered from that in the earlier stages. Although the nuclei are still scattered generally throughout the finer yolk-matter around the blastoderm, yet they are especially aggregated at one or two points. In the first place a special collection of them may be noticed immediately below the floor of the segmentation cavity. They here form a distinct row or even layer. If the presence of this layer is coupled with the fact that at this period cells are beginning to appear on the floor of the segmentation cavity, a strong argument is obtained for the supposition that around these nuclei cells are being produced, which pass into the blastoderm to form the floor. Of the actual formation of cells at _this_ period I have not been able to obtain any satisfactory example, so that it remains a matter of deduction rather than of direct observation. Another special aggregation of nuclei is generally present at the periphery of the blastoderm, and the same amount of doubt hangs over the fate of these as over that of the previously mentioned nuclei. The next stage is the most important in the whole history of the formation of the layers. Not only does it serve to shew, that the process by which the layers are formed in Elasmobranchii can easily be derived from a simple gastrula type like that of Amphioxus, but it also serves as the key by which other meroblastic types of development may be explained. At the very commencement of this stage the embryonic swelling becomes more conspicuously visible than it was. It now projects above the level of the yolk in the form of a rim. At one point, which eventually forms the termination of the axis of the embryo, this projection is at its greatest; while on either side of this it gradually diminishes and finally vanishes. This projection I propose calling, as in my preliminary paper[129], the embryonic rim. Footnote 129: _Qy. Journal Microsc. Science_, Oct. 1874. [This Edition, No. V.] The segmentation cavity can still be seen from the surface, and a marked increase in the size of the blastoderm may be noticed. During the stage last described, the growth was but very slight; hence the rather sudden and rapid growth which now takes place becomes striking. Longitudinal sections at this stage, as at the earlier stages, are the most instructive. Such a section on the same scale as Pl. 7, fig. 4, is represented in Pl. 7, fig. 5. It passes parallel to the long axis of the embryo, through the point of greatest development of the embryonic ring.