This part of the subject may be conveniently supplemented by a short description of the manner in which the blastoderm encloses the yolk. It has been already mentioned that the growth of the blastoderm is not uniform. The part of it in the immediate neighbourhood of the embryo remains comparatively stationary, while the growth elsewhere is very rapid. From this it results that that part of the edge of the blastoderm where the embryo is attached forms a bay in the otherwise regular outline of the edge of the blastoderm. By the time that one-half of the yolk is enclosed the bay is a very conspicuous feature (Pl. 9, fig. 1). In this figure _bl._ points to the blastoderm, and _yk._ to the part of the yolk not yet enclosed by the blastoderm. Shortly subsequent to this the bay becomes obliterated by its two sides coming together and coalescing, and the embryo ceases to lie at the edge of the yolk. This stage is represented on Pl. 9, fig. 2. In this figure there is only a small patch of yolk not yet enclosed (_yk_), which is situated at some little distance behind the embryo. Throughout all this period the edge of the blastoderm has remained thickened, a feature which persists till the complete investment of the yolk, which takes place shortly after the stage last figured. In this thickened edge a circular vein arises, which brings back the blood from the yolk-sac to the embryo. The opening in the blastoderm (Pl. 9, fig. 2, _yk._), exposing the portion of the yolk not yet enclosed, may be conveniently called the blastopore, according to Professor Lankester's nomenclature. The interesting feature which characterizes the blastopore in Elasmobranchii is the fact of its not corresponding in position with the opening of the anus of Rusconi. We thus have in Elasmobranchii two structures, each of which corresponds in part with the single structure in Amphioxus which may be called either blastopore or anus of Rusconi, which yet do not in Elasmobranchii coincide in position. It is the blastopore of Elasmobranchii which has undergone a change of position, owing to the unequal growth of the blastoderm; while the anus of Rusconi retains its normal situation. In Osseous Fishes the blastopore undergoes a similar change of position. The possibility of a change in position of this structure is peculiarly interesting, in that it possibly serves to explain how the blastopore of different animals corresponds in different cases with the anus or the mouth, and has not always a fixed situation[179]. Footnote 179: For a fuller discussion of this question vide Self, "A comparison of the early stages of development in vertebrates." _Quart. Journ. of Micr. Science_, July, 1875. [This Edition, No. VI.] EXPLANATION OF PLATES 8 and 9. COMPLETE LIST OF REFERENCE LETTERS.