In the woodcut, fig. 1, _A_, _B_, _C_[159], are represented three diagrammatic longitudinal sections of an Elasmobranch embryo. _A_ nearly corresponds with the longitudinal section represented on Pl. 7, fig. 4, and _B_ with Pl. 7, fig. 7. In Pl. 7, fig. 7, the segmentation cavity has however completely disappeared, while it is still represented as present in the diagram of the same period. If these diagrams, or better still, the woodcuts fig. 2 _A_, _B_, _C_ (which only differ from those of the Elasmobranch fish in the smaller amount of food-yolk), be compared with the corresponding ones of Bombinator, fig. 3, _A_, _B_, _C_, they will be found to be in fundamental agreement with them. First let fig. 1, _A_, or fig. 2, _A_, or Pl. 7, fig. 4, be compared with fig. 3, _A_. In all there is present a segmentation cavity situated not centrally but near the surface of the egg. The roof of the cavity is thin in all, being composed in the Amphibian of epiblast alone, and in the Elasmobranch of epiblast and _lower layer cells_. The floor of the cavity is, in all, formed of so-called yolk (vide Pl. 7, fig. 4), which in all forms the main mass of the egg. In the Amphibian the yolk is segmented, and, though it is not segmented in the Elasmobranch, it contains in compensation the nuclei so often mentioned. In all, the sides of the segmentation cavity are formed by lower layer cells. In the Amphibian the sides are enclosed by smaller cells (in the diagram) which correspond exactly in function and position with the lower layer cells of the Elasmobranch blastoderm. Footnote 159: This figure, together with figs. 2 and 3, are reproduced from my paper upon the comparison of the early stages of development in vertebrates. [Illustration: FIG. 1. Diagrammatic longitudinal sections of an Elasmobranch embryo. _Epiblast_ without shading. _Mesoblast_ black with clear outlines to the cells. _Lower layer cells_ and _hypoblast_ with simple shading. _ep._ epiblast. _m._ mesoblast. _al._ alimentary cavity. _sg._ segmentation cavity. _nc._ neural canal. _ch._ notochord. _x._ point where epiblast and hypoblast become continuous at the posterior end of the embryo. _n._ nuclei of yolk. _A._ Section of young blastoderm, with segmentation cavity in the middle of the lower layer cells. _B._ Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary slit has appeared. The segmentation cavity is still represented as being present, though by this stage it has in reality disappeared. _C._ Older blastoderm with embryo in which neural canal has become formed, and is continuous posteriorly with alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast.] [Illustration: FIG. 2. Diagrammatic longitudinal sections of embryo, which develops in the same manner as the Elasmobranch embryo, but in which the ovum contains far less food-yolk than is the case with the Elasmobranch ovum.