" 33. Development of paired fins of Elasmobranchii 714 " 34.} " 35.} " 36.} " 37.} " 38.} Structure and development of Lepidosteus 738 " 39.} " 40.} " 41.} " 42.} " 43.} " 44.} Germinal layers of the Chick 854 " 45.} " 46.} " 47.} " 48.} " 49.} Anatomy and development of Peripatus Capensis 871 " 50.} " 51.} " 52.} " 53.} EXPLANATION OF PLATES. EXPLANATION OF PLATE 1. Figs. 1-5 and 9-12. (II. p. 29.) Fig. 1. Section through an unincubated blastoderm, shewing the upper layer, composed of a single row of columnar cells, and the lower layer, composed of several rows of rounded cells in which no nucleus is visible. Some of the "formative cells," at the bottom of the segmentation cavity, are seen at (_b_). Fig. 2. Section through the periphery of an eight hours' blastoderm, shewing the epiblast (_p_), the hypoblast (_h_), and the mesoblast commencing to be formed (_c_), partly by lower-layer cells enclosed between the epiblast and hypoblast, and partly by formative cells. Formative cells at the bottom of the segmentation cavity are seen at _b_. At _s_ is one of the side folds parallel to the primitive groove. Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated with silver nitrate, shewing the great variation in the size of the cells at this period. An hour-glass shaped nucleus is seen at _a_. Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for cell the junction between the hypoblast (_h_) and white-yolk spheres (_w_). Fig. 5. Junction between the white-yolk spheres and the hypoblast cells at the passage from the area pellucida to the area opaca. The specimen was treated with silver nitrate to bring out the shape of the cells. The line of junction between the opaque and pellucid areas passes diagonally. Fig. 9. Section through the primitive streak of an eight hours' blastoderm. The specimen shews the mesoblast very much thickened in the immediate neighbourhood of the primitive streak, but hardly formed at all on each side of the streak. It also shews the primitive groove just beginning to be formed (_pr_), and the fusion between the epiblast and the mesoblast under the primitive groove. The hypoblast is completely formed in the central part of the blastoderm. At _f_ is seen one of the side folds parallel to the primitive groove. Its depth has been increased by the action of the chromic acid. Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo, treated with silver nitrate, shewing the regularity and elongated shape of the cells over the embryo and the smaller cells on each side. Fig. 11. Epiblast cells from an unincubated blastoderm, treated with silver nitrate, shewing the regular hexagonal shape of the cells and the small spherules they contain.