The figures further shew (1) the formation of the spinal nerves (_spn_) as small bodies of cells closely applied to the upper and outer edge of the neural canal. (2) The commencing formation of the cells which form the axial skeleton from the inner (splanchnopleuric) layer of the muscle-plate. Sections _b_ and _c_ are given more especially to shew the mode of formation of the oviduct (_ov_). In _b_ it is seen as a _solid knob (ov)_, arising from the point where the somatopleure and splanchnopleure unite, and in _c_ (the section behind _b_) as a _solid rod (ov)_ closely applied to the epiblast, which has grown backwards from the knob seen in _b_. N.B. In all three sections only one side is completed. Fig. 12_a_ and _b_. Two transverse sections of an embryo just before the appearance of the external gills. (Magnified 96 diam.) In _a_ there is seen to be an involution on each side (_pwd_), while _b_ is a section from the space between two involutions from the pleuro-peritoneal cavity, so that the Wolffian duct (at first solid) (_wd_) is not connected as in _a_ with the pleuro-peritoneal cavity. The further points shewn in the sections are-- (1) The commencing formation of the spiral valve (_al_). (2) The suprarenal body (_sur_). (3) The oviduct (_ov_), which has acquired a lumen. (4) The increase in length of the muscle-plates, the spinal nerves, &c. Fig. 13. Section through the dorsal region of an embryo in which the external gills are of considerable length. (Magnified 40 diam.) The chief points to be noticed: (1) The formation of the Wolffian body by outgrowths from the Wolffian duct (_wd_). (2) One of the still continuing connections (primitive involutions) between the Wolffian duct and the pleuro-peritoneal cavity (_pwd_). (3) The oviduct largely increased in size (_ov_). N.B. On the left side the oviduct has been accidentally made too small. (4) The growth downwards of the muscle-plate to form the muscles of the abdomen. (5) The formation of an outgrowth on each side of the mesentery (_pov_), which will become the ovary. (6) The spiral valve (_al_). Fig. 14. Transparent view of the head of an embryo shortly before the appearance of the external gills. (Magnified 20 diam.) The chief points to be noticed are-- (1) The relation of the cranial nerves to the visceral clefts and the manner in which the glosso-pharyngeal (_gl_) and vagus (_vg_) are united. (2) The remnants of the pleuro-peritoneal cavity in the head (_pp_). (3) The eye (_op_). The stalk, as well as the bulb of the eye, are supposed to be in focus, so that the whole eye has a somewhat peculiar appearance. EXPLANATION OF PLATE 5. (VI. p. 112.) COMPLETE LIST OF REFERENCE LETTERS.