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. VI. A COMPARISON OF THE EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES[19]. Footnote 19: From the _Quarterly Journal of Microscopical Science_, Vol. XV. 1875. With Plate 5. If the genealogical relationships of animals are to be mainly or largely determined on embryological evidence, it becomes a matter of great importance to know how far evidence of this kind is trustworthy.