Fig. 35B. Longitudinal section through part of the Wolffian body and the anterior end of the ovary of an eighteen days' embryo, to shew the derivation of tubuliferous tissue (_t._) from the Malpighian bodies, close to the anterior extremity of the ovary. Zeiss A, ocul. 1. Fig. 36. Transverse section through the ovary of an embryo rabbit of twenty-two days, hardened in osmic acid. It is coloured in the same manner as fig. 35. Zeiss A, ocul. 2. Fig. 36A. Transverse section through a small part of the ovary of an embryo, from the same female as fig. 36, hardened in picric acid, shewing the relation of the germinal epithelium to the stroma of the ovary. Zeiss D D, ocul. 2. Figs. 37 and 37A. The same parts of an ovary of a twenty-eight days' embryo as figs. 36 and 36A of a twenty-two days' embryo. Fig. 38. Ovary of a rabbit five days after birth, coloured in the same manner as figs. 35, 36 and 37, but represented on a somewhat smaller scale. _Picric acid._ Fig. 38A. Vertical section through a small part of the surface of the same ovary as fig. 38. Zeiss D D, ocul. 2. Fig. 38B. Small portion of the deeper layer of the germinal epithelium of the same ovary as fig. 38. The figure shews the commencing differentiation of the cells of the germinal epithelium into true ova and follicle cells. Zeiss D D, ocul. 2. Fig. 39A. Section through a small part of the middle region of the germinal epithelium of a rabbit seven days after birth. Zeiss D D, ocul. 2. Fig. 39B. Section through a small part of the innermost layer of the germinal epithelium of a rabbit seven days after birth, shewing the formation of Graafian follicles. Zeiss D D, ocul. 2. Figs. 40A and 40B. Small portions of the middle region of the germinal epithelium of a rabbit four weeks after birth. Zeiss D D, ocul. 2. Fig. 41. Graafian follicle with two ova, about to divide into two follicles, from a rabbit six weeks after birth. Zeiss D D, ocul. 2. EXPLANATION OF PLATES 27 AND 28. (XIII. p. 618.) COMPLETE LIST OF REFERENCE LETTERS. _ao._ Aorta. _cv._ Cardinal vein. _gl._ Glomerulus. _gr_1. First groove of head-kidney. _gr_2. Second groove of head-kidney. _gr_3. Third groove of head-kidney. _ge._ Germinal epithelium. _mrb._ Malpighian body. _me._ Mesentery. _md._ Müllerian duct. _r_1. First ridge of head-kidney. _r_2. Second ridge of head-kidney. _r_3. Third ridge of head-kidney. _Wd._ Wolffian duct. _x._ Fold in germinal epithelium. PLATE 27. SERIES A. Sections through the head-kidney at our second stage. Zeiss 2, ocul. 3 (reduced one-third). The second and third grooves are represented with the ridge connecting them, and the rod of cells running backwards for a short distance. No. 1. Section through the second groove. No. 2. Section through the ridge connecting the second and third grooves. No. 3. Section passing through the same ridge at a point nearer the third groove. Nos. 4, 5, 6. Sections through the third groove.