No. 7. Section through the point where the third groove passes into the solid rod of cells. No. 8. Section through the rod when quite separated from the germinal epithelium. No. 9. Section very near the termination of the rod. No. 10. Last section in which any trace of the rod is seen. SERIES B. Sections passing through the head-kidney at our third stage. Zeiss C, ocul. 2. Our figures are representations of the following sections of the series, section 1 being the first which passes through the anterior groove of the head-kidney. No. 1 SECTION 3. No. 8 SECTION 13. " 2 " 4. " 9 " 15. " 3 " 5. " 10 " 16. " 4 " 6. " 11 " 17. " 5 " 8. " 12 " 18. " 6 " 10. " 13 " 19. " 7 " 11. " 14 " 20. The Müllerian duct extends through eleven more sections. The first groove (_gr_1.) extends to No. 3. The second groove (_gr_2.) extends from No. 4 to No. 7. The third groove (_gr_3.) extends from No. 11 to No. 13. The first ridge (_r_1.) extends from No. 2 to No. 5. The second ridge (_r_2.) extends from No. 8 to No. 11. The third ridge (_r_3.) extends from No. 13 backwards through twelve sections, when it terminates by a pointed extremity. FIG. C. Section through the ridge connecting the second and third grooves of the head-kidney of an embryo slightly younger than that from which Series B was taken. Zeiss C, ocul. 3 (reduced one-third). The fold of the germinal epithelium, which gives rise to a deep groove (_x._) external to the head-kidney is well marked. SERIES G. Sections through the rod of cells constituting the termination of the Müllerian duct at a stage in which the head-kidney is still present. Zeiss C, ocul. 2. PLATE 28. SERIES D. Sections chosen at intervals from a complete series traversing the peritoneal opening of the Müllerian duct, the remnant of the head-kidney, and the termination of the Müllerian duct. Zeiss C, ocul. 3 (reduced one-third). Nos. 1 and 2. Sections through the persistent anterior opening of the head-kidney (abdominal opening of Müllerian duct). The approach of the Wolffian duct to the groove may be seen by a comparison of these two figures. In the sections in front of these (not figured) the two are much more widely separated than in No. 1. No. 3. Section through the Müllerian duct, just posterior to the persistent opening. Nos. 4 and 5. Remains of the ridges, which at an earlier stage connected the first and second grooves, are seen passing from the Müllerian duct to the peritoneal epithelium. No. 6. Rudiment of the second groove (_gr_2.) of the head-kidney. Between 6 and 7 is a considerable interval. No. 7. All traces of this groove (_gr_2.) have vanished, and the Müllerian duct is quite disconnected from the epithelium.