The third groove (gr3.) extends from No. 11 to No. 13.
The first ridge (r1.) extends from No. 2 to No. 5.
The second ridge (r2.) extends from No. 8 to No. 11.
The third ridge (r3.) 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.
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.