EXPLANATION OF PLATE IT. 



Series D. — Sections chosen at intervals from a complete series tra- 

 versing the peritoneal opening of the Miillerian duct, the remnant of the 

 head-kidney, and the termination of the Miillerian duct. Zeiss, c, ocul. 3 

 (reduced one-third). 



Nos. I and 3. — Sections tlirough the persistent anterior opening of the 

 head -kidney (abdominal opening of Miillerian 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 Miillerian duct, just posterior to the per- 

 sistent opening. 



Nos. 4 and 5. — Kemains of the ridges, which at an earlier stage connected 

 the first and second grooves, are seen passing from the Miillerian duct to 

 the peritoneal epithelium. 



No. 6. — Rudiment of the second groove i^gr^?^ of the head-kidney. 



Between 6 and 7 is a considerable interval. 



No. 7. — All traces of this groove {gr^?} have vanished, and the Miillerian 

 duct is quite disconnected from the epithelium. 



No. 8. — Hudiment of the third groove {gr^.). 



No. 9. — Miillerian duct quite free in the space between the peritoneal 

 epithelium and the Wolffian duct, in which condition it extends until near 

 its termination. 



Between Nos. 9 and 10 is an interval of eight sections. 



No. 10. — The penultimate section, in which the Miillerian duct is seen. 

 A lumen cannot be clearly made out. 



No. 11. — The last section in which any trace of the Miillerian duct is 

 visible. No line of demarcation can be seen separating the solid end of the 

 Miillerian duct from the ventral wall of the Wolffian duct. 



Figs. E. and F. — Sections through the glomerulus of the head-kidney 

 from an embryo prior to the appearance of the head-kidney. Zeiss, b, 

 ocul. 2. A comparison of the two figures shows the variation in the thick- 

 ness of the stalk of the glomerulus. E. — Section anterior to the foremost 

 Malpighian body. F. — Section through both the glomerulus of the head- 

 kidney and that of a Malpighian body. The two are seen to be connected. 



Seeies H. — Consecutive sections through the hind end of the Miillerian 

 duct, from an embryo in which the head-kidney was only represented by a 

 rudiment. (The embryo was, perhaps, very slightly older than that from 

 which Series D was taken.) Zeiss, c, ocul. 3 (reduced one third). 



No. 1. — Miillerian duct is without a lumen, and quite distinct from the 

 Wolffian wall. 



No. 2. — The solid end of the Miillerian duct is no longer distinct from 

 the internal wall of the Wolffian duct. 



No. 3. — All trace of the Miillerian duct has vanished. 



Series I. — Sections through the hinder er-d of the Miillerian duct from 

 an embryo of about the middle of the sixth day. Zeiss, c, ocul. 2 (reduced 

 one third). 



No. 1. — The Miillerian duct is distinct and small. 



No. 2. — Is posterior by twelve sections to No. 1. The Miillerian duct 

 is dilated, and its cells are vacuolated. 



No. 3. — Penultimate section, in which the Miillerian duct is visible ; it 

 is separated by three sections from No. 2. 



No. 4. — Last section in which any trace of the Miillerian duct is visible ; 

 the lumen, which was visible in the previous section, is now absent. 



No. 5. — No trace of Miillerian duct. Nos. 3, 4, and 5, are consecutive 

 sections. 



Fig. K. — Section .hruugh the hind end of the abdominal opening of the 

 Miillerian duct of a chick of 123 hours. Zeiss, c, ocul. 2 (reduced one- 

 third). It illustratea the peculiar cord connecting the Miillerian and Wolffian 

 ducts. 



