142 PROFESSOR MARSHALL AND EDWARD J. BLES. 



the duct is an important factor in causing the dilatation of the 

 tubules of the head kidney, which forms the first stage in their 

 degeneration. 



The succeeding stages we have only followed imperfectly. According 

 to Hoffmann, who lays stress on the great difficulty of the investiga- 

 tion, they occur as follows : — Towards the close of the metamorphosis, 

 at a period when the greater part of the tail is absorbed, i.e., a stage 

 corresponding to our Fig. 5, the archinephric duct separates com- 

 pletely from the head kidney, which latter still communicates with 

 the body cavity through the third or hindmost nephrostome. The 

 peritoneal epithelium opposite the degenerating head kidney becomes 

 columnar, the patch of columnar cells extending outwards as a 

 narrow transverse strip some distance beyond the outer edge of the 

 head kidney. With this columnar epithelium the blind anterior end 

 of the archinephric duct, which is now completely separate from 

 the head kidney, comes in contact, and then fuses. 



After the completion of the metamorphosis and the entire dis- 

 appearance of the tail, the part of the archinephric duct in front 

 of the Wolffian body splits into two, which become respectively 

 the Wolffian duct and the Miillerian duct. Of these, the Wolffian 

 duct commences blindly in front, and runs backwards through the 

 Wolffian body as the ureter, receiving the collecting tubules of the 

 kidney, and in the male the seminal tubules as well. The Miillerian 

 duct is at first short, and lies entirely in front of the Wolffian body 

 or kidney ; its anterior end is blind, and fused as described above 

 with the columnar peritoneal epithelium opposite to the head kidney; 

 its posterior end is also blind, and lies to the outer side of the 

 anterior end of the Wolffian duct. 



The anterior end of the Miillerian duct soon acquires an opening 

 into the body cavity. This opening lies close to, but is independent 

 of, the third nephrostome, which is still present, but which closes 

 and disappears very shortly afterwards. The anterior opening of the 

 Miillerian duct does not, as might be expected, become directly the 

 mouth of the oviduct, but undergoes first a series of complicated 

 changes. From the opening a groove extends outwards along the 

 columnar strip of peritoneum already noticed ; by closure of the lips 

 of this groove it becomes converted into a tube continuous at its inner 

 end with the Miillerian duct. Later on, through unequal growth of the 



