hall: MESONEPHROS and MiJLLERIAN DUCT IN AMnilBIA. 81 



(evg. 1, Fig. 42). It is a simple outpocketing, which is continued caudad, 

 first as a duct, then as a cord which soon dwindles and disappears. 

 Figure 53 shows this cord {dt. 2), more highly magnified, eight sections 

 behind the evagination shown in Figure 44, and three sections anterior 

 to its distal end. As the figure shows, it is entirely free from the 

 Wolffian duct {dt IF/.). 



Along the course of this posterior duct the peritoneal epithelium is 

 slightly thickened (Fig. 53, ci's.o'dt.). Posterior to the distaP end of 

 the cord, this thickening accompanies the Wolffian duct mediad and then 

 caudad, and is continued as that enigmatical welt which lies near the 

 Wolffian duct throughout its entire length and has been called the 

 " Tubenleiste " by German authors (see Fig. 54, Plate 5, crs. ddt.). Its 

 appearance always precedes the formation of the Miillevian duct, and 

 Wilson ('94), Semon ('92), and others have seen in it the fundament of 

 that structure. I cannot find that it has anything to do with the devel- 

 opment of the essential part of the Miillerian duct, — the epithelial lining. 

 It often seems, however, at least in the anterior region of the body, to be 

 proliferating cells to form the outer layers of the duct. The " Tuben- 

 leiste," or oviducal welt, as I have termed it, later disappears entirely. 



Larva VII, 35 mm. 



The pronephros of this larva shows undoubted signs of degeneration. 

 The first and second nephrostomes have drawn quite near to each other, 

 a very common condition during degeneration. Both pairs of nephro- 

 stomes have migrated slightly caudad : the first on one side, and the sec- 

 ond on the other have become closed, and remain connected with the 

 peritoneum merely by cords. With the closure of the first nephrostome, 

 the associated nephrostomal cavity is obliterated.^ This obliteration of 

 the nephrostomal cavity does not affect the anterior evagination, how- 

 ever, as it has already moved out of the cavity by a migration caudad. 

 (Compare Fig. L, p. 88.) 



The conditions of the evaginations on the two sides of the body are as 

 follows : On the right side the first nephrostome and nephrostomal cav- 

 ity are still present. The anterior evagination is behind them ; it is still 



1 I use the term "distal" to designate the ^oriion farthest frovi the origin. 



2 It would seem that the obliteration of the nephrostomal cavity takes place not 

 by a flattening out of its walls to merge with tlie general epithelium of the glomeru- 

 lar cavity, but rather by the closure of its mouth, followed by an actual dissolution 

 of the walls, similar to that which takes place in the nephrostomal canals and 

 pronephric tubules. If this is true, it forms a suggestive parallel to the process in 

 liana (see page 90). 



