80 JESSE LE ROY CONEL 



branch going to and entering the right postcardinal vein. In 

 Bdellostoma the vein varies in diameter from 0.35 by 0.40 mm. to 

 0.65 by 0.85 mm. at a point about the middle of the pronephros. 

 The vein is always between the two folds of pericardium, by 

 means of which the pronephros is attached to the dorsal aorta 

 and to the wall of the alimentary tract (fig. 5). 



The pericardium completely envelopes the pronephros. It en- 

 circles each tubule where the latter extends into the pericardial 

 cavity. At the distal end of each tubule the pericardium is con- 

 tinuous with the columnar epithehum which lines the tubule. 

 This enveloping pericardium extends farther down between the 

 tubules in Bdellostoma than in Myxine. 



In all the specimens examined the right pronephros was lo- 

 cated in somites 31 and 32, counting the somite in which the eye 

 is located as the first. In only one specimen, a young female, 

 did the pronephros occupy but one somite. In all the other 

 specimens it began in somite 31 and projected into somite 32. 



The left pronephros lies in the left pericardial cavity immedi- 

 ately dorsal to the auricle and along the dorso-lateral surface of 

 the alimentary tract, parallel to the dorsal aorta. The left pro- 

 nephros, also, is connected to the aorta and to the alimentary 

 tract by a fold of pericardium (fig. 5). The fine of attachment 

 of this fold of pericardium embraces from one-fourth to one-half 

 of the width, and about three-fourths of the length of the median- 

 lateral surface of each pronephros. In all the specimens the left 

 pronephros was slightly posterior to the right, usually lying in 

 somites 32 and 33. Both right and left pronephroi lie about one 

 millimeter to the right and left of the dorsal aorta, as shown in 

 figures 45 and 46. 



The left pronephros lies along a vein extending posteriorly 

 from the left anterior cardinal. From his sections the writer 

 could not trace this vein to any posterior connection in Bdello- 

 stoma, and concludes, mth Price ('10), that it is merely a diver- 

 ticulum. In Myxine, however, the writer found that this vein 

 continues posteriorly and enters the large vein formed by the 

 union of the two posterior cardinals (fig. 4), the left ductus 

 Cuvieri of Cole ('14)'. Like the right vein, this vein along the 



