UROGENITAL SYSTEM OF MYXINOIDS 85 



from another lobule. In section 60 the duct enters the proneph- 

 ric vein, and remains in it to section 80 where it ends blindly. 

 From its beginning this segment of the duct is also surrounded by 

 a central mass like the one around the first segment of the duct. 

 In transverse section the mass is circular in outline and is dis- 

 tributed somewhat regularly around the lumen of the duct. At 

 its widest part the mass measures 0.16 mm. from its outer sur- 

 face to the lumen of the duct. Only two lobules of the proneph- 

 ros are connected with this segment of the central duct. 



The third segment of the duct appears in section 90, and is 

 formed by the main tube of one lobule. In this section the 

 lumen of the duct is 0.05 mm. in diameter, but becomes as wide 

 as 0.20 mm. This segment of the duct extends to section 150 

 and there breaks up to form the tubules of three lobules. Alto- 

 gether there are four lobules connected with this segment. The 

 duct runs parallel to the pronephric vein, and up to section 115 

 is separated from the latter by a wide layer of connective tissue. 

 In this distance one vein 1 mm. wide and another 3 mm. wide 

 permit blood to flow into the pronephric vein from capillaries 

 which run between the tubules of the pronephros. At section 

 115 the duct widens abruptly and extends downward in the pro- 

 nephric vein. In the vein the duct becomes sheathed in a cen- 

 tral mass which re.se mbles in all respects the masses which 

 surround the first and second segments of the duct. In section 

 117 the duct opens through the central mass into the lumen of 

 the pronephric vein. This opening is 0.05 mm. wide. Another 

 such opening 0.09 mm. wide occurs in section 250, and a third 

 0.04 mm. wide in section 320. These openings are not breaks 

 or tears, but are natural apertures lined by a single layer of 

 squamous epithelium which is continuous with the columnar 

 epithelium of the duct on the one hand and with the endothelium 

 of the pronephric vein on the other. Figure 20 represents one 

 of these openings. At least one, often more, such opening con- 

 necting the lumen of the central duct with the lumen of the 

 pronephric vein was found in every pronephros of Bdellostoma 

 examined. These openings occur only through the central mass, 

 never through the walls of the central duct when not surrounded 

 by the central mass. 



