86 JESSE LE ROY CONEL 



To return to the description, the duct remains in the central 

 mass to section 137 where it leaves the vein and breaks up into 

 the tubules of three lobules. The central mass, however, con- 

 tinues in the lumen of the vein independent of the duct, and in 

 section 147 the fourth segment of the central duct appears in it. 

 The central mass projects into the pronephric vein as indicated 

 in figure 12 and has a cylindrical shape. Between sections 137 

 and 147 it is 0.30 by 0.40 mm. in transverse section. 



The fourth segment of the central duct begins as the result of 

 the anastomoses of the tubules of four lobules whose main tubes 

 unite near section 150 and form the central duct. In this sec- 

 tion the duct is 0.05 mm. wide. At first the duct lies in the con- 

 nective tissue at the base of the pronephros, but in section 157 

 it bends toward and enters the central mass in the vein. Here 

 the lumen of the duct -is 0.27 mm. wide. In this segment one 

 side of the duct remains closely apphed to the connective tissue 

 at the base of the pronephros and retains its columnar lining, 

 while the other side extends along the central mass to section 

 200 where it bends toward its fellow, thus narrowing the lumen 

 of the duct considerably, and the segment ends by the duct 

 branching into the tubules of a single lobule. There are eleven 

 lobules connected with this segment of the duct. The central 

 mass continues without interruption in the pronephric vein to 

 section 240 where the fifth segment of the duct enters it. That 

 is to say, the central mass extends from section 200 to section 

 240 (a distance of 0.40 mm.) entirely disconnected from the cen- 

 tral duct. In the region of the fourth segment the central mass 

 resembles in every respect that around the preceding segments 

 of the duct. In the portion between sections 200 and 240, how- 

 ever, the mass changes its shape in transverse section from cir- 

 cular to elongated, measuring 0.15 by 0.55 mm. in the largest 

 portion. Figure 20 represents the shape of the mass at this place. 

 This figure also indicates the relation of the size of the mass to 

 the lumen of the vein. In this particular pronephros the duct 

 and the mass together in the largest parts never occupy more 

 than about one-half of the lumen of the vein as viewed in trans- 

 verse section. In one specimen, however, the central duct was 



