84 JESSE LE ROY CONEL 



basis for the description of the minute structure of the head- 

 kidney of Bdellostoma, as follows : 



Histology. The tubules in each of the two most anterior lob- 

 ules anastomose inwardly and form a single tubule which unites 

 with one similarly formed from the other lobule, and the singles 

 tube thus formed constitutes the central duct of the pronephros. 

 The first appearance of the central duct occurs in the 10th sec- 

 tion from the anterior end of the pronephros. All of the pro- 

 nephroi of Bdellostoma stouti examined by the writer have a 

 central duct, which is so evident that it can not possibly be in- 

 terpreted as anything else. Price ('10) found a central duct in 

 the head-kidney of Bdellostoma stouti, and Weldon ('84) de- 

 scribed one for Bdellostoma forsteri. Semon ('96) thought that 

 Weldon was mistaking the venous sinus (pronephric vein) for a 

 central duct, but Weldon's description of the duct coincides so 

 nearly with the condition in Bdellostoma stouti that it seems 

 very probable that Bdellostoma forsteri has a central duct in the 

 pronephros. In the 10th section from the anterior end of the 

 pronephros the lumen of the duct is 0.02 mm. wide, but t en- 

 larges immediately and is 0.15 mm. at the widest part. The 

 duct proceeds posteriorly, entering the pronephric vein in sec- 

 tion 20 and ending blindly in the vein in section 48. From its 

 beginning this short segment of the central duct is surrounded 

 by a mass of large, deeply stained nuclei, which will be referred 

 to hereafter in this discussion as the 'central mass' and which 

 will be described in detail later. This mass is distributed some- 

 what regularly around the central duct, now more on one side, 

 now more on the other. Some idea of its gross appearance may 

 be had from figure 12. At its widest part the mass measures 

 0.21 mm. from its outer edge to the lumen of the duct, and it 

 extends 0.14 mm. beyond the posterior end of the duct. 



No trace of the central duct is again seen until section 52 is 

 reached. Here the tubules of one lobule unite into a single 

 tube, and the latter forms the beginning of the central duct. 

 The lumen of this segment of the duct is 0.05 mm. in diameter 

 at first, but measures 0.09 mm. at the widest part. The duct 

 proceeds posteriorly to section 60, receiving in its course the tube 



