100 JESSE LE ROY CONEL 



these lines are always streaming toward a more or less large, 

 loosely arranged mass of nuclei, bordering or lying in the pro- 

 nephric vein. 



Investigators do not agree as to the existence of a central duct 

 in the pronephros of Myxine. W. Miiller and Kirkaldy affirm, 

 while Semon, Spengel and Maas deny the presence of the duct. 

 After studying the pronephros of Bdellostoma, it is inevitable 

 for one to conclude that a central duct at one time existed and 

 that fragments of it still exist in Myxine. It is not at all promi- 

 nent and might easily be overlooked except in a comparative 

 study. Along the entire extent of the pronephros the inner ends 

 of the tubules, as stated above, end as a loosely arranged mass of 

 nuclei in which a cavity frequently appears (fig. 28). This cav- 

 ity is the remains of the lumen of the central duct. In occasional 

 places it opens into the lumen of the pronephric vein, and blood 

 corpuscles occur in it and in the tubules near it (fig. 29). Here 

 and there the central duct is cut in longitudinal section and is 

 then unmistakable (fig. 30). The youngest specimen (Myxine 

 no. 18) shows the duct but imperfectly (fig. 31). Likewise, in 

 the oldest animal examined (no. 10) the duct has almost entirely 

 disappeared, the lumen being visible for only a short distance in 

 the posterior part of the pronephros and having but a small piece 

 of columnar wall (fig. 32) . Extending along the wall of the vein 

 next to the pronephros is a large number of heavily stained nu- 

 clei, which are interpreted as remains of the central mass; or, 

 in other words, . remains of the central duct and the inner ends 

 of the tubules. Waste particles, nuclei, and granules are occa- 

 sionally seen in the lumen of the duct and in the tubules. The 

 tubules are filled with a coagulum which is more or less drawn 

 out into granular processes, and these are almost always directed 

 inwards. In only a few places do the walls of the duct still re- 

 main entire (fig. 33). Where the duct has entirely disappeared, 

 the tubules often border directly on the vein and their inner 

 ends are degenerated. 



The central mass in the pronephros of Myxine is decidedly dif- 

 ferent from that in Bdellostoma. In the young, immature ani- 

 mals (Myxine no. 18) it approaches in structure and appearance 



