UROGENITAL SYSTEM OF MYXTNOIDS 97 



supports this hypothesis somewhat. The diffused condition of 

 the cells of the duct where the latter is in contact with the cen- 

 tral mass suggests that the duct is breaking down, and the en- 

 larged nuclei in the mass, which are without granules, are suggest- 

 ive of disintegrating cells, while the rounded masses of small 

 nuclei and granules found in the lumen of the duct may be inter- 

 preted as nuclei which have been broken down. These may 

 either be ejected into the bod}^ cavity thi'ough the tubules and 

 pericardial cavity, or, what is more probable, they may be swept 

 into the blood stream and be engulfed by leucocytes. These 

 masses occur only in or near the openings in the central mass 

 which connect the central duct with the pronephric vein. In one 

 specimen, however, a small ball of these nuclei was observed in a 

 tubule. Furthermore, in those pronephroi in which the central 

 duct is most continuous there is no central mass in the anterior 

 part of the duct where its columnar walls remain entire, but the 

 mass is limited to the posterior region where the walls of the duct 

 have lost their columnar structure and the lumen of the duct is 

 surrounded by the mass of nuclei. Also in those specimens in 

 which the duct is broken up into segments and the palisade wall 

 has broken down, each segment, anterior as well as posterior, is 

 surrounded by the central mass, but even in these specimens the 

 bulk of the central mass is found at the posterior end of the pro- 

 nephros, where the crowding of the original tubules is the great- 

 est. On the other hand, the quantity of the central mass in the 

 older animals seems greater than would be the case if it were 

 all derived from merely a crowding-together of the central duct, 

 especially if it were disintegrating all the while. 



The central mass is the same in appearance and structure 

 wherever it occurs in Bdellostoma stouti. No lymphatic tissue 

 such as Weldon ('84) describes is present at the posterior end of 

 the pronephros of Bdellostoma stouti, unless he refers to the cen- 

 tral mass around the duct. 



The pronephros of Myxine differs strikingly from that of 

 Bdellostoma in the fact that it contains much more connective 

 tissue. The pericardial sheath around the tubules is farther from 

 the wall of the tubule than in Bdellostoma, and the intervening 



JOURNAL OF MORPHOLOGY, VOL. 29, NO. 1 



