98 • JESSE LE ROY CONEL 



space is usually filled with connective tissue. Also, there is 

 much more connective tissue at the base of the pronephros, sepa- 

 rating the latter from the pronephric vein, than in Bdellostoma, 

 and bands of connective tissue even extend into the vein and into 

 the central mass. In the older animals there is often considerable 

 connective tissue between the walls of the tubule and the sur- 

 rounding sheath of pericardium (figs. 23 and 24). The pericar- 

 dial sheath is heavier and its nuclei larger than in Bdellostoma. 

 Figure 25, a cross section of one lobe of a pronephros of an adult 

 female, shows the proportion of connective tissue with reference 

 to the size of the tubules which it surrounds. Not all of the 

 tubules in the pronephroi of this animal, however, have connec- 

 tive tissue between their walls and the pericardial sheath. In 

 the youngest specimen examined, a young female 31 cm. long (no. 

 18), there is very little, and often no connective tissue at all, 

 near the distal ends of the tubules. In the oldest specimen 

 (Myxine no. 10) the connective tissue around the tubules and in 

 the entire pronephros is more abundant than in any of the other 

 animals and contains many blood vessels. In this specimen 

 there is, in transverse section of almost every large tubule, a 

 single large vessel which partially or entirely surrounds it (fig. 

 26). A blood sinus, which completely fills the space between 

 the tubules, as is frequent in Bdellostoma, is rare in Myxine. 

 The blood around the tubules is almost always confined in ves- 

 sels which lie in the connective tissue surrounding the tubules. 



Unhke Bdellostoma, the tubules in an adult Myxine are not 

 all of the same size (fig. 27) . They are practically of the same 

 size in the youngest specimen examined (Myxine no. 18), but in 

 the older animals there is a gradually increasing number of tu- 

 bules which are slightly larger than others, and in the oldest speci- 

 men (Myxine no. 10) there are present, in the pronephros, tubules 

 which range in size from the smallest to very large ones. In Myx- 

 ine no. 18 the average dimensions of the tubules are: Width of 

 lumen 0.0162 imn., width of each wall 0.0189, outside diameter 

 0.054 mm. Only a few of this size are present in the pronephros 

 of Myxine no. 10, many approximating the following dimen- 

 sions: Width of lumen 0.108 mm., width of each wall 0.027, out- 



