UROGENITAL SYSTEM OF MYXINOIDS 87 



SO large that it, together with the central mass, almost entirely 

 obstructs the lumen of the vein. Between sections 205 and 230 

 the pronephric vein opens directly between the tubules as a large 

 sinus (fig. 20), and another such sinus lies between sections 255 

 and 300. At all other places, however, there is a strip of con- 

 nective tissue from 0.01 to 0.05 mm. wide, or even wider, be- 

 tween the pronephros and the vein. In one specimen, though, 

 the vein opens directly into the spaces between tubules along the 

 entire base of the pronephros. But even here the pronephros 

 stands upon the vein as illustrated in figure 20, so that in no 

 sense can the head-kidney be said to he buried within the vein, 

 a misleading statement which occurs very frequently in the Ut- 

 erature. Where the vein opens between the tubules as a sinus, 

 the capillaries, which otherwise carry the blood between the 

 tubules, have broken down and only the pericardial sheath and 

 the endothelium of the sinus confine the blood in the intertubular 

 space. These vascular spaces between the tubules are' not 'sinu- 

 soids' according to Minot's use of the term. The pronephric 

 tubules do not invade the vein, for, as Price ('10) has shown, 

 the branching to form new tubules always occurs in a direction 

 away from the pronephric vein, ''just back of the nephrostomes." 

 The fifth segment of the central duct begins in section 212 

 where the tubules of one large lobule unite into a single tube 

 which widens to form the duct. The latter extends posteriorly 

 in the connective tissue at the base of the pronephros parallel to 

 the pronephric vein to section 240, increasing gradually in width 

 from 0.04 mm. to 0.15 mm. In this section the duct bends down- 

 ward so that one side enters the central mass, which still remains 

 in the vein. The wall of the duct which lies toward the proneph- 

 ros retains its columnar structure, but the wall which lies in 

 the central mass breaks down completely soon after entering 

 the mass and presents a jagged appearance, and the nuclei bor- 

 dering the lumen have no definite arrangement. In sections 243 

 to 255 th6 central duct opens through the central mass into the 

 lumen of the vein. The mass is similar in all respects to that 

 surrounding the preceding segments of the duct. In section 260 

 the duct divides, one branch entering a lobule and breaking up 



