UROGENITAL SYSTEM OF MYXINOIDS 



81 



left pronephros lies between the folds of pericardium which 

 attach the pronephros to the dorsal aorta and to the digestive 

 tract. Since no name has been given to these veins, hereafter 

 in this discussion they will be referred to as the right and left 

 pronephric veins. The position of the left pronephros in the 

 pericardial cavity is represented in. figures 2 and 4. 



In shape the pronephros is never exactly the same in any two 

 specimens nor on both sides of the same specimen. In most 

 cases, however, it is somewhat three-sided, though it is sometimes 

 flat on the attached side only and the remaining surface is 

 rounded. The ends always taper more or less, the anterior end 

 usually being the more pointed. This is due to the fact that at 

 its anterior end the pronephros always invariably consists of 

 only one or two small lobules, while the posterior end is usually 

 composed of several lobules crowded together in a compact 

 mass. Figures 7 to 11 represent pronephroi taken at random 

 and illustrate the variation that may occur in the shape. Figure 

 11a represents a cross section of a pronephros and illustrates 

 the three-sided form. When three-sided the head-kidney is 

 more or less wedge-shaped, flattened dorso-ventrally with the 

 sharp edge of the wedge opposite the point of attachment. In 

 this condition the pronephros has the appearance of having 

 been subjected to pressure, probably due to the distended portal 

 heart and auricle pushing it against the dorsal body-wall. 



The size of the pronephros is also quite variable, as the follow- 

 ing few measurements will indicate: 



JOURNAL OF MORPHOLOGY, VOL. 20, NO. 



