UROGENITAL SYSTEM OF MYXINOIDS 101 



the condition existing in Bdellostoma (fig. 34). Here the mass 

 contains the more or less oval nuclei, no blood corpuscles, but the 

 cytoplasmic groundwork is replaced by connective tissue. Also, 

 one side of the mass projects into the lumen of the pronephric 

 vein and has a more or less rounded form, while the other side 

 remains attached to the base of the pronephros, but is not as 

 compact as in Bdellostoma. The mass is distributed along the 

 entire length of the vein, from the anterior to the posterior end 

 of the pronephros, and extends behind the posterior end of the 

 latter about 0.18 mm. in one specimen. This posterior exten- 

 sion of the mass is cylindrical, has a tubule which ends blindly in 

 it, and contains several large blood corpuscles in capillaries. 



In the older animals the central mass is much more unlike that 

 of Bdellostoma. In the anterior part of the pronephros it con- 

 sists of a loose collection of nuclei which extends along the base 

 of the tubules, embracing their inner ends, either lying outside 

 the vein or projecting slightly into it. The mass is not at all 

 compact, and has no definite form. Many of the nuclei are dis- 

 tributed in an irregular manner along the sides of the vein. At 

 the base of the pronephros the mass is more or less exposed to the 

 blood in the vein, even though it does not project into the lumen 

 of the latter. In occasional places the lumen of the vein opens 

 through the mass (fig. 32) and communicates with the lumen of 

 the central duct, and here blood corpuscles are present in the 

 duct, but are rarely found in the tubules. In the posterior part 

 of the pronephros the central mass becomes cylindrical and com- 

 pact, and projects into and extends posteriorly in the lumen of the 

 vein, completely surrounded by blood. It is bound to the sides 

 of the vein by broad bands of connective tissue which extends 

 from the side of the mass here and there (fig. 35). Blood vessels 

 run from the connective tissue at the base of the pronephros into 

 these bands of connective tissue. The fibers of the latter con- 

 tinue to the center of the cyhndrical mass, and enlarged nuclei 

 and blood capillaries are arranged in a cellular structure around 

 this central core of fibers. The giant blood corpuscles are rounded 

 in shape and are contained in capillaries. They are very num- 

 erous in the central mass in Alyxine (as many as fifteen in one sec- 



