UROGENITAL SYSTEM OF AU'XINOIDS 93 



40 sections, 0.4 mm.) beyond the end of the lumen of the duct. 

 The mass is more or less cyhndrical, though in places it may be 

 considerably widened as if bent upon itself, and then it is elon- 

 gated in transverse section. In none of the specimens does it 

 entirely fill the lumen of the vein. In those pronephroi in which 

 the duct is abnost continuous, however, the lumen is so great in 

 diameter, that the duct, together with the central mass surround- 

 ing it, almost entirely fills the lumen of the vein, leaving only a 

 very narrow space between the mass and the vascular endothel- 

 ium. The endothehum of the pronephric vein is continuous 

 around the mass (figs. 17 and 22), but is here much thinner than 

 when Uning the vein, and its nuclei can be observed only now and 

 then. The surface of the mass is smooth and convoluted. 



Figure 18 represents a transverse section through the central 

 mass, and illustrates the typical condition in Bdellostoma. It is 

 composed of a large number of cells, the nuclei of which are more 

 or less oval in shape. The latter are of various sizes, the largest 

 and most numerous averaging 0.0054 by 0.0108 mm. These 

 nuclei are not distributed in any regular manner, large, small 

 and intermediate sizes are mingled in all parts of the mass. Also, 

 there is no regularity in the manner of their arrangement ; some 

 are isolated completely from surrounding neighbors, others are 

 collected in groups of two or three, and again six or eight may be 

 closely crowded together in a mass. There is no definite cellular 

 structure observable in the central mass. Most of the nuclei 

 contain many large, deeply stained granules, but many have only 

 a nuclear membrane 'V'vdth no granules inside. Many of the nu- 

 clei have no cytoplasm surrounding them, while others are im- 

 bedded in what resembles the granular cytoplasm in the cells 

 of the central duct and tubules. This granular cytoplasmic 

 ground-work is scattered throughout the central mass and, in 

 places, is drawn out into slender processes which resemble those 

 found in the tubules. There is no connective tissue in the cen- 

 tral mass. In an occasional transverse section of the mass there 

 are one to three or four giant nuclei, but these are not frequent 

 in Bdellostoma. About one hundred sections of the mass were 

 examined before a section containing one of these giant nuclei 



