90 JESSE LE ROY CONEL 



(fig. 14), The columnar cells which constitute the wall of the 

 tubules are approximately the same size in all the tubules, vary- 

 ing somewhat around 0.0135 mm. high by 0.004 mm. wide. 

 The nucleus is always at the base of the cell and occupies the en- 

 tire width and from one-half to two-thirds its length. At the 

 base of the cells the cell-walls fuse to form a thin basal membrane, 

 the outer limit of the tubule. When connective tissue occupies 

 the space between the tubules, it lies next to the basal membrane, 

 and when a blood sinus fills the inter-tubular space, the endo- 

 thelium of the sinus usually lies against the basal membrane, 

 completely surrounding the tubule (fig. 15). The lumen is prac- 

 tically the same in diameter in all of the tubules, and the outside 

 diameter of the tubules is likewise approximately the same for 

 all tubules. 



The specimens of Bdellostoma examined were within a few 

 centimeters of being the same length, and the dimensions of the 

 tubules are about the same in all the pronephroi examined. At 

 the mouth of the tubule the lumen narrows slightly, but other- 

 wise is approximately the same diameter throughout. Often, 

 however, a tubule is constricted in one or two places. The lu- 

 men is approximately 0.0216 mm. wide, and the outside diameter 

 of the tubule is about 0.054 mm. 



The protoplasm of the columnar cells of each tubule is coarsely 

 granular, and the boundary of the cell facing the lumen is usually 

 marked by many granules and one or two long processes which 

 consist of granules (fig. 16). The nuclei are more or less oval and 

 always contain many large deeply stained bodies. The cells of 

 the tubules are continuous with those of the central duct at the 

 proximal end, and with those of the pericardium at the distal 

 end. 



The cells of the central duct are also columnar and are usually 

 longer than those of the tubules. The character of the cytoplasm 

 and nuclei in the duct cells is practically the same as in the tu- 

 bules, and the basal membrane of the latter is continuous with the 

 basal membrane of the former. The cells of the duct in a given 

 pronephros vary in length at different points, but any increase 

 or decrease is gradual. They may be as long as 0.0324 mm. in 



