PLATE 3 



EXPLANATION OP FIGURES 



16 Enlarged drawing of columnar epithelial cells of a pronephric tubule. 

 Bdellostoma no. 15. 



17 Transverse section through pronephros of Bdellostoma no. 10, showing 

 the endothelium of the pronephric vein continuing around the central mass. 



18 Transverse section through the central mass. Bdellostoma no. 15. 



19 Enlarged drawing of the central mass. Bdellostoma no. 15. 



20 Transverse section through pronephros of Bdellostoma no. 15, showing 

 communication between the central duct and the pronephric vein, with nuclear 

 mass in the opening; also shows pronephric vein opening into the space between 

 the tubules, thus making a blood sinus. Not all the pronephric tubules are in- 

 cluded in the drawing. 



21 Transverse section in another part of pronephros of Bdellostoma no. 15, 

 which also shows the central duct communicating with the pronephric vein and 

 the opening filled with the nuclear mass. Not all of the tubules are shown. 



22 Transverse section through pronephros of Bdellostoma no. 6, showing the 

 central duct in communication with the pronephric vein through a large opening 

 which is filled with a nuclear mass. 



23 Transverse section through a pronephric tubule of Myxine no. 10, showing 

 connective tissue surrounding the epithelium. 



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