MORPHOLOGY OF CYCLOSTOMATA. 363 



somite (see fig. 63) far backward of the posteriormost tubule 

 which is found in the ninth somite. 



The segmentally arranged Auhigen of the segmental duct 

 are secondarily united with one another just as in the case of the 

 collecting duct in the pronephric region. This union seems to 

 take place during the separation of the Anlage from the myotome 

 and is finished before it is separated from the lateral plate ; for, 

 when the Anlage first comes int(j view, there is no intersomitic 

 cord as in the case of the pronephric tubules and the duct is seen 

 already consisting of radially arranged cells (fig. 58) when it is 

 cut off from the lateral plate. When established, the duct is the 

 same in structure in both the somitic and intersomitic spaces ; a 

 cross-section of such a duct in the intersomitic portion is shown 

 ou the right side of iig. 75, while that in the somitic portion 

 is seen on the left of the same figure. 



This condition of the duct is already traceable, in Stage iir, 

 for no fewer than 10 somites from the hindmost pronephric tubule 

 backwards, and it forms a direct posterior continuation of the 

 collecting duct. The duct remains awhile as a solid cord of cells 

 arranged radially in cross-section, but it soon acquires a lumen 

 (figs. 75, 86, and 87, .sd). The further development of the duct 

 goes on more promptly than that of the tubules in the hind part, 

 and therefore, the embryos at such a stage (Stage iii) have a well 

 developed duct and more or less primitive tubules (compare fig. 

 74 with fig. 75). 



In the hind region, where yolk-cells are crowded, the pro- 

 cess is much delayed and more or less modified. Instead of the 

 diilerentiation of the cells in dtu, it seems to me, a few cells 

 are detached from tlie nephrotome; a number of cells is produced 

 by repeated division of these cells (fig. lU, aM.) and becomes ar- 



