MORPHOLOGY OF CYCLOSTOMAïA. 361 



are caused, therefore, by the growth of the tubule at tlie point 

 of bending. 



The curvature in tlie vcntro-hiteral direction is ver}^ simple 

 and undergoes no remarkable change ; its projection is showw in /'. 



B. — The Segmental Duet (cml the Genital Cells. 



For the sake of simplicity, the development of the segmental 

 duct and of the vascular system in the pronephros haa been 

 entirely put aside in the description given above. 



As already alluded to, the origin of the segmental duct in 

 Petroiny^on is extremely difficult to make out, because its formation 

 goes on rapidly at a comparatively young stage. The early 

 process of its formation is essentially the same as in the prone- 

 pliric tubules. In the anterior region, the infer //lediate cell-mass 

 or the nejjJiTotome (see p. o40) behaves itself in precisely the same 

 manner as in the Anlage of the pronephric tubules ; the difference 

 is that it is cut off from the lateral plate and is transformed into 

 the duct, while in the case of the tubule it retains the continuity 

 with the lateral plate. If fig, 31, which represents the section 

 through the tenth somite {i.e. the somite, from which backwards 

 the Anlagen are converted to the segmental duct) be compared 

 with the left half of figs. 2, 5, 6, 14, and the right half of 

 fig. 16, in which the Anlagen all develop to the pronephric 

 tubules, it will be found that there is no difference between them ; 

 in fact, they are morphologically equivalent to one another. 

 Such an Anlage is, posterior to the pronephros, not confined to 

 the tenth somite, but, as has been already repeatedly said (pp. 317, 

 320, and 327), is observed for some segments further backwards 

 (see fig. 17). 



