348 DEVELOPMENT OF ELASMOBRANCH FISHES. 



of them ; the first of them arising in the segment immediately 

 behind the front end of the oviduct (PI. 12, fig. 17, st\ and two 

 of them being formed in segments just posterior to the hinder 

 extremity of the oviduct. 



PL 12, figs. 16 and 18 represent two longitudinal sections 

 shewing the segmental nature of the involutions and their 

 relation to the segmental duct. 



Many of the points which have been mentioned can be seen 

 by referring to PL 1 1 and 1 2. Anteriorly the segmental duct 

 opens into the pleuro-peritoneal cavity. In the sections behind 

 this there may be seen the segmental duct with a distinct lumen, 

 and also a pair of segmental involutions (PL 12, fig. 140). In 

 the still posterior sections the segmental duct would be quite 

 without a lumen, and would closely adjoin the epiblast. 



It seems not out of place to point out that the modes of 

 the development of the segmental duct and of the segmental 

 involutions are strikingly similar. Both arise as solid involu- 

 tions, from homologous parts of the mesoblast. The segmental 

 duct arises in the vertebral segment immediately in front of 

 that in which the first segmental involution appears ; so that the 

 segmental duct appears to be equivalent to a single segmental 

 involution. 



The next stage corresponds with the first appearance of the 

 external gills. The segmental duct now communicates by a 

 wide opening with the body-cavity (PL 11, fig. 9,^). It pos- 

 sesses a lumen along its whole length up to the extreme hind 

 end (PL u, fig. 9#). It is, however, at this hinder extremity 

 that the most important change has taken place. This end has 

 grown downwards towards that part of the alimentary canal 

 which still lies behind the anus. This downgrowth is begin- 

 ning to shew distinct traces of a lumen, and will appear in the 

 next stage as one of the horns by which the segmental ducts 

 communicate with the cloaca (PL u, fig. gb}. All the anterior 

 segmental involutions have now acquired a lumen. But this 

 is still absent in the posterior ones (PL II, fig. 90). 



Owing to the disappearance of the body-cavity in the region 

 behind the anus, the primitive involutions there remain as simple 

 masses of cells still disconnected with the segmental duct (PL TI, 

 figs. <)b,<)c and 9 d). 



