DEVELOPMENT OF ELASMOBRANXH FISHES. 129 



appear in the form of solid outgrowths from the intermediate 

 cell-mass just at the most dorsal part of the body-cavity. 



The outgrowths correspond in numbers with the vertebral 

 segments, and are at first quite disconnected with the seg- 

 mental duct. At this stage they are only distinctly visible 

 in the first few segments behind the front end of the segmental 

 duct. A full description of them will come more conveniently 

 in the next stage. 



By a stage somewhat earlier than K important changes 

 have taken place in the urinary system. 



The segmental duct has acquired a lumen in its anterior 

 portion, which opens at its front end into the body-cavity. 

 (PL XI. fig. 9 sd). The lumen is formed by the columnar cells 

 spoken of in the last stage, acquiring a radiating arrangement 

 round a central point, at which a small hole appears. After the 

 lumen has once become formed, it rapidly increases in size. 



The duct has also grown considerably in length, but its 

 hind extremity is still as thin, and lies as close to the epiblast, 

 as at first. The segmental involutions which commenced to 

 be formed in the last stage, have now appeared for every 

 vertebral segment along the- whole length of the segmental 

 duct, and even for two or three segments behind this. 



They are simple independent outgrowths arising from the 

 outer and uppermost angle of the body -cavity, and are at first 

 almost without a trace of a lumen, though their cells are arranged 

 as two layers. They grow in such a way as to encircle the 

 oviduct on its inner and upper side (PL X. fig. 8 and PL xi. fig. 

 14 b. st). When the hindermost ones are formed, a slight trace 

 of a lumen is perhaps visible in the front ones. At a stage 

 slightly subsequent to this, in Scyllium Canicula, I noticed 29 

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

 behind the front end of the oviduct (PL XI. fig. 17 5^' ^^'^ two 

 of them being formed in segments just posterior to the hinder 

 extremity of the oviduct. 



PL XI. fig. 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 X. and XI. Anteriorly the segmental 

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



B. 9 



