THE URINOGEN1TAL SYSTEM. 347 



rod the cells are still rounded. In no part of it has a lumen 

 appeared. 



At this period also the knob, partly owing to the com- 

 mencing separation of the muscle-plate from the remainder of 

 the mesoblast, begins to pass inwards and approach the plcuro- 

 peritoneal cavity. 



At the same stage the first not very distinct traces of the 

 remainder of the urinary system become developed. These 

 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 segmental 

 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. (PI. 

 u, fig. 9, sd.) The lumen is formed by the columnar cells 

 spoken of in the last stage, acquiring a radiating arrange- 

 ment 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 (PI. u, fig. 8 and PI. 12, fig. 

 \^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 



232 



