THE URINARY SYSTEM: KIDNEYS 



427 



are distributed in continuous bands round their peripheries — broken, however, where 

 the partitioning into individual renculi is beginning to take place. This is seen in 

 kidneys of foetuses about 1 -o m. in length. In kidneys of foetuses about 2-0 m. in length, 

 in which the renculi have rounded themselves off from the lobules, the embryonic area 

 forms a continuous but only just discernible band round the outer edge of the cortex. 

 The band becomes less distinct as the renculus grows. In kidneys from foetuses about 

 3-0 m. long the embryonic areas are no longer found. 



Fig. 28. Diagrams illustrating the formation of the capsule and tubule in the Fin whale. 



A 1 . Collecting tube approaching the periphery of the lobule, producing a thickening of the periphery, 



the " Anlage" of the capsule. 



A 2 . A bifurcate collecting tube approaching the periphery, with the formation of two basin-shaped 



thickenings. 



B. A further stage. The " Anlagen" have begun to grow inwards. 



C. The cavity has appeared in each "Anlage", as it continued to grow inwards. 



D. The "Anlage" has undergone rotation and inward growth. The inner wall of the basin now looks 

 towards the periphery and is beginning to thicken. 



E. The young capsule is now connected to the periphery by a slender cord of cells, and the thickened 

 wall shows signs of folding to form the future glomerulus. The collecting tube has meanwhile grown round 

 towards the tip of the young capsule, from which a process is extending to meet it. 



F. The glomerular wall of the capsule shows marked folding and the opposite wall of the capsule is thin. 

 A long U-shaped extension has grown out from the capsule to the collecting tube. At the junction of the 

 collecting tube and this extension, which is the rudiment of the future tubule, a thick plug of cells still 

 occludes the canal. 



G. A stage farther than F. The young tubule has rotated farther in a clockwise direction. 



H. Young glomerulus, capsule and tubule, as they appear before their orientation becomes irregular. The 

 glomerular wall of the capsule is now trilobed, and the junction of the tubule and collecting tube is nearly 

 but not quite complete. 



Wherever the blind end of a collecting tube approaches the surface of a lobule or 

 renculus it apparently causes the formation of a basin-shaped thickening (Fig. 28 A 1 ) 

 immediately below the surface. Frequently two forks of the same branch approach the 

 surface in close proximity and cause two such basin-shaped thickenings (Fig. 28 A 2 ). 

 These are the rudiments of future capsules. The next stage observable in their develop- 

 ment (Fig. 28 B) involves the extension inwards of one side of the basin, the other side 

 still remaining in connection with the periphery. The inwardly extended portion is 



