THE URINARY SYSTEM: KIDNEYS 429 



thick and deeply staining. The blind end of the collecting tube now takes an inward 

 direction (Fig. 28 C). In the centre of the inward extension of the embryonic area a slit 

 now appears (Figs. 28 C, 29 C). It appears in the deepest part of the thickening and 

 grows outwards, producing a basin composed of two layers of cells instead of one 

 (Figs. 28 D, E, 29 E). The thickening grows farther into the body of the lobule or 

 renculus and considerably enlarges, so that the slit becomes a rounded cavity still 

 retaining a slender connection with the periphery (Figs. 28 E, F, 29 E). During its 

 inward growth the young capsule, as shown in Fig. 28, undergoes rotation through 

 180 , so that the original outer wall of its cavity now faces inwards into the interior of 

 the lobule and thins out to become the future capsular wall. Similarly the original 

 inner wall rotates so as to look outwards. It thickens up and presently begins to show 

 signs of the folding to form the glomerulus (Fig. 28 E, F, G). The tip of the collecting 

 tube now has grown round so as to look slightly inwards towards the interior of the 

 lobule (Fig. 28 C-H). The capsule is still connected with the periphery by a slender 

 cord of cells, and from the side of the capsule opposite to that at which this cord is 

 attached, a pouch grows outwards towards the blind tip of the collecting tube (Fig. 28 E). 

 This is the rudiment of the renal tubule. It elongates, and in its early stages usually 

 exhibits a U-shape (Fig. 28 F, G), fusing with the end of the collecting tube, where for 

 some time a plug of cells (Fig. 28 F, G, H) still occludes the junction of the two tubes. 

 The cord of cells from the capsule to the periphery is presently severed, the outer 

 (originally the inner) wall of the capsule thickens markedly, and becomes folded in a 

 bilobed, and presently in a trilobed manner — the earliest sign of the future glomerulus. 

 Further rotation of the capsule now takes place, so that the U of the young tubule passes 

 inwards (Fig. 28 G) and becomes inverted in position with regard to the periphery. 

 At the end of this rotation the inverted U of the young tubule has widened considerably 

 and the glomerular wall of the capsule is folded in a trilobed manner. Further move- 

 ments of the capsule and tubule appear to be irregular and governed by the amount of 

 space available to them. 



Fig. 30 A and B were drawn from a series of sections through a young kidney from 

 a foetus 18-5 cm. in length (B. musculus) and through an incipient renculus of a kidney 

 from a foetus 0-9 m. in length (B. physalas) respectively. Fig. 30^ represents the 

 arrangement of the capsules and tubules in one lobe about to form three incipient lobules. 

 The embryonic areas are shown at the peripheries of the lobules, together with young 

 capsules and tubules in various stages. The collecting tubes are shown dotted. It is 

 seen that the tubules run in a complicated and apparently random manner. In the 

 section through the renculus of the 0-9 m. foetus they are beginning to show traces of 

 a radial disposition, foreshadowing the future cortex and medulla. 



In older foetal kidneys the capsules are orientated so that their glomeruli in general 

 look centripetally ; but in the adult they appear to have lost this arrangement and to 

 lie more or less at random in the cortex, which is traversed by the medullary rays of 

 collecting tubes. These appear in kidneys of foetuses between 2-5 and 3-0 m. in length. 

 The manner in which the renal tubules are coiled was not studied in detail in the 



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