THE DEVELOPMENT OF THE UROGEXITAL SYSTEM. 397 



continuous with the pyramidal epithelium of the distal convoluted tubule. 

 The entire structure is a renal corpuscle. The formation of renal corpuscles 

 begins in embryos of 30 mm. and continues until after birth. 



The Renal Pyramids and Renal Columns. The tubules arising from 

 the four primary evaginations of the renal pelvis together form four distinct 

 groups or primary renal (Malpighian) pyramids one cephalic, one caudal, and 

 two central. The central pyramids are crowded in between the end pyramids, 

 (cephalic and caudal) and do not develop as rapidly as the latter which soon 

 bend around toward the ureter, thus resulting in the formation of the convex 

 side of the kidney and a depression or hilus opposite (compare Figs. 352 and 

 358) . Between these four pyramids the mesenchyme remains for some time as 



Primary renal pyramid 



'Primary renal column 

 Cephalic straight tubule- 

 Primary renal pyramid 

 Central straight tubule = 



___^^^^^_^^_^^^^_ Primary renal column 

 Caudal straight tubule 



Urett 



Primary renal pyramid 

 FIG. 358. Frontal section of the kidney of a human foetus of 3! months (10 cm.). Hauch. 



rather distinct septa, forming the primary renal columns (columns of Bertini) 

 which are marked by corresponding depressions on the surface of the kidney 

 and extend to the renal pelvis. The four primary pyramids may be considered 

 as lobes (Fig. 358). It should also be stated that the parts of the tubules 

 derived from the mesenchyme form the bases of the renal pyramids. Be- 

 tween the groups of straight tubules derived from evaginations of the second or 

 third order (see p. 392) there are also septa of mesenchyme which divide each 

 primary pyramid into two or three secondary pyramids. These septa may 

 be considered as secondary renal columns (Fig. 359). Thus the entire kidney 

 is divided into from eight to twelve secondary pyramids. Tertiary renal 

 columns then divide incompletely the secondary pyramids into tertiary pyra- 



