434 TEXT-BOOK OF EMBRYOLOGY. 



that a single flat mass is formed. This occupies a medial position or lies at 

 either side of the medial line, and may be situated at the normal level or lower. 

 The renal pelvis may be single or double, with one or two ureters. In cases of 

 double ureters and pelves it seems most likely that the anlagen of the kidneys 

 have fused secondarily, that is, after the evagination from the mesonephric 

 ducts (p. 394). In cases where the pelvis and ureter are single, the fusion may 

 have occurred secondarily, although there is the possibility that only a single 

 anlage appeared. 



Occasionally in children and even in adults the kidneys show a distinct 

 tabulation. This is due to the persistence of the lobulation that normally 

 exists in the foetus (p. 401). 



The kidney may be more or less movable owing to laxity of the surrounding 

 tissue, or it may Refloating, in which case it has a distinct mesentery. These 

 cases should be distinguished from those in which similar conditions have been 

 acquired, usually as the result of trauma. 



Congenital cysts of the kidney are not uncommon. They vary in size and 

 number, sometimes being so numerous that they crowd out the greater part 

 of the renal tissue. Rarely they are so large and numerous that the affected 

 organ fills a large part of the abdominal cavity, resulting in serious or even 

 fatal disturbances of the functions of other organs. There are three views con- 

 cerning the origin of these cysts, (i) They may be the result of dilatation of 

 certain renal tubules derived from the nephrogenic tissue, which failed to unite 

 w r ith the straight tubules (p. 396). (2) Inflammation in the medulla of the 

 fcetal kidney may effect a closure of the lumina of some of the tubules, with 

 subsequent dilatation of the portions (tubules or renal corpuscles) that are cut 

 off from communication with the renal pelvis. (3) Normally some of the renal 

 corpuscles and tubules degenerate (p. 402) , and the cysts may arise as dilatations 

 of incompletely degenerated corpuscles or tubules or both. While these views 

 appear reasonable, none of them has been proven. All three views express 

 possibilities, and there is no good reason for believing that any one of them 

 expresses the only possibility. 



THE URETERS. The renal pelvis is sometimes absent, the calyces uniting 

 to form two or more tubes which in turn unite to form the ureter. This prob- 

 ably is the result of abnormal branching of the ureter during development and 

 the failure of the ends of the branches to become dilated. Occasionally the 

 ureter is double or triple throughout the whole or a part of its length. The 

 most reasonable explanation of two or three complete ureters on either side is 

 that two or three separate evaginations arose from the mesonephric duct (p. 

 394.) Where the tube is double in only a part of its length, an abnormal 

 branching of the single original evagination is indicated. 



Atresia of one or both ureters is occasionally met with. This probably 



