XI, B, 5 Garcia: Absence of Kidneys in a Pig Embi-yo 197 



single organ which is laid down and gradually brought up to 

 its definitive completion, but to a saltatory development of three 

 different organs, the pronephros, the mesonephros, and the me- 

 tanephros, which are developed and formed in succession, each 

 being apportioned and adapted to a definite period of one entire 

 development. Of these organs the pronephros and the mesone- 

 phros are merely provisional kidneys; their activities become 

 superfluous and partially or totally disappear and are eventually 

 supplanted by the last and third organ, the metanephros, which 

 becomes the adult organ of urinary excretion. 



The anlage of these three organs is the same; the cells of 

 the intermediate mass of mesoderm of the embryo, from the 

 cephalic portion of which develops the pronephros, from the 

 middle zone the mesonephros, and from the caudal end the 

 metanephros. 



In the present case the pronephros has evidently undergone 

 its full development and degeneration, leaving only behind its 

 primary collecting duct from which the two ureters have ap- 

 parently normally developed. 



That the mesonephros has also undergone its normal course 

 of development and degeneration is evidenced by the presence 

 of distinct mesonephric tubules in the region of the rete ovarii 

 forming the parovarium, the normally formed uterus arid 

 oviduct, and the two well-developed ureters, which are all 

 derivatives of the mesonephros. 



In the case of the metanephros, the ureter in the right side 

 has apparently developed upward to its normal length, and its 

 upper extremity has started to expand into a funnel-shaped 

 primitive pelvis and to divide into several primary collecting 

 tubules. No evidences can be seen, however, of a blastemal cap 

 accompanying it in this upward growth. The left ureter has 

 apparently become arrested at the brim of the pelvis, becoming 

 entangled there in the retroperitoneal mesenchyma. 



Whether these ureters were accompanied by blastemal tissue 

 from the nephrogenic cord which might have undergone com- 

 plete regression or not is a question which cannot be fully 

 determined with the present findings. 



SUMMARY AND CONCLUSIONS 



The review of the literature has shown the comparative rarity 

 of congenital absence of kidneys, especially in man. Only a 

 few cases have so far been reported, one or two of which are 

 inconceivable. 



