CHAPTER VIII 



THE DEVELOPMENT OF THE UROGENITAL SYSTEM 



The excretory and reproductive systems are intimately associated in develop- 

 ment. Both arise from the mesoderm of the intermediate cell mass (nephrotome), 

 which unites the primitive segments with the lateral somatic and splanchnic 

 mesoderm (p. 52; Fig. 205). 



Vertebrates possess excretory organs of three distinct types. The pronephros 

 is the functional kidney of amphioxus and certain lampreys, but appears only in 

 immature fishes and amphibians, being replaced by the mesonephros. The em- 

 bryos of amniotes (reptiles, birds, and mammals) possess first a pronephros, and 

 then a mesonephros, whereas the permanent kidney is a new organ, the meta- 

 nephros. Whether these glands represent modifications of an originally continu- 

 ous organ, or whether they are three distinct structures, is undecided, but how- 

 ever this may be, the pro-, meso-, and metanephroi of amniotes develop suc- 

 cessively in the order named, both as regards time and place. 



THE PRONEPHROS 



The pronephros, when functional, consists of paired, segmentally arranged 

 tubules, one end of each tubule opening into the ccelom, the other into a longitu- 

 dinal pronephric duct which drains into the cloaca (Fig. 204 A). Near the 

 nephrostome (the opening into the coelom) knots of arteries project into the coelom, 

 forming glomeruli. Fluid from the coelom and glomeruK and excreta from the 

 cells of the tubules are carried by ciliary movement into the pronephric ducts. 



The human pronephros is vestigial. It consists of about seven pairs of rudi- 

 mentary pronephric tubules, formed as dorsal sprouts from the nephrotomes (Fig. 

 205) in each segment, from the seventh to the fourteenth, and perhaps from more 

 cranial segments as well. The nodules hollow out and open into the coelom. 

 Dorsally and laterally, the tubules of each side bend backward and unite to form 

 a longitudinal collecting duct (Fig. 204 B, A). The tubules first formed in the 

 seventh segment begin to degenerate before those of the fourteenth segment have 

 developed. Caudal to the fourteenth segment no pronephric tubules are devel- 

 oped, but the free end of the collecting duct, by a process of terminal growth, ex- 

 tends caudad beneath the ectoderm and lateral to the nephrogenic cord, until it 



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