CHAPTER VIII 

 THE DEVELOPMENT OF THE UROGENITAL SYSTEM 



THE excretory and reproductive systems are intimately associated 

 in development. Both arise from the mesoderm of the intermediate 

 cell mass (nephrotome), which unites the primitive segments with the 

 lateral somatic and splanchnic mesoderm (p. 53; 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 embryos of amniotes (reptiles, birds, and mammals) 

 possess first a pronephros, and then a mesonephros, whereas the permanent 

 kidney is a new organ, the metanephros. Whether these glands represent 

 modifications of an originally continuous organ, or whether they are 

 three distinct structures, is undecided, but however this may be, the pro-, 

 meso-, and metanephroi of amniotes develop successively in the order 

 named, both as regards time and place. 



THE PRONEPHROS 



The pronephros, when functional, consists of paired, segmentally ar- 

 ranged tubules, one end of each tubule opening into the ccelom, the other 

 into a longitudinal pronephric duct which drains into the cloaca (Fig. 

 204 A}. Near the nephrostome (the opening into the coelom), knots 

 of arteries project into the ccelom, forming glomeruli. Fluid from the coelom 

 and glomeruli, 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 rudimentary 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 ccelom. 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 developed, 

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

 extends caudad beneath the ectoderm and lateral to the nephrogenic cord, 

 until it reaches the lateral wall of the cloaca and perforates it.' Thus are 



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