82 



Basic Structure of Vertebrates 



neural tube 



notochord, 



dorsal aorta 

 afferent vessel 

 glomerulus 

 efferent vessel 



gut 



posterior 



cardinal vein 



collecting vein 



connecting 

 sub and post, 

 cardinals 



capsule 



mesonephric 

 duct 

 subcardinal 



Fig. 79. Diagrams showing the relations of the blood-vessels to an embryonic 

 mesonephric tubule. (After McCallum. Courtesy, Patten: "Embryology of the 

 Pig," Philadelphia, The Blakiston Company.) 



partly from branching outgrowths of the expanded end of the ureter 

 (Fig. 83). These derivatives of the ureter push out into mesoderm lying 

 posterior to that which gives rise to mesonephric tubules and differ- 

 ing in no obvious way from the mesonephric mesoderm. In this 

 nephrogenic tissue (Fig. 82) adjacent to the end of the ureter 

 are developed tubules each of which acquires connection with 

 one of the tubules produced by the ureter (Fig. 84). The structures 

 produced by the nephrogenic tissue are essential excretory tubules, 

 while the tubules derived from the ureter serve to collect the 

 excreted fluid and convey it into the ureter. In the kidney of a mammal 

 the excretory tubules, which are very long and much coiled, constitute 

 the outer layer, or cortex, of the kidney. The deeper medulla con- 

 sists of the relatively straight and branching collecting tubules 

 (Fig. 85). 



In fishes and amphibians the mesonephric tubules of the embryo 

 usually open into the coelom, the wall of the tubule being continuous 

 with the peritoneum (Fig. 81). The aperture (nephrostome) is sur- 

 rounded by a ciliated zone of the wall. Beating of the cilia produces a 

 current draining fluid from the coelom into the tubule, whence it passes 

 via the Wolffian duct out to the cloaca. This arrangement resembles the 

 nephridial excretory mechanism of an earthworm and other inverte- 



