REPRODUCTION 



71 



Meanwhile, as the pronephric tubules form, the mesomere material 

 on each side of the embryo gives rise to a longitudinal tube (Fig. 59) 

 which extends from the pronephric region to the cloaca into which it 

 finally opens. The pronephric tubules of each side join the corresponding 

 longitudinal pronephric duct (Fig. 62) thus putting the coelom into com- 

 munication with the exterior by way of the cloaca. The coelomic open- 

 ings or nephrostomes (Figs. 59^, n and 62) of the pronephros are 

 ciliated. The arrangement apparently serves for drainage from the 

 coelom to the exterior. 



The mesonephric tubules acc|uire connection with the already-formed 

 longitudinal duct which, as the pronephros degenerates, then serves, at 

 least in part, as the mesonephric or Wolffian duct. In Anamnia usually 

 each mesonephric tubule has a ciliated nephrostome opening into the 

 coelom. In the kidneys of amniotes, nephrostomes rarely appear. 



Mesonephric and metanephric tubules usually form specialized excre- 

 tory structures. The tubule (Figs. 63, 64) gives rise to a cup-shaped 

 expansion (Bowman's capsule). The hollow of the cup is occasioned by 

 ingrowth of a dense network of fine blood-vessels, the glomerulus. The 

 capsule and glomerulus together constitute a renal (or malpighian) 

 corpuscle. The part of the tubule between the corpuscle and the mes- 

 onephric duct eventually becomes much elongated, coiled and locally 

 differentiated. 



Fig. 64. — Diagram of renal (Malpighian) corpuscle, a, artery; b. Bowman's 

 capsule; gl, glomerulus; n, nephrostome; t, nephridial tubule; v, vein. (From Kingsley, 

 "Comparative Anatomy of Vertebrates.") 



In the absence of nephrostomes drainage of waste from the coelom 

 does not occur and the function of excretion must be confined to the renal 

 corpuscle, where the glomerulus brings blood-vessels into close relation 

 to the lumen of a kidney tubule, and to other vascular regions of the 

 tubule. 



The amniote metanephros has outlet by way of a duct, the ureter, 

 which develops as a forward-growing branch from the cloacal end of the 

 mesonephric duct of the same side of the embryo. The tubular struc- 

 tures of the metanephros are formed largely by outgrowth from the 

 anterior end of the ureter. 



