122 MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



and here, by means of the cubical cells, loses its nitrogenous 

 waste (uric acid, urates, etc.). By means of the cUia surround- 

 ing the nephrostomata, watery matter is also taken from the coe- 

 lom, and all of these waste products are passed via the pronephric 

 duct to the exterior. 



In teleosts and ganoids all of the mesonephros is excretory ; 

 but in elasmobranchs and amphibians the anterior end loses this 

 function and becomes largely degenerate (females), or enters 

 into the service of the reproductive structures (males), as will 

 be described below. In the amniotes the whole mesonephros 

 degenerates and disappears, except in so far as it enters into 

 connection with the gonads, and is represented by the paradich- 

 mis and parovarium {infra). 



To compensate for this disappearance a third excretory 

 organ, the metanephros, or kidney proper, is developed in the am- 

 niotes. Its developmental history is not so well known as that 

 of the pro- and mesonephros, and the following statement is only 

 tentative. A hollow diverticulum arises from the dorsal sur- 

 face of each pronephric duct, near its entrance into the cloaca. 



This grows rapidly forward near 

 the aorta, and develops into the 

 excretory duct (ureter) of the 

 metanephros. As it grows for- 

 ward the mesoderm behind the 

 \\'olffian body rapidly proliferates, 

 and becomes richly vascular. 

 When the ureter reaches the hin- 

 der end of the Wolffian body it 

 expands, giving rise to the pelvis 

 of the kidne}', and produces, by 

 budding from its tip, cords of cells 

 which soon become tubular, and 

 form the collecting tubules of the 

 kidney. In the proliferated meso- 

 derm other tubules also appear 

 (the method of their formation is not clear) connected with 

 Malpighian bodies, essentially like those of the mesonephros. 

 These metanephric tubules become greatl)' convoluted, and ai 



Fig. 130. Kidneys (i) and supra- 

 renals (j) of a human embryo, after 

 Wiedersheim. The figure shows the 

 lobulated appearance of the early 

 kidney. 



