. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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 o
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 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 emb
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