194 THE EXCRETORY SYSTEM 



kidney is usually an elongated body up uniler the peritoneum, close 

 to the backbone. In the urodeles the kidneys become somewhat 

 elongated, but in frogs and toads they are shorter and l^roader. In 

 these latter forms, the kidney becomes free in the body above the 

 viscera but is connected with the body wall by peritoneal sheets of 

 tissue. 



The mesonephric tubule is more com])lex than the preceding 

 pronephric tubule. In the embryonic tubules the proximal end 

 has a distinct nephrostome, as in the case of the pronei)hric tu})ule, 

 but difi'ers in that a glomerulus projects into each ne])hridial tubule 

 in the region near the nephrostome. The arterial tuft is thus sur- 

 rounded by a double capsule of reflected tu])ular tissue. Bowman's 

 capsule, and the whole is called a renal corjiuscle. In such an 

 arrangement the fluid from the blood is filtered through the inner 

 capsule and ])asses directly into the lumen of the mesonephric 

 tubule without first passing into the coelomic fluid, as in the case 

 of the pronephros. With development the nephrostomes may close; 

 in urodeles they persist through life but many of them degenerate 

 in anuran kidneys. Continuing from the glomerulus there is a 

 proximal secreting ])ortion of tul)ule that in turn connects with the 

 mesonephric duct. 



The mesonephros is the functional kidney of most adult cyclos- 

 tomes, elasmobranchs, teleosts, and Amphibia. It is functional for 

 a short time in certain young lizards and for a short time after birth 

 in the monotreme, Echidna, and in the marsupial, ])idel])hys. The 

 kidney of the frog may be taken as an exam])le of the functional 

 mesone])hros. 



Kidney of the Frog.^The kidney of the frog is a flattened, rela- 

 tively broad, elongated body lying within the body ca\'ity dorsal 

 to the viscera. It is composed of an aggregate of mesone])hric 

 tubules whose distal ends unite with the main collecting duct, the 

 mesonephric duct, which runs along the outer lateral edge and 

 continues ])osteriorly to join the cloaca. 



Each tubule has several different regions. At the proximal end 

 of each is the renal corpuscle (Eig. 123), formed by a glomerulus of 

 arterial capillaries encapsulated by the double layer of thin, flat, 

 epithelial cells forming Bowman's ca])sule. As the tubule contimies 

 distally the cells liecome cuboidal, or low cohunnar, and are more 

 distinctly outlined. The region of the tubule just distal to the 

 capsule is formed by ciliated cuboidal cells and may receive the 

 short l)ran(Ii composed of ciliated cuboidal cells that oi)ens to 



