244 ZOOLOGY. 



arise on either side of the cloacal aperture and project back 

 alongside the tail. 



The kidney (mesonephros) of the adult frog first appears 

 as a metameric series of tubules, called the mesonephric (or 

 Wolffian) tubules. These differ from the pronephric tubules 

 in being formed from the mesoblast of the somites, and not 

 from the somatic mesoblast below the somite. At first they 

 are solid masses of cells, but they burrow their way, as it 

 were, till they meet the archinephric duct. Then each 

 acquires a lumen (central cavity), and this opens into the 

 duct. Then the dorsal aorta sends out branches ending 

 in little loops of blood-vessels, and these meeting the other 

 ends of the tubules, the typical Malpighian bodies and 

 glomeruli of an adult kidney are established. The glome- 

 rulus differs from the pronephric glomus in that it comes 

 in actual contact with its tubule, instead of being 'separated 

 from it by the width of the ccelom. 



The mesonephric tubules acquire a nephrostome each, by 

 growing out towards the coelom ; but this is temporary 

 only. The nephrostomes remain, but lose their connexion 

 with the tubules and open into the renal veins instead a 

 very astonishing change. 



The mesonephric tubules are formed in the seventh and 

 hinder somites ; but they soon lose their metamerism, 

 increasing greatly in number and branching, and gradually 

 becoming compacted into a single pair of kidneys. 



As the mesonephric tubules take over the functions of 

 the pronephric, the latter begin to degenerate, and lose 

 their connexion with the archinephric duct, which now 

 undergoes a remarkable change, about the exact nature of 

 which there is some dispute. It is usually stated * to become 

 split longitudinally, or obliquely, into two tubes a 

 Miillerian duct, which includes all the anterior part, and a 

 Wolffian or mesonephric duct, which includes all the posterior 

 part. The Miillerian duct is said to grow back alongside 

 the Wolffian duct and acquire an opening into the cloaca ; 

 while anteriorly it acquires an opening into the coalom, 

 close to the position of the pronephric tubules. The 

 Miillerian duct becomes the oviduct in the adult female, 

 * There is considerable doubt about this. 



