UROGENITAL SYSTEM 809 



mesonephros. The point of origin of the mesonephric tubules varies in 

 different animals. Some lie dorsal to the pronephric tubules while two 

 arise from the same nephrotome one above the other. In fish and 

 amphibians the nephrostome consists of the opening of the nephrotome 

 into the metacoele. As this opening, however, is closed in the amniotes, 

 even before the tubules are formed, there are no nephrostomes, and con- 

 sequently there is no connection between tubules and the peritoneal 

 cavity. 



"Segmental arteries grow out from the aorta to the splanchnic wall 

 of each nephrotome, forming there a network of capillaries at a higher 

 level than the pronephric glomeruli. The glomerulus thus formed 

 presses the wall before it, while the rest of the nephrotome closes around 

 it as a Bowman's capsule, the whole forming a Malpighian body (in some 

 rodents the glomeruli are rudimentary or absent). In most ichthyopsida 

 the Malpighian body is connected on one side with the metacoele by the 

 nephrostome, and on the other with the mesonephric tubule. 



"Thus at first the mesonephros is a metameric structure, extending 

 over a much larger number of somites than does the pronephros and 

 reaching nearly to the posterior limits of the metacoele. As the develop- 

 ment of the embryo proceeds, the number of tubules, in all vertebrates 

 except the myxinoids, increases by budding in a manner not readily 

 described. These tubules unite with those first formed, so that the distal 

 part of these become collecting tubules. Each of these secondary tubules 

 forms its own Malpighian body and all of the tubules elongate, become 

 convoluted and the mesonephros loses its primitive metameric character. 



"At the same time changes are introduced into the mesonephric 

 circulation. The veins emerging from the renal corpuscles extend out 

 into the region of the tubules, each breaking up there into a second sys- 

 tem of capillaries which envelop the tubules before returning the blood 

 to the postcardinal vein. The subcardinal vein brings the blood from the 

 caudal region (and usually from the hind limbs) to the Wolffian body 

 and this is also returned via the postcardinals to the heart." 



THE MESONEPHRIC DUCTS. 



In those animals seemingly more primitive, such as the elasmo- 

 branchs and in some of the amphibia, the pronephric duct divides longi- 

 tudinally from its most caudal end forward, almost to the cephalic end of 

 the Wolffian body. This occurs at the time the mesonephros develops. 

 There are thus two ducts formed one of which, called Wolffian or Ley- 

 dig's duct, remains connected with the tubules of the mesonephros and 

 forms its excretory canal, while the other called the Mullerian duct is 

 also quite closely related to the pronephros, but forms the oviduct in the 

 female. In the amniotes the pronephric duct does not divide but be- 

 comes the Wolffian duct, while the oviduct arises in another manner. 

 This same thing holds true in many of the amphibians and in all of the 

 teleosts. 



