008 THE EMBRYOLOGY OF THE 



can be seen before degeneration sets in. The pronephric duct close: 

 just posterior to the pronephros, and the tubules break up and disappear 

 The nephrostomes, however, approach one another, and finally mee 

 to open into a common cavity called the common nephrostome. This 

 then, also closes so that the nephrostomes are entirely cut off from al 

 communication with the body cavity. The glomeruli also disappear 

 although traces of these can still be seen for some months after meta 

 morphosis. 



THE MESONEPHROS OR WOLFFIAN BODY 



Just about the time the pronephros attains its full size, or a littl 

 before, the mesonephros or Wolffian body begins to form in the regioi 

 of the nephrotomes or intermediate cell-mass of the seventh to twelftl 

 somites. It is both somatic and splanchnic in origin. These nephro 

 tomes fuse in a continuous longitudinal strip of irregularly .arrange< 

 cells which lie between the pronephric duct and the dorsal aorta, alonj 

 the posterior cardinal vein. Little swellings appear in this mass, whicl 

 are the beginnings of the mesonephric vesicles. They are more numerou 

 than the mesodermal segments, and so are not strictly metameric. 



Each of these swellings divides into a large ventral and a smalle 

 dorsal chamber ; the larger one being called the primary mesonephri 

 unit and the smaller the secondary mesonephric unit. 



The secondary units divide later to form a tertiary mesonephri 

 unit. 



Each of the three units develops much alike. Figures 349 and 35' 

 show this development. There are two outgrowths, an inner tubul 

 which grows dorso-laterally to the pronephric duct where it opens, an< 

 an outer tubule which grows ventro-medially to the peritoneum wit' 

 which it fuses, and then it empties into the body cavity. 



The inner tubules connecting with the mesonephric duct conver 

 the portion where such connections are formed into the mesonephros o 

 Wolffian body the true kidney of the adult frog. The duct itself, i 

 the region of the connections, is known as the Mesonephric or Wolffia: 

 Duct, while the duct posterior to this region is now the ureter. 



The inner tubules become elongated and coiled to. form the tubula 

 portion of the mesonephros, w r hile the outer tubules form outgrowth 

 which later form the capsule around the glomeruli, known as Bowman' 

 capsules. 



A small twig from the dorsal aorta connects with each glomerulu 

 in a similar manner to the way the glomeruli were formed in the prc 

 nephros. The proximal portion of the outer tubule, from which the oul 

 growths arise to form the capsule, now separates from the remainin 

 tubule, but retains its connection with the inner tubule, and the distc 

 portion of the outer tubule comes to lie in connection with the bod 

 cavity. This latter connection becomes ciliated and forms a typicr 



