606 



THE EMBRYOLOGY OF THE FROG 



fold, which develop the long convoluted vessels of the glomerulus 

 proper, and also a definite vessel which connects with a branch from tne 

 dorsal aorta. 



This region of the body cavity is later cut off from the pronephric 

 chamber by the lungs, projecting laterally, and carrying a fold of peri- 

 toneum across to the peritoneum which covers the pronephros, fusing 

 with it for a short distance. The pronephric chamber remains open into 

 the coelom both anteriorly and posteriorly to the lung region. 



The pronephric capsule is derived from two sources, namely: from 



otphrostome 



gonad 



MESONEPIIROS 



Fig. 349. 



A and B. Diagrams of the development of the excretory system of the frog. 

 A, The system of a tadpole about 12 mm, Jong, showing the pronephros and 

 origin of the mesonephric tubules ; B, the system at the end of metamorphosis. 

 The broken line represents approximately the position of the strip of peritoneal 

 epithelium which gives rise to the oviduct, cl., Cloaca ; d.ao., dorsal aorta ; f.b., 

 fat body ; gl., glomerulus ; gr., gential ridge ; mcs., mesonephro.3 ; ms.t., 

 mesonephric tubules ; bd., oviduct ; ov/., position of oviducal opening ; pn.f., 

 pronephric funnels; pnp., pronephros; sg., segmental duct. (From Bourne.) 



C, Diagram to show the structure of the pronephros and the mesonephros. 

 Pronephros on the right and mesonephros on the left. The chief difference is in 

 the relation of the glomerulus ; in the pronephros it projects into the coelom ; 

 in the mesonephros it projects into the tubule, which forms the Bowman's capsule 

 about it. (From Wiedersheim. ) 



