222 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



reduced in forms with richly yolked eggs where the development is 

 not larval. Its reduction or disappearance in the last-mentioned 

 forms may probably be due to the facilities afforded for getting rid 

 of excretory products by simple diffusion from the blood circulating 

 on the surface of the yolk-sac into the surrounding medium. 



The opisthonephros forms the functional kidney in the adults of 

 most if not all anamniotic Vertebrates. 



Distinct mesonephros and metanephros are found in the Amniota 



— the mesonephros being functional during the later embryonic 



'period and in the Reptiles during the first few months after hatching, 



Fig. 120. — Renal organs of the Frog {Rana temporaria) as seen from the ventral side after 

 the ventral wall of the splanchnocoele and the portion of the alimentary canal 

 contained within it have -been removed. (After Marshall, 1893.) 



A, 12 mm. tadpole ; B, 40 mm. tadpole ; C, frog at time of metamorphosis. A, dorsal aorta ; A.r, 

 aortic root ; F t fatty body ; gl, glomerulus ; op, opisthonephros ; pii t pronephros. 



while the metanephros forms the definitive kidney of the adult, its 

 excretory activities being reinforced during the first few months in 

 the case of Reptiles by the still functional mesonephros. 



The Pronephros. — A typical functional pronephros is well seen 

 in a frog tadpole of about half an inch in length (Fig. 120, A, pn). 

 It consists of a massive organ lying dorsal to the anterior portion of 

 the splanchnocoele on each side. It consists mainly of a much con- 

 voluted tube the anterior portion of the archinephric duct, and into, 

 this there open three segmentally arranged pronephric tubules also, 

 except the anterior one, much coiled and twisted. While the organ 

 as a whole is retroperitoneal, i.e. outside the coelomic lining, there 

 exists an opening leading from the splanchnocoele into each tubule 

 — the nephrostome (Fig. 121, ns). Duct and tubules are lined with 

 cubical, or almost columnar, epithelium and in the neighbourhood 



