iv OPISTHONEPHEOS 251 



see what is probably the expression of a general tendency for the 

 portion of coelome containing the glomerulus to become more and 

 more completely isolated from the main splanchnocoele as the renal 

 unit becomes more and more highly evolved. Eventually, in the 

 adult of the majority of Elasmobranchs, the peritoneal canal becomes 

 completely obliterated, but in a considerable number of others 1 this 

 happens, if at all, only towards the anterior and posterior ends of the 

 opisthonephros so that the greater part of the organ retains open 

 peritoneal funnels throughout life. Bles (1897) has made the 

 interesting suggestion that there is a physiological correlation between 

 the persistence of open peritoneal funnels and the absence of 

 abdominal pores — secondary perforations of the wall of the 

 splanchnocoele in the neighbourhood of the anus which make their 

 appearance, at a late period of development, in various Elasmobranchs 

 and other Vertebrates. 



Ukodela. — The third type of development of the opisthonephros 

 amongst the more primitive Vertebrates is found in the Amphibians, 

 especially in the Urodeles. The excellent account given by Fiir- 

 bringer (1877) still forms a thoroughly adequate basis for the 

 description. 



The Amphibians possess, as has already been shown, a large and 

 highly developed pronephros amply sufficient for their excretory 

 needs during early periods of development. In correlation with 

 this there is marked delay in the development of the opisthonephros, 

 the myotomes having already become separated and their stalks or 

 nephrotomes breaking up into mesenchyme before the opisthonephric 

 units make their appearance. The rudiments of these units — the 

 nephrotomes — become reconstituted in the midst of the mesenchyme 

 as solid cellular strands which may retain their metameric arrange- 

 ment (Amphiuma — Field, 1891; anterior segments in Triton, 

 Amhly stoma, etc.) but usually have completely lost it. Each of these 

 nephrotome rudiments is a solid strand of cells which curves out- 

 wards dorsal to the duct. In the anterior region where, as is specially 

 clear in Triton, the inner end of the strand is for a time continuous 

 with the lining of the splanchnocoele, the general arrangement is 

 clearly the same as that of the Elasmobranch (cf. Pig. 136, A, with 

 Fig. 134). The splanchnocoelic end of the nephrotome disappears 

 for a time while the main portion develops a cavity in its interior 

 and becomes converted into a vesicle with epithelial wall lying 

 immediately dorsal ,to the duct (Fig. 136, B). This vesicle becomes 

 elongated in a mediolateral direction (? by active growth of its outer 

 wall) and then assumes a characteristic curvature first <*V- and then 

 v/5-like in shape (Fig. 136, C). The mesial end of the </> gives rise 

 to the Malpighian body, the remainder to the actual tubule, its 

 outer end undergoing fusion with the wall of the duct (Fig. 136, D). 

 The tubule grows rapidly in length and is forced into complicated 



1 E.g. Cestracion philippi, Rhina squatina, Scyttium canicula,S. stellarc, Pristiurus 

 melanostomus, Spinaxniger, Acanthias vulgaris, Scymnus lichia — (Bles, 1897). 



