3o8 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



anterior end of the organ backwards. As a matter of fact this sequence 

 does occur, but its regularity is interfered with by a tendency of the tubes 

 to develop in batches. First a few tubules at the front end of the series 

 develop one after the other — constituting what is termed the pronephros 

 (Fig. 128, B, pn). In those vertebrates which have a larval stage the 

 tubules of the pronephros become greatly enlarged, so that it is able to 

 overtake the whole excretory needs of the individual during this stage. 

 The tubules which would develop in the next succeeding segments do 

 not make their appearance — being unnecessary owing to the great 

 development of the tubules in front of them. It is only when it comes 

 to be the turn of a segment further back in the series to develop a tubule 



n 



and 



an. 



A 



Fig. 127. 



Diagram illustrating the arrangement of the nephridial tubes of Annelids (A) and Vertebrates (B). 

 an Anal opening ; a.n.d, archinephiic duct ; c, coelome ; ent, enteron ; n, nephridium. 



that the tubule actually makes its appearance. Successive tubules now 

 go on developing right back to the hinder end of the coelome, and the 

 whole of these constitute the opisthonephros (Fig. 128, B, op). The 

 opisthonephros takes on the excretory function hitherto performed by 

 the pronephros^ and the latter, no longer required, rapidly dwindles 

 away. The opisthonephros forms the functional excretory organ or 

 kidney of the fishes and other lower vertebrates. 



In the higher vertebrates, from the Reptiles onwards, the opistho- 

 nephros becomes again differentiated into two portions — an anterior — 

 the mesonephros — which loses its excretory function in the adult and be- 

 comes related to the reproductive process — and a hinder portion — the 

 metanephros (Fig. 128, D, mn) — composed of the hinder tubule or tubules 

 which become immensely enlarged and added to by the development of 



