iv OPISTHONEPHROS 249 



of outward growth on the part of the somatic wall of the nephro- 

 tome — the terminal portion becoming the tubule rudiment. Eiickert 

 (1888) figures a remarkably interesting variation which he came 

 across in Torpedo in which the separation of nephrotome from myo- 

 tome had been delayed. In this (Fig. 134, B) the nephrotome still 

 forms a distinct stalk continuous with the myotome and the tubule 

 rudiment is visible as a pocket-like projection of its somatic wall, 

 agreeing exactly with the assumedly primitive type of tubule rudi- 

 ment as it occurs in the pronephros of one of the lower holoblastic 

 Vertebrates. To correlate this specimen with the normal condition 

 all that is necessary is to imagine the portion of the stalk next the 

 myotome to have disappeared by becoming resolved into mesenchyme. 

 The rest of the stalk together with the tubule rudiment would then 

 remain as a curved blindly ending pocket 1 the tip of which would 

 represent the tip of the tubule rudiment. This curved pocket-like 

 structure increases in length, its tip comes into contact with, and 

 later fuses with, the dorsal wall of the duct and it is in this way 

 converted into a short tube opening at its inner end into the 

 splanchnocoele and at its outer into the duct. The tubular structure 

 so arising does not retain its simple tubular shape but undergoes 

 the series of changes shown in Fig. 135. Its cavity dilates in the 

 middle to form the definitive nephrocoele, the cavity of the Mal- 

 pighian body (m.b) ; its splanchnocoelic end becomes relatively 

 narrow to form the peritoneal canal (p.c) 2 : its outer end becomes 

 also relatively narrow and it is this outer portion which undergoes 

 an immense increase in length and becomes the functional tubule. 



Opisthonephric rudiments appear in the fashion above indicated 

 throughout the greater part of the length of the body where the 

 splanchnocoele is present. They commence behind the pronephros 

 (about the 8th or 9th segment) and extend back to the cloaca or a 

 few segments posterior to it. In the latter case the postcloacal 

 rudiments do not come to anything. Their occurrence is to be 

 looked on as a reminiscence of a period when the alimentary canal 

 and splanchnocoele extended farther tailwards. It is to be noted 

 also that the group of tubules at the front end which subserve a 

 genital function in the male similarly appear only as transient 

 rudiments in the female. 



The portions of the opisthonephros which perform an active 

 renal function increase much in bulk and this, as elsewhere, is 

 brought about not merely by the great increase in length of the 

 individual tubules, but also by the addition of numerous new tubules, 

 each with its Malpighian body etc., of the second, third and so on, 

 order. Probably (Balfour, 1878) these arise by a process of budding 

 of the nephrotomes of a similar type to that which occurs in Hypo- 



1 Care should be taken to avoid the not uncommon error of referring to the whole 

 of this structure as the " tubule-rudiment." 



2 Attention has already been drawn (p. 227) to the undesirability of applying the 

 misleading adjective " nephrostomal " to this eanaj. 



