30 ADAM SEDGWICK. 



As the mesonephros becomes more developed the pronephros 

 retrogrades, and is eventually entirely, as far as its function is 

 concerned, replaced by the former. 



The development of the mesonephros in Teleostei, Marsi- 

 pobranchii, Ganoidei, is similary described as taking place in the 

 free young (larva) from strings of cells derived from the peri- 

 toneal epithelium. InMarsipobranchii as in Amphibia the young 

 are hatched with a functional pronephros, and no trace of the 

 mesonephros ; but the former is, in the further growth of the 

 young animal, gradually replaced functionally by the latter, and 

 more or less retrogrades. In the Teleostei, however, and 

 Ganoidei, it persists with the mesonephros as an important 

 functional organ in the adult. In some Teleostei the pronephros 

 is the only functional adult kidney, the mesonephros not being 

 developed. 



I have made some observations on the development of the 

 mesonephros in the Prog {Rana temporand), Salmon and Stur- 

 geon, and my observations lead me very strongly to doubt whether 

 Tiirb ringer and other observers are right in describing the origin 

 of the cells which give rise to the mesonephros as actual 

 ingrowths from the peritoneal epithelium. 



In the case of the Prog this is certainly not the case. In 

 fig. 25 is represented a section through a Tadpole of 11 mm., 

 showing the first trace of the cells (k b) from which the Wolffian 

 tubules arise. At their first appearance they are independent 

 of the peritoneum, and only secondarily become connected 

 with it. Piirbringer figures from the Salamander a section 

 in support of his statement ; I have also seen such appearances 

 in the Tadpole, but in this animal these strings are only found 

 in that part of the animal in which, I am confidently able to state, 

 no Wolffian tubules are ever developed. I have examined and 

 compared segment with segment of Tadpoles of various ages, 

 and have never found these strings of cells developing into 

 Wolffian tubules. The cell strings appear to me to arise from a 

 blastema of cells developed in siM becoming connected with the 

 peritoneal epithelium, and they are, no doubt, rudimentary 

 tubules. 



Piirbringer in his paper gives no evidence of the origin of 

 these cells from the peritoneal epithelium, except a drawing of 

 a stage in which the blastema is connected with the peritoneal 

 epithelium.-^ I have also seen this stage, as mentioned above, 



' Gotte also, in his latest writings on the subject, agrees with Piirbringer 

 as to the origin of the cells wliich give rise to the mesonephros. But 1 may 

 draw attention to the fact that Gotte has held three views on this point, 

 the last of which did not appear (see Tiirbringer, loc. cit.) till 1875, i.e. 

 after the publication of Balfour and Semper's works on 'Elasmobranchi.' 



