100 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



complete cutting off of the glomus from the body cavity by an out- 

 growtli ^ from the radix mesenterii, which passes ventral to the glomus 

 and fuses with the prone[)hric wall. 



The second nephrostome remains after the complete disappearance of 

 the rest of the pronephros. The Wolffian duct degener9,tes back almost 

 to the niesonephros, even while the first nephrostome is still present. 

 The glomus remains, in a modified condition, in half-grown animals, 

 where it is entirely enclosed in the radix mesenterii. 



In a short paper, which has not, so far as I know, been followed by 

 any more extended account, Wilson ('94) gives the following descrip- 

 tion of the formation of the Mlillerian duct in the Axolotl (Siredon pis- 

 ciformis) : " In a 25 mm. long larva one finds the coelomic epithelium 

 of the portion of the body cavity that surrounds the glomerulus of the 

 pronephros partially modified to form a hand of cylmdric cells, that runs 

 close to the outer boundary of tlie space, in contact with the limit formed 

 by the fusion of the lung and pronephros. This hand is a direct contin- 

 uation hacJiivards of the ciliated epithelium that forms the first 'pronephric 

 nep)hrostome, and where the lung frees itself from the pronephros the 

 band spreads out laterally to form a plate of cylindric epithelium that 

 extends far beyond the lateral boundary of the pronephros, but only to 

 narrow again in the region of the second nephrostome, with the epithe- 

 lium of which it fuses. Posterior to the second nephrostome the cylin- 

 dric epithelium rapidly narrows to a thread of cells that lie outside the 

 segmental duct. There can be no doubt as to the origin of these cells, 

 for (1) the coelomic epithelium is markedly thickened and proliferating, 

 and (2) the segmental duct is rounded and Avell defined, and shows no 

 sign of budding off new cells or splitting. Sometimes the thickening is 

 only one cell deep and three or four in breadth ; sometimes it is several 

 cells deep. It extends back at least as far as the niesonephros." 



It will be seen that the condition here described is very similar to an 

 early stage in Amblystoma. The thickening of the epithelium between 

 the nephrostomes corresponds to what I have called tlie " thickened 

 band," and that which "extends back at least as far as the mesone- 

 phros " is what I have termed the oviducal welt. 



The author then goes on to say that in a larva twenty-seven milli- 

 metres in length the thickening in the pronephric region is very similar 

 to that in the younger animal, although not so marked. Just behind 

 the second nephrostome, however, there is a more marked proliferation 



1 Evidently this corresponds to the structure wliich I have designated as 

 " shelf." 



