100 BULLETIN: MUSEUM OF COMPARATIVE Z05lOGY. 



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

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

 and fuses with the pronephric wall. 



The second nephrostome remains after the complete disappearance of 

 the rest of the pronephros. The AVolffian duct degenerates back almost 

 to the mesonephros, 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- 

 eiformis) : " 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 band of cylindric cells, that runs 

 close to tiie 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 backtuards of the ciliated epithelium that forms the first pronejihric 

 nephrostome, 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 tlie origin of these cells, 

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

 and (2) the segmental duct is rounded and well 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 mesonephros." 



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 the "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 which I have designated as 

 " shelf." 



