516 MR GREGG WILSON ON THE DEVELOPMENT 



The immediately following stages in the development of the Miillerian duct may 

 better be followed in Axolotl than in Salamandra, to which, however, I shall return 

 later on. 



In an Axolotl of 25 mm. (a) I find the Miillerian duct in a condition similar to that 

 just described in Salamandra atra " D." The anterior nephrostome opens into the 

 usual dorsal diverticulum, along the lateral margin of which there runs a band of 

 thickened epithelium continuous with the heightened columnar epithelium of the two 

 nephrostomes (fig. 14). The connection of the gut and lung with the body-wall ceases 

 on one side slightly anterior, on the other posterior to the second nephrostome, and in 

 each case there is an immediate lateral expansion ; in the posterior region of the prone- 

 phros the epithelial plate spreads over the whole surface of the pronephros, and beyond 

 it upon the lateral wall of the body-cavity (fig. 15). On the one side there is no 

 middle diverticulum remaining, and the plate lies entirely posterior to the attachment of 

 the gut and lung ; on the other side there is a slight middle diverticulum, and in it a 

 lateral anterior band of thickened epithelium is found. In the latter case, where the 

 lung is fusing with the epithelium, there is a division of the lateral plate ; so that here 

 again, while one process runs in the line between the two nephrostomes, another passes 

 along the ventral diverticulum (fig. 14). 



The two segments of the plate unite opposite, or on one side some few sections 

 anterior to the second nephrostome, and on both sides the lateral plate ends opposite or 

 just behind the second nephrostome ; but the brick-shaped epithelium is continued 

 backwards as a thin thread that lies outside the segmental duct. 



It is in this region, immediately posterior to the second nephrostome, that the split- 

 ting or budding of the segmental duct has been alleged to occur in Amphibians, and I 

 have carefully examined my sections to find if any evidence of such splitting or budding 

 could be discovered. My next stage in the development shows a well-marked rod of 

 cells in this neighbourhood as the precursor of the Miillerian duct, and I have had to 

 consider what tissue of the slightly earlier " a " stage gives rise to this rod. Here there 

 is only an undifferentiated band of cells ventral to the segmental duct ; it is a direct 

 continuation of the thickened epithelium of the pronephros, and it runs back into a 

 thread of cells that is equally clearly of epithelial origin. In some sections the cells 

 that represent the Anlage of the Miillerian duct are closely applied to the segmental 

 duct, but the segmental duct is well marked and distinct from the Miillerian duct 

 Anlage, both in outline and in its susceptibility to stains ; and there is no differentia- 

 tion in the cells that lie to the outside of the duct ; there is one layer apparent — the 

 epithelium. 



A specimen ( " B " ) 27 mm. in length differs essentially from the last described 

 specimen only in the region behind the second nephrostome. The two nephrostomes 

 are still open ; there is the same thickened epithelium ; * but where the lateral band 



* See fig. 1 in my Preliminary Note on the Development of the Miillerian Ducts in Axolotl, Anat. Anzeiger, ix. 

 Bd., 1894. 



