742 



Fig. 16. 



the posterior end, and afterwards describe the formation of the anterior 

 end. There is a difference in the development that can only be seen 

 and studied in somewhat late stages, when the plate of epithelial cells 

 and the thin backward thread have given place to a grooved anterior 

 ridge with a rodlike posterior prolongation. 



In Axolotls of 35 — 50 mm the development of the posterior end 

 of the Miillerian duct seems to follow the course traced in the more 

 anterior parts in younger stages. A 37 mm specimen shows along 

 the course of the duct many parts where the epithelial thickening ex- 

 hibits a rod-like structure in its middle, but again and again this 

 merges in the epithelial thickening which surrounds it. Still larger 

 forms also show irregularity of development: there are parts where 

 the tube is complete, bounded on both sides by regions where it is 

 only appearing. And at the very posterior end I have found, in all 

 cases, a simple epithelial thickening, which leads forward to a dis- 

 tinct rod, and finally to the more or less distinct tube (Figs. 16, 15, 

 14, 22 and 21). 



The anterior of the Anlage of the Miillerian duct of a 37 mm 

 Axolotl differs chiefly from that of the earlier stages in that the 

 connection with the pronephric nephrostomes is no longer visible. The 

 pronephros is greatly reduced by degeneration, and is covered by a 

 thick layer of connective tissue; the nephrostomes have been closed. 

 At this stage, then, one finds in the region of the pronephros merely 

 a band of thickened epithelium that passes obliquely backwards from 

 the lateral wall of the coelom almost to the mid-dorsal line. An 

 Axolotl of 45 mm still shows a plate of cylindric epithelium on the 

 lateral wall of the body-cavity, opposite the degenerate pronephros, 

 just where the lung becomes free. In a specimen of 51 mm, however, 



