242 BULLETIN OF THE 



duct pursues a direct course through the gland. The duct is situated 

 in the dorsal portion of the mesonephros adjacent to the lower borders 

 of the myotomes ; its relations are therefore difi'erent from those of the 

 longitudinal canal of the pronephros, since, as we have seen, the common 

 trunk in the pronephros is greatly convoluted, and its windings occupy 

 the ventral portions of the gland. 



The marked signs of degeneration which the pronephros presents in 

 this stage prevented my reconstructing the gland, since it proved to be 

 impossible to follow any given tube throughout the entire series of sec- 

 tions. Indeed, I am convinced that the tubules are no longer strictly 

 continuous. I must therefore content myself with a brief description 

 of the histological features noticed. 



The lumen of the tubules is greatly enlarged, and is frequently filled 

 with a dense coagulum which stains similarly to protoplasm. The cell 

 walls are very thin and show a tendency to become shredded or frayed 

 along the interior surface. The membranes between the cells in the 

 wall have become indistinct, and the number of nuclei in a given area 

 is far less than in a corresponding portion of the wall in Stage VI. The 

 nuclei are stained only feebly, but contain deeply staining granules, and 

 seem to be disappearing, since one can observe numerous gradations 

 between the typical nuclei and those which have become so pale as to 

 be nearly invisible. The ground substance of the walls is slightly vacu- 

 olated and contains numerous scattered dark granules. Between this 

 remnant of the cellular wall of the tube and the basement membrane, 

 I have frequently seen small cells with deeply stained nuclei. These 

 may possibly represent intrusive connective-tissue elements. 



I regret that I have not been able to make an extended study of the 

 degeneration of the pronephros ; but the limit which I have set to my 

 work is perhaps the least arbitrary which I could easily make. 



B. Bufo. 



The development of the pronephros and the segmental duct in Bufo is 

 very similar to that which I have described for Rana. For this reason, 

 I can treat the development in Bufo much more briefly, and shall lay 

 principal stress upon those features which are unlike in the two genera. 



Stage I. 

 In the case of Rana, this stage included embryos which showed an ill 

 defined somatopleural thickening lying immediately posterior to the 

 cranial ganglionic mass. This proliferation proved, on comparison with 



