322 Proceedmgs of the Royal Physical Society. 



originally part of the co3lom, into which the tubules open 

 and the glomeruli project. The pronephros of Ccratodus 

 agrees with amphibians and differs from bony fishes and 

 Ganoids in these respects. 



My earliest stage shows two tubules on each side, with 

 nephrostomes opening freely into the coelom. Between the 

 nephrostomes, on either side of the mesentery, there is a 

 lobulated fold, or glomerulus, hanging into the body-cavity. 

 The tubules communicate externally with the segmental 

 duct. 



Later stages show the same tubules greatly coiled, but 

 with no diverticulum or internal pronephric chamber de- 

 veloped on them. There is, however (as in amphibians), 

 an external pronephric chamber formed on each side by 

 secondary fusion of the gut with the body-wall. 



My second stage is shown in a specimen three weeks old. 

 Here, in the region immediately anterior to the pronephros, 

 the gut is fused with the body-wall, so as to form a slight 

 anterior dorsal diverticulum of the coelom on either side. 

 This is most distinct opposite the anterior end of the 

 pronephros, and ends exactly opposite the first nephrostome, 

 where it passes into the general body-cavity. (See Fig. 2 

 in the text of preceding paper, on the " First Foundation of 

 the Lung of Ccratodus " ; one side of the figure shows the 

 chamber still complete, just where the first nephrostome is 

 met; the other side shows the relations of parts slightly 

 farther back.) 



A later stage shows partial obliteration of the pronephric 

 chambers; but these persist far enough forward to receive 

 the nephrostomes. 



Still later stages show further obliteration of the chambers 

 anteriorly and a closure of the anterior nephrostomes. (See 

 Fig. 3 in the text of the preceding paper ; on one side the 

 second nephrostome still opens into a reduced diverticulum 

 of the coelom; on the other side the first nephrostome is 

 seen to be quite closed up by the secondary connection 

 between the gut and the body- wall.) 



After this stage the pronephros appears in a degenerating 

 condition. 



