737 



been supposed to be derived, being a very much earlier formation, is 

 the less likely to have had a common origin with the Miillerian 

 duct. On the other hand, as the Miillerian duct is formed at a 

 time when the pronephros is still complete, there can be no suggestion 

 of such a mode of origin as was asserted for the oral end of the duct 

 by van Wijhe, who found that it was formed in Selachians and Mam- 

 mals from cells derived from the degenerated pronephros. 



In a 25 mm long larva one finds the coelomic epithelium of the 

 portion of the body cavity that surrounds the glomerulus of the pro- 

 nephros partially modified to form a bandofcylindric cells, that 

 runs close to the outer boundary of the space, in contact with the 

 limit formed by the fusion of the lung and pronephros. This band 

 is a direct continuation backwards of the ciliated epi- 

 thelium that forms the first pronephric 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 epithelium 

 of which it fuses. 



Posterior to the second nephrostome the cylindric epithelium 

 rapidly narrows to a thread of cells that lie outside the segmental 

 duct. There can be no doubt as to the 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. My sections are stained 

 with borax-carmine and bleu de Lyon, and a very obvious colour 

 differentiation is visible: the crimson of the epithelial thickening is 

 in sharp contrast to the purplish hue of the segmental duct. Some- 

 times 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. 



A specimen 27 mm in length shows similar conditions. The 

 thickening is not so marked anteriorly as in the smaller larva, being 

 only one cell deep; but just anterior to where the lung becomes free 

 from the pronephros (Fig. 1) there is again a well-defined plate of 

 thickened epithelium that covers the lateral wall of the body-cavity, 

 and can be followed back to the second nephrostome (Fig. 2 w), and 

 then along the dorsal aspect of the coelom, as a thread of cells run- 

 ning ventral or latero-ventral to the segmental duct, which in this 

 case again is quite plainly neither budding nor splitting. 



An important difference between the 25 mm specimen and the 



