DEVELOPMENT OF PETROMYZON FLUVIATILIS. 345 



The segmental duct throughout its course runs in close 

 connection with the post-cardinal vein, lying in contact with 

 it, almost in its wall in the under and inner side. In 

 the anterior region this vein has so grown round the pro- 

 nephros that the tubuli really lie inside it {fig. 29). The 

 tubuli are covered by a few flattened cells whose presence 

 becomes more obvious about the twenty-fifth day by a deposit 

 of dark brown pigment. The tubuli have thus a venous 

 blood supply. The glomerulus on the other hand is supplied 

 from the aorta. There is only one glomerulus on each side, 

 stretching each side of the alimentary canal and extending 

 through about the same space as the glandular part of the 

 kidney. Each glomerulus is a diverticulum of the peritoneum, 

 which generally becomes sacculated ; it receives its blood by a 

 single vessel on each side directly from the aorta. 



Since the time of Bowman it has been known that the 

 kidneys of Fishes, Frogs, and Snakes have a double blood 

 supply, the tubuli uriuiferi being surrounded by a capillary net- 

 work of vessels which receive their blood from the renal portal 

 veins, and the glomerulus which is supplied with blood from 

 the aorta by the renal artery. It is an interesting fact to find 

 that a similar blood supply is present from the very first in 

 such a temporary organ as the pronephros of the Lamprey. 



In the great majority of cases I found fine ciliated funnels 

 in each pronephros. The whole gland did not extend over a 

 greater space than that occupied by three myomeres, although 

 in some cases the ciliated funnels, which were of some length, 

 overlapped into a fourth myomere, but I was unable to confirm 

 the relationship alleged to exist between the number of ciliated 

 funnels and the number of somites through which the pro- 

 nephros extended. 



The Skeleton. 

 The skeletons of the oldest larva at my disposition consisted 

 of the notochord derived from the endoderm, and of certain carti- 

 lages in the head and branchial region derived from the lateral 

 mesoblast. The origin of the notochord has been completely 



