DEVELOPMENT OF ELASMOBRANCH FISHES. 227 



Coincidently with the formation of the hepatic network, 

 the umbilical vein (PI. XI. fig. 9, u. v.) which unites wdth the 

 subintestiual or splanchnic vein (PI. X. fig. 8 V.) breaks up into 

 a series of channels, which form a second network in the spaces 

 of the hepatic network. These vascular channels of the liver 

 appear to me to have from the first distinct walls of delicate 

 spindle-shaped cells, and I have failed to find a stage similar 

 to that described by Gotte for Amphibians in which the blood- 

 channels are simply lacunar spaces in the hepatic parenchyma. 



The changes of the median duct of the liver are of rather 

 a passive nature. By stage its anterior end has dilated into 

 a distinct gall-bladder, whose duct receives in succession the 

 hepatic ducts, and so forms the ductus choledochus. The ductus 

 choledochus opens on the ventral side of the intestine immedi- 

 ately in front of the commencement of the spiral valve. 



It may be noted that the liver and pancreas are correspond- 

 ing ventral and dorsal appendages of the part of the alimentary 

 tract immediately in front of its junction with the yolk-sack. 



The subnotochordal rod. 



The existence of this remarkable body in Yertebrata was 

 first made known by Dr Gotte \ who not only demonstrated 

 its existence, but also gave a correct account of its develop- 

 ment. Its presence in Elasmobranchs and mode of deve- 

 lopment w^ere mentioned by myself in my preliminary ac- 

 count of the development of these fishes^, and it has been 

 independently observed and described by Professor Semper^ 

 No plausible suggestion as to its function has hitherto been 

 made, and it is therefore a matter of some difiiculty to 

 settle with what group of organs it ought to be treated. 

 In the presence of this difficulty it seemed best to deal with 

 it in this chapter, since it is unquestionably developed from 

 the wall of the alimentary canal. 



At its full growth this body forms a rod underlying the 

 notochord, and has nearly the same longitudinal extension as 



1 Archiv fvr Micros. Anatomie, Bd. v., and Entioicldungsgeschiclite d. TJnke. 



2 Quarterly Journal of Microscopic Science, Oct. 1874. 



^ Stammvencandschaft d. Wirhelthicre u. JVirbelloseu and Das Urogehital- 

 system d. Flayiostomen, Arb. Zool. Zoot. Institut. z. Wiirzburg, Bd. 11. 



