DEVELOPMENT OF THE ELASMOBRANCH LIVER 335 



figured a cleared embryo of Squalus acanthias approximately 

 18 mm. in length in which the liver is pictured as a dark irregular 

 mass lying just posterior to the heart. He described the cells of 

 this organ as arranged in lobules and containing numerous fat 

 droplets embedded in a homogeneous ground substance. He 

 also traced the course of the omphalo-mesenteric veins through 

 the liver. 



Balfour ('76) working with Scyllium, Pristiurus and Torpedo, 

 described the liver as arising in Stage I, when forty-eight pairs 

 of somites and three pairs of gill pouches are present, as a ventral 

 outgrowth from the ' duodenum' directly anterior to the umbilical 

 canal. This outpouching gives off at once two lateral diverticula 

 which are the rudimentary lobes of the liver, while the remainder 

 of the original ventral median pouch forms the gall bladder and 

 ductus choledochus. The hepatic tubule diverticula appear 

 as hollow buds by stage K, and increasing rapidly both in length 

 and number soon anastomose forming a regular network. In 

 the course of these changes the lumina of the tubules become 

 much reduced in size. The gall bladder arises as dilatation of the 

 anterior end of the median pouch and its duct joining with the 

 hepatic ducts forms the ductus choledochus. 



Hammar ('93) figured and described the first series of recon- 

 structions of the selachian liver. These are of embryos of Tor- 

 pedo ocellata, the first of forty segments, and the remainder, 

 9, 11, 15 and 18 mm. long respectively. These specimens corre- 

 spond roughly to Balfour's stages J, K, L, M, and O. From the 

 study of these models Hammar concludes that the liver arises 

 primarily from three diverticula, two lateral and one median in 

 position, and not from a single median and ventral pouch as stated 

 by Balfour. He also considered the liver proper to arise from the 

 lateral diverticula, the median giving rise to the gall bladder and 

 its duct only. He noted further that there was a twisting of the 

 fore gut from left to right, a point apparently overlooked by other 

 observers. The account of the formation of the common bile 

 duct and gall bladder is fairly complete, but he traced the hepatic 

 ducts no farther than their entrance into the lateral masses of 

 hepatic trabeculae. 



