ALIMENTARY CANAL. 



769 



The liver. The liver is the earliest formed and largest 

 glandular organ in the embryo. 



It appears in its simplest 



form in Amphioxus as a single 

 unbranched diverticulum of the 

 alimentary tract, immediately 

 behind the respiratory region, 

 which is directed forwards and 

 placed on the left side of the 

 body. 



In all true Vertebrata the 

 gland has a much more compli- 

 cated structure. It arises as a 

 ventral outgrowth of the duode- 

 num (fig. 420, /). This out- 

 growth may be at first single, FIG. 421. SECTION THROUGH THE 

 and then grow out into two VENTRAI - PART F THE TRUNK OF A 



6 YOUNG EMBRYO OF SCYLLIUM AT THE 



lobes, as in Elasmobranchii (fig. LEVEL OF THE UMBILICAL CORD. 

 42l) and Amphibia, Or have from b. pectoral fin; ao. dorsal aorta; 



the first the form of two some- " cardinal vein; . vitelline ar- 



tery ; uv. vitelline vein united with 



what unequal diverticula, as in sawntestinal vein; al. duodenum; 



Birds (fig. 422), or again as in 



the Rabbit (Kolliker) one di- cle-plate ; /. umbilical canal. 

 verticulum may be first formed, and a second one appear 

 somewhat later. The hepatic diverticula, whatever may be 

 their primitive form, grow into a special thickening of the 

 splanchnic mesoblast. 



From the primitive diverticula there are soon given off a 

 number of hollow buds (fig. 421) which rapidly increase in 

 length and number, and form the so-called hepatic cylinders. 

 They soon anastomose and unite together, and so constitute an 

 irregular network. Coincidently with the formation of the 

 hepatic network the united vitelline and visceral vein or veins 

 (u.v\ in their passage through the liver, give off numerous 

 branches, and gradually break up into a plexus of channels 

 which form a secondary network amongst the hepatic cylinders. 

 In Amphibia these channels are stated by Gotte to be lacunar, 

 but in Elasmobranchii, and probably Vertebrata generally, they 

 arc from the first provided with distinct though delicate walls. 

 B. in. 49 



