ALIMENTARY CANAL. 



769 



(TO 



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, 



into two 



~u w 



and then grow out 



FIG. 421. SECTION THROUGH THE 

 VENTRAL PART OF THE TRUNK OF A 

 YOUNG EMBRYO OF SCYLLIUM AT THE 

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



421) and Amphibia, or have from 

 the first the form of two some- 



/'. pectoral fin ; an. dorsal aorta ; 

 cav. cardinal vein ; ita. vitelline ar- 

 tery ; nv. vitelline vein united with 

 what Unequal divertiCLlla, as in subintestinal vein ; al. duodenum ; 



Birds (fig. 422), or again as in Lt imo t ^^ 



the Rabbit (Kolliker) one di- de-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 



