032 



THE LIVER. 



dorsal bide of the rectum, aud in many other forms there is a ca;cum 

 at the commencement of the rectum or of the large intestine. 



\\\ Teleostei, the Sturgeon and Lepidosteus there opens into the 

 front end of the intestine a number of ca^cal pouches known as 

 the pancreatic caeca. In the adult iSturgeon these pouches unite to 

 form a compact gland, but in the embryo they arise as a series of 

 isolated outgrowths of the duodenum. 



Connected with the anterior portion of the middle region of the 

 alimentary canal, which may be called the duodenum, are two very 

 important and constant glandular organs, the liver and the pancreas. 

 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 un- 

 branched 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 complicated 

 structure. It arises as a ventral outgrowth of the duodenum (fig. 

 420, I). This outgrowth may be at first single, and then grow out 

 into two lobes, as iu Elasmobranchii (fig. 421) and Amphibia, or have 

 from the first the form of two somewhat unequal diverticula, as in 

 Birds (fig. 422), or again as in the Rabbit (Kolliker) one diverticulum 

 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 ir- 

 regular network. Coincidently with 

 the formation of the hepatic network 

 the united vitelline and visceral vein 

 or veins (w.v), in their passage through 

 the liver, give off numerous bran- 

 ches, and gradually break up into a 

 plexus of channels which form a 

 secondary network amongst the he- 

 patic cylinders. In Amphibia tliese 

 channels are stated by Gotte to be 

 lacunar, but in Elasmobranchii, and 

 probably Vertebrata generally, they 

 are from the first provided with dis- 

 tinct thouoh delicate walls. 



Fig. 421. Section through the 

 ventral part of the trunk of a 

 young embryo of scyllium at the 

 level of the umbilical cord. 



h. pectoral fin ; ao. dorsal aorta ; 

 cav. cardinal vein ; ua. vitelline ar- 

 tery; uv. vitelline vein united with 

 subintestinal vein ; al. duodenum ; 

 /. liver; »d. opening of segmental 

 duet into the body-cavity ; vq}. mus- 

 cle-plate ; um. umbilical canal. 



