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.
Fig. 421. Section through the ventral part of the trunk of a young embryo of Scyllium at the level of the umbilical cord.
b. pectoral fin; ao. dorsal aorta; cav. cardinal vein; ua. vitelline artery; uv. vitelline vein united with subintestinal vein; al. duodenum; l. liver; sd. opening of segmental duct into the body-cavity; mp. muscle-plate; um. umbilical canal.
In all true Vertebrata the gland has a much more complicated structure. It arises as a ventral outgrowth of the duodenum ([fig. 420], l). This outgrowth may be at first single, and then grow out into two lobes, as in 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 (Kölliker) 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 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 Götte to be lacunar, but in Elasmobranchii, and probably Vertebrata generally, they are from the first provided with distinct though delicate walls.
It is still doubtful whether the hepatic cylinders are as a rule hollow or solid. In Elasmobranchii they are at first provided with a large lumen, which though it becomes gradually smaller never entirely vanishes. The same seems to hold good for Amphibia and some Mammalia. In Aves the lumen of the cylinders is even from the first much more difficult to see, and the cylinders are stated by Remak to be solid, and he has been followed in this matter by Kölliker. In the Rabbit also Kölliker finds the cylinders to be solid.
The embryonic hepatic network gives rise to the parenchyma of the adult liver, with which in its general arrangement it closely agrees. The blood-channels are at first very large, and have a very irregular arrangement; and it is not till comparatively late that the hepatic lobules with their characteristic vascular structures become established.