The liver arises during stage I as a ventral outgrowth from the duodenum immediately in front of the opening of the umbilical canal (duct of the yolk-sack) into the intestine. Almost as soon as it is formed this outgrowth develops two lateral diverticula opening into a median canal.
The two diverticula are the rudimentary lobes of the liver, and the median duct is the rudiment of the common bile-duct (ductus choledochus) and gall-bladder (Pl. 11, fig. 9).
By stage K the hepatic diverticula have begun to bud out a number of small hollow knobs. These rapidly increase in length and number, and form the so-called hepatic cylinders. They anastomose and unite together, so that by stage L there is constructed a regular network. As the cylinders increase in length their lumen becomes very small, but appears never to vanish (Pl. 19, fig. 5).
The mode of formation of the liver parenchyma by hollow and not solid outgrowths agrees with the suggestion made in the Elements of Embryology, p. 133, and also with the results of Götte on the Amphibian liver. Schenk has thrown doubts upon the hypoblastic nature of the secreting tissue of the liver, but it does not appear to me, from my own investigations, that this point is open to question.
Coincidently with the formation of the hepatic network, the umbilical vein (Pl. 11, fig. 9, u.v.) which unites with the subintestinal or splanchnic vein (Pl. 11, fig. 8, V.) breaks up into a series of channels, which form a second network in the spaces of the hepatic network. These vascular channels of the liver appear to me to have from the first distinct walls of delicate spindle-shaped cells, and I have failed to find a stage similar to that described by Götte for Amphibians in which the blood-channels are simply lacunar spaces in the hepatic parenchyma.
The changes of the median duct of the liver are of rather a passive nature. By stage O its anterior end has dilated into a distinct gall-bladder, whose duct receives in succession the hepatic ducts, and so forms the ductus choledochus. The ductus choledochus opens on the ventral side of the intestine immediately in front of the commencement of the spiral valve.
It may be noted that the liver and pancreas are corresponding ventral and dorsal appendages of the part of the alimentary tract immediately in front of its junction with the yolk-sack.
The subnotochordal rod.
The existence of this remarkable body in Vertebrata was first made known by Dr Götte[310], who not only demonstrated its existence, but also gave a correct account of its development. Its presence in Elasmobranchii and mode of development were mentioned by myself in my preliminary account of the development of these fishes[311], and it has been independently observed and described by Professor Semper[312]. No plausible suggestion as to its function has hitherto been made, and it is therefore a matter of some difficulty to settle with what group of organs it ought to be treated. In the presence of this difficulty it seemed best to deal with it in this chapter, since it is unquestionably developed from the wall of the alimentary canal.
At its full growth this body forms a rod underlying the notochord, and has nearly the same longitudinal extension as this. It is indicated in most of my sections by the letter x. We may distinguish two sections of it, the one situated in the head, the other in the trunk. The junction between the two occurs at the hind border of the visceral clefts.