94-4 OF GENERATION EMBRYONIC DEVELOPMENT. 



rudiments of the Liver, which is formed in the chick at about the 60th hour 

 of incubation by the thickening of the meso- and hypoblastic cells in the 

 wall of the duodenum, at the spot at which the hepatic duct is subsequently 

 to discharge itself. This thickening (Fig. 349), which is situated a little 



FIG. 349. 



Origin of the Liver from the intestinal wall, in the embryo of the Fowl, on the fourth day of incuba- 

 tion : a, heart; 6, intestine; c, everted portion giving origin to liver; </, liver; e, portion of vitelliue 

 vesicle. 



behind the heart, soon presents two diverticula which embrace the meatus 

 venosus or united trunk of the omphalo-mesaraic veins, which, as it passes 

 between them, exhibits numerous small bulgiugs. Towards the end of the 

 third day, according to Foster and Balfour, there may be observed in the 

 greatly thickened mesoblastic investment of each diverticulum a number of 

 solid cylindrical masses of cells which are apparently outgrowths from the 

 hypoblast of the diverticulum. These unite and form a network ; the meso- 

 blast contained in the meshes becoming at the same time largely converted 

 into bloodvessels. In addition to this network of solid hypoblastic cylinders, 

 the diverticula also send out hollow processes lined with hypoblast. On the 

 fourth day, the' two masses meet beneath the meatus venosus. The blood- 

 vessels of the mesoblast enter into communication with the meatus venosus. 

 " It may soon be observed that in those vessels which are connected with the 

 posterior part of the liver the stream of blood is directed from the meatus 

 veuosus into the network of the liver. In those connected with the anterior . 

 part the reverse is the case ; here the blood flows from the liver into the 

 meatus venosus. The thick network of solid cylinders represents the hepatic 

 parenchyma of the adult liver, while the hollow processes of the deuticula 

 are the rudiments of the biliary ducts." (Foster and Balfour.) As this is 

 going on, the hepatic mass is gradually removed to a distance from the wall 

 of the alimentary canal ; and the ciecum is narrowed and lengthened, so as 

 to become a mere connecting pedicle, forming, in fact, the main trunk of the 

 hepatic duct. In the Human embryo, the formation of the Liver begins at 

 about the third week of intrauteriue existence ; the organ is from the first 

 of very large size, when compared with that of the body ; and between the 

 third and fifth weeks, it is one-half the weight of the entire embryo. It is 

 at that period divided into several lobes. By the third lunar month, the liver 

 extends nearly to the pelvis, and almost fills the abdomen ; the right side now 

 begins to gain upon the left: the gall-bladder makes its first appearance at 

 this time. The subsequent changes chiefly consist in the consolidation of 

 the viscus, and the diminution of its proportional size. Up to the period of 

 birth, however, the bulk of the liver, relatively to that of the entire body, 

 is much greater than in the adult; the proportion being as 1 to 18 or 20 in 

 the newborn child, whilst it is about 1 to 06 in the adult; and the difference 

 between the right and left lobes is still inconsiderable. During the first 

 year of extrauterine life, however, a great change takes place ; the right 



