Development of the liver in the embryo.— The development of the liver in the embryo commences so early in Mammiferous animals, hurries so rapidly through its different phases, and is completed so soon, that it has hitherto been impossible to obtain any connected and precise information with regard to its progress. The observations of eminent physiologists made from time to time have, however, shewn that the mode of its development is in all respects similar to the development of the liver in the chick. Indeed, the egg of the bird is in the highest degree favourable to anatomical exa mination, both on account of its large size and the facility with which the incubated egg may be obtained from hour to hour, and from day to day. The principle of development there fore being the same in the ovum of the bird as in Marnmifera, I shall here trace the progress of the liver in the chick according to the most recent researches of Baer.
In the embryo of the fowl at the commence ment of the third day, the common vein of the body is embraced by two pyramidal ccecal pouches which communicate by their bases with the intestinal canal, and which shoot for wards so as to carry before them a fold of the vascular layer of the germinal membrane, in which they begin to ramify by giving off ccecal branches from their sides and extremities. These two ccecal tubuli with their correspond ing ramifications form two flattened processes, which represent the two lateral lobes of the liver. By the end of the third day the two processes resemble .folds of the vascular layer in which the tubuli are seen ramifying; they have increased in size and almost surround the vein. On the fourth day the liver has the appear ance of two flattened processes which enclose the vena The hepatic tubuli have be come lengthened and further removed from the intestine, and have ramified more freely in the vascular layer. By their bases the hepatic tubuli approach nearer to each other, and at the end of the fourth day they coalesce and form a common tube. On the fifth day the liver has attained considerable size; its two lobes have become thick and appear to possess a spongy texture in their interior. The hepatic ducts are connected with the intestine by a commun duct, the ductus communis chole dochus; and the portal vein gives off large branches which are distributed among the ra mifications of the ducts. On the sixth and seventh days the liver receives an abundance of blood and is nearly as red as the auricle of the heart. The left lobe is sensibly smaller than the right. On the eightlr,-nhinth, and tenth days the liver has lost its great redness and presents a yellowish brown tint; the vessels have diminished in calibre, while the paren chyma has increased, and the gall-bladder has become apparent. The succeeding days aug ment the size of the organ, and mould it to the form which it possesses after the escape of the chick From the egg; it begins to secrete bile; and the gall-bladder assumes the pyri form shape which it retains in after-life.
In the human ovum the formation of the embryo commences visibly at about the third week of intrauterine existence; the parietes which separate the embryo from the ovum begin to be developed, and rudiments of the intestinal canal, the liver, and the heart soon become distinctly visible. Upon its earliest appearance
the liver is of large size, and between the third and the fifth week is one-half the weight of the entire body, divided into several lobes of a reddish grey colour, and receives a large pro portion of blood from the omphalo-mesenteric vein. From the fifth to the eighth week the liver extends as low as the margin of the pelvis; it is soft, almost pulpy, and greyish in colour. The gall-bladder is developed in the form of a lengthened filiform cord, having an extremely minute canal through its centre. By the third lunar month the liver extends nearly to the pelvis and almost fills the abdomen, and the right lobe has increased somewhat beyond the left. The texture is more firm and of a redder colour, and the gall-bladder is long and conical. At the fourth lunar month the liver is still prolonged nearly to the margin of the pelvis, but the left lobe is evidently shorter than the right. The gall-bladder is elongated, straight, and vertical in direction, and contains a little mucus. Upon its internal surface a few rugre begin to be perceived ; it receives no bile, although a small quantity of that fluid is secreted by the liver and poured into the in testine. By the fifth lunar month the liver has acquired an increased consistence and deeper colour. It no longer descends so low as the pelvis, but appears to have diminished in bulk in proportion with the size of the abdomen. The gall-bladder assumes a more horizontal direction, and the contained mucus has a yel lowish green tint. The openings of the ductus choledochus and pancreatic duct, at first placed at a considerable distance from each other, approximate and produce less projection of the mucous membrane. By the sixth lunar month the descent of the liver is still more curtailed, the foetus increases in development from before backwards, and the organ becomes more horizontal. By the seventh lunar month the gall-bladder contains bile, and the mucous membrane becomes rugous and reticulated. At the eighth month, and during the ninth and tenth months, the liver becomes still more ho rizontal in position and of a deep red colour. The bile is more abundant and of a clear green tint. At the tenth month, that is, at birth, the relative proportion of the liver to the rest of the body is as I to 18 or 20; the average in the adult being as I to 36. After birth the size and weight of the liver diminish until the end of the first year, for, according to Meckel, the liver of the newly born infant weighs one fourth heavier than at the age of eight or ten months. The borders of the liver are rounded in the foetus, and the inferior surface is convex. The lobes are nearly equal until birth, after which the left diminishes in size, the right re maining stationary or growing but little, and at the age of one year the left lobe is scarcely one-half so large as at birth. The texture of the liver in the fcetus is soft and fragile and apparently homogeneous in structure; during the earlier periods its colour is a light brownish grey ; at about the mid-period it becomes deeply red, and after birth loses a portion of its colour from a diminution of the quantity of blood circulating through it.