DEVELOPMENT OF THE EMBRYO. 999 



which was before carried on upon the plan of that of the higher Reptiles, now 

 becomes that of the complete Bird or Mammal. It is by no means unfrequent, 

 however, for some arrest of development to prevent the completion of these 

 changes; and various malformations, involving an imperfect discharge of the 

 function, may hence result. 



1005. The Alimentary Canal has been shown ( 997) to have its origin in 

 the blastodermic vesicle ; being a portion pinched off (as it were) from that part 

 of it which is just beneath the spinal column of the embryo, whilst the remain- 

 der, which is at that time the largest part of it, forms the vitelline or umbilical 

 vesicle. In its earliest form it is merely a long narrow tube (Fig. 269, m), 

 nearly straight, and communicating with the umbilical vesicle (n, n) at about 

 the middle of its length; thus it may be regarded as composed of the union of 

 two divisions, an upper and a lower. At first, neither mouth nor anus exists; 

 but these are formed early in the second month, if not before. The tube 

 gradually manifests a distinction into its special parts, oesophagus, stomach, 

 small intestine, and large intestine ; and the first change in its position occurs 

 in the stomach, which, from being disposed in the line of the body, takes an 

 oblique direction. The curves of the large and small intestines present them- 

 selves at a later period. It is at the lower part of the small intestine, near its 

 termination in the large, that the entrance of the vitelline duct exists ; and a 

 remnant of this canal is not unfrequently preserved throughout life, in the form 

 of a small pouch or diverticulum from that part of the intestine. In immediate 

 connection with the intestinal tube, we find the first rudiment of the Liver, 

 which is formed by the thickening of the cells in the wall of the canal, at the 

 spot at which the hepatic duct is subsequently to discharge itself. This thick- 

 ening increases, so as to form a projection upon the exterior of the canal ; and 

 soon afterwards the lining membrane of the intestine dips down into it, so that 

 a kind of caecum is formed, surrounded by a mass of cells, as shown in Fig. 

 270. The increase of the organ seems to take place by a continual new bud- 

 ding forth of cells from its peripheral portion; and a considerable mass is thus 

 formed, before the caecum in its interior undergoes any extension by ramifica- 

 tions into it. Gradually, however, the cells of the exterior become metamor- 

 phosed into fibrous tissue for the investment of the organ ; those of the interior 

 break down into ducts, which are developed in continuity with the caecum 

 derived from the intestine, and which are lined by muscular and fibrous tissues 

 developed from the primitive cellular blastema; whilst those which occupy the 

 intervening space, and which form the bulk of the gland, give origin to the 

 proper secreting cells, which are now to come into active operation. As this 

 is going on, the hepatic mass is gradu- 

 ally removed to a distance from the Fig. 270. 

 wall of the alimentary canal ; and the 

 caecum 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 intra-uterine 

 existence ; the organ is from the first of 

 very large size, when compared with 

 that of the body ; and between the third Origin of the Liver from the intestinal ^ in ^ 



and the faith weeks, it IS One-half _ the embryo of the Fowl, on the fourth day of incubation ; 

 Weight of the entire embryo. It is at a, heart; &, intestine; c, everted portion giving 

 that period divided into several lobes, origin to liver ;d, liver ;e, portion of vitelline vesicle. 



By the third lunar month, the liver ex- 

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



