7S0 



DEVELOPMENT OF THE ABDOMINAL GLANDS. 



more than the lining epitlielium of the bile-ducts and gall-ljladder, and 

 the hepatic or glandnlar cells are entirely derived from mesoblast ; but, 

 according to Foster and Balfour, following Reiuak and the earlier 

 observers, the cellular elements of the gland are stated to derive their 

 origin from the h3'poblast, and the mesoblast is mainly converted into 

 blood-vessels and the fibrous tissue of the ducts. 



Fig. 580. 



Fig. oSO. — E.VRLT Con- 

 dition OF THE Liver 

 IN THE Chick on thk 

 Third Bxy of Incu- 

 bation (from J. ]\Iiil- 

 Lr.) \" 



] , the lieai-t as a .sim- 

 ple curved tube ; 2, 2, 

 tlie intestinal tube : 3, 

 conical protrusion of the 

 coat of the commencing 

 intestine, on which the 

 blastema of the liver (4) 

 is formed ; 4, portion of 

 the layers of the germinal 

 membrane, passing into 

 the yolk-sac. 



The gall bladder is formed by extension from the wall of the main 

 duct. 



The blood-vessels formed in the liver become branches of the main 

 vein, which passes tlumigh the cellular mass. These are distinguish- 

 able as an anterior and posterior set, the arrangement of which is such 

 that the blood flows from stem to branches in the anterior, and from 

 branches to stem in the posterior. Thus the distinction is estab- 

 lished between portal and hepatic veins (sec the Development of the 

 Yeins). 



The solid cylinders of the blastema represent the hepatic lobular 

 tissue, the hollow processes the hepatic ducts ; but the origin of the 

 finest ducts is not known. Perhaps each cellular cylinder may be 

 looked upon as a collection of hepatic cells, in the centre of which is 

 the minute duct, according to the view now taken of the structure of 

 the adult liver (Foster and Balfour). 



The gall-bladder is at first tubular, and then has a rounded form. 

 The aU'eoli in its interior appear about the sixth month. At the 

 seventh month it first contains bile. In the fcetus its direction is more 

 horizontal than in the adult. 



The following are the principal peculiarities in the liver of the 

 foetus : — 



Size. — In the human fcetus, at the fifth or sixth week, the liver is so largo that 

 it is said to constitute one -half of the weight of the -n-hole body. This propor- 

 tion gradually decreases as development advances, until at the full pei'iod the 

 relative weight of the foetal liver to that of the body is as 1 to IS. 



In early foetal life, the right and left lobes of the liver are of equal, or nearly 

 equal, size. Later, the right preponderates, but not to such an extent as after 

 birth. Immediately before birth the relative weight of the left lobe to the riglit 

 is nearly as 1 to I'G. 



Position. — In consequence of the nearly equal size of the two lobes, the posi- 

 tion of the foetal liver in the abdomen is more symmetrical than in the adult. 



