THE BILIARY APPARATUS. 



1725 



S-^if- Common bile-duct 



latter, with a portion of those without a kimen, are converted into the system of 

 bile-canals, while the remaining cylinders give ofi additional sprouts which reduce 

 the intervening meshes and increase the solidity of the organ. The solid cylinders 

 of secreting tissue at first contain no bile-capillaries. The latter are hollowed out 

 between two adjacent cells as extensions of the meanwhile differentiating biliary 

 ducts. Differentiation of the developing liver into lobules does not occur until the 

 beginning of the fourth foetal month, by which time the larger blood-vessels and 

 bile-ducts become surrounded by condensations of the mesoderm which form the 

 capsule of'Glisson. 



The details of the formation of the hepatic blood-vessels are considered in con- 

 nection with the de\elopment of the veins (page 928). It may be here men- 

 tioned, however, that the primary circulation of the liver, including the portal vein, 

 the intralobular capillary net-work, and the hepatic veins, is derived from the modi- 

 fication of the vitelline veins, in conjunction with their tubularies from the digestive 

 organs. The relations of the placental circulation to the liver are secondary. The 

 left umbilical vein for a time pours practically all the blood returned from the placenta 

 into the portal vessel ; when the latter is no longer capable of receiving the entire 

 amount of the placental 



blood, the development Fig. 1460. 



of the ductus venosus 

 brings relief by establish- 

 ing a short cut by w^hich 

 the excess of placental 

 blood passes directly into 

 the ascending vena cava. 



The development of 

 the gall-bladder and its 

 duct proceeds, as already 

 indicated, from the more 

 caudally placed cystic di- 

 vision of the primary he- 

 patic diverticulum. The 

 subsequent changes in- 

 clude the growth and ex- 

 pansion of the terminal 

 portion of the primitive 

 cystic canal to form the 

 bile-sac, its elongated 

 stalk becoming the cystic 

 duct, while differentiation 

 of the entoblastic lining 



and the surrounding mesoblast produces the distinguishing details of the fully formed 

 organs. 



With the conversion of the primary liver-mass into the more definite organ, the 

 relations of the ventral mesentery, into which the early liver-anlage grows, become 

 chang'ed. For a time the developing liver lies within the septum transversum, but 

 later, with the formation of the diaphragm, it separates from the latter and projects 

 into the body-cavity. This projection results in a differentiation of the ventral 

 mesentery into three parts : (a) the middle portion, the layers of which become 

 separated by the growing liver to form its serous investment ; (fi) the anterior portion, 

 which extends from the front surface of the liver to the umbilicus and becomes the 

 falciform ligament enclosing the umbilical vein, later the ligamentum teres ; (c) the 

 posterior portion, which stretches between the digestive tube and the liver and, as 

 the gastro-hepatic, or lesser omentum, maintains similar relations and encloses the 

 biliary ducts. 



In the foetus the liver is relatively immense, especially at an early period. At 

 the fourth foetal month it practically fills the whole of the top of the abdomen. 

 Although it increases absolutely after this, it relatively diminishes, but at birth is still 

 considerably above the relative size of the adult organ, forming approximately one- 



,Gut-tube 





Portion of sagittal section of rabbit embryo, showing developing liver and 



ducts. X 95- 



